queue runner: fix nullptr deref on build exception after releasing a machine reservation

Add metric for builds waiting for download slot
(cherry picked from commit f23ec71227911891807706b6b978836e4d80edde)
2025-02-16 13:27:26 +01:00 · 2025-02-12 10:35:17 +01:00 · 2025-02-12 10:35:17 +01:00 · 2025-02-12 10:35:17 +01:00 · 2025-02-12 10:35:17 +01:00 · 2025-02-12 10:35:17 +01:00
35 changed files with 636 additions and 240 deletions
--- a/flake.lock
+++ b/flake.lock
@@ -1,10 +1,51 @@
 {
  "nodes": {
+    "flake-parts": {
+      "inputs": {
+        "nixpkgs-lib": [
+          "nix-eval-jobs",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1722555600,
+        "narHash": "sha256-XOQkdLafnb/p9ij77byFQjDf5m5QYl9b2REiVClC+x4=",
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "rev": "8471fe90ad337a8074e957b69ca4d0089218391d",
+        "type": "github"
+      },
+      "original": {
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "type": "github"
+      }
+    },
+    "libgit2": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1715853528,
+        "narHash": "sha256-J2rCxTecyLbbDdsyBWn9w7r3pbKRMkI9E7RvRgAqBdY=",
+        "owner": "libgit2",
+        "repo": "libgit2",
+        "rev": "36f7e21ad757a3dacc58cf7944329da6bc1d6e96",
+        "type": "github"
+      },
+      "original": {
+        "owner": "libgit2",
+        "ref": "v1.8.1",
+        "repo": "libgit2",
+        "type": "github"
+      }
+    },
    "nix": {
      "inputs": {
        "flake-compat": [],
        "flake-parts": [],
        "git-hooks-nix": [],
+        "libgit2": [
+          "libgit2"
+        ],
        "nixpkgs": [
          "nixpkgs"
        ],
@@ -12,58 +53,88 @@
        "nixpkgs-regression": []
      },
      "locked": {
-        "lastModified": 1739393420,
-        "narHash": "sha256-jGFuyYKJjJZsBRoi7ZcaVKt1OYxusz/ld1HA7VD2w/0=",
+        "lastModified": 1726787955,
+        "narHash": "sha256-XFznzb8L4SdUm9u+w3DPpMWJhffuv+/6+aiVl00slns=",
        "owner": "NixOS",
        "repo": "nix",
-        "rev": "970942f45836172fda410a638853382952189eb9",
+        "rev": "a7fdef6858dd45b9d7bda7c92324c63faee7f509",
        "type": "github"
      },
      "original": {
        "owner": "NixOS",
-        "ref": "2.26-maintenance",
+        "ref": "2.24-maintenance",
        "repo": "nix",
        "type": "github"
      }
    },
    "nix-eval-jobs": {
-      "flake": false,
+      "inputs": {
+        "flake-parts": "flake-parts",
+        "nix-github-actions": [],
+        "nixpkgs": [
+          "nixpkgs"
+        ],
+        "treefmt-nix": "treefmt-nix"
+      },
      "locked": {
-        "lastModified": 1739500569,
-        "narHash": "sha256-3wIReAqdTALv39gkWXLMZQvHyBOc3yPkWT2ZsItxedY=",
+        "lastModified": 1733814344,
+        "narHash": "sha256-3wwtKpS5tUBdjaGeSia7CotonbiRB6K5Kp0dsUt3nzU=",
        "owner": "nix-community",
        "repo": "nix-eval-jobs",
-        "rev": "4b392b284877d203ae262e16af269f702df036bc",
+        "rev": "889ea1406736b53cf165b6c28398aae3969418d1",
        "type": "github"
      },
      "original": {
        "owner": "nix-community",
+        "ref": "release-2.24",
        "repo": "nix-eval-jobs",
        "type": "github"
      }
    },
    "nixpkgs": {
      "locked": {
-        "lastModified": 1739461644,
-        "narHash": "sha256-1o1qR0KYozYGRrnqytSpAhVBYLNBHX+Lv6I39zGRzKM=",
+        "lastModified": 1726688310,
+        "narHash": "sha256-Xc9lEtentPCEtxc/F1e6jIZsd4MPDYv4Kugl9WtXlz0=",
        "owner": "NixOS",
        "repo": "nixpkgs",
-        "rev": "97a719c9f0a07923c957cf51b20b329f9fb9d43f",
+        "rev": "dbebdd67a6006bb145d98c8debf9140ac7e651d0",
        "type": "github"
      },
      "original": {
        "owner": "NixOS",
-        "ref": "nixos-24.11-small",
+        "ref": "nixos-24.05-small",
        "repo": "nixpkgs",
        "type": "github"
      }
    },
    "root": {
      "inputs": {
+        "libgit2": "libgit2",
        "nix": "nix",
        "nix-eval-jobs": "nix-eval-jobs",
        "nixpkgs": "nixpkgs"
      }
+    },
+    "treefmt-nix": {
+      "inputs": {
+        "nixpkgs": [
+          "nix-eval-jobs",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1723303070,
+        "narHash": "sha256-krGNVA30yptyRonohQ+i9cnK+CfCpedg6z3qzqVJcTs=",
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "rev": "14c092e0326de759e16b37535161b3cb9770cea3",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "type": "github"
+      }
    }
  },
  "root": "root",
--- a/flake.nix
+++ b/flake.nix
@@ -1,25 +1,25 @@
 {
  description = "A Nix-based continuous build system";

-  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.11-small";
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.05-small";

-  inputs.nix = {
-    url = "github:NixOS/nix/2.26-maintenance";
-    inputs.nixpkgs.follows = "nixpkgs";
+  inputs.libgit2 = { url = "github:libgit2/libgit2/v1.8.1"; flake = false; };
+  inputs.nix.url = "github:NixOS/nix/2.24-maintenance";
+  inputs.nix.inputs.nixpkgs.follows = "nixpkgs";
+  inputs.nix.inputs.libgit2.follows = "libgit2";

-    # hide nix dev tooling from our lock file
-    inputs.flake-parts.follows = "";
-    inputs.git-hooks-nix.follows = "";
-    inputs.nixpkgs-regression.follows = "";
-    inputs.nixpkgs-23-11.follows = "";
-    inputs.flake-compat.follows = "";
-  };
+  inputs.nix-eval-jobs.url = "github:nix-community/nix-eval-jobs/release-2.24";
+  inputs.nix-eval-jobs.inputs.nixpkgs.follows = "nixpkgs";

-  inputs.nix-eval-jobs = {
-    url = "github:nix-community/nix-eval-jobs";
-    # We want to control the deps precisely
-    flake = false;
-  };
+  # hide nix dev tooling from our lock file
+  inputs.nix.inputs.flake-parts.follows = "";
+  inputs.nix.inputs.git-hooks-nix.follows = "";
+  inputs.nix.inputs.nixpkgs-regression.follows = "";
+  inputs.nix.inputs.nixpkgs-23-11.follows = "";
+  inputs.nix.inputs.flake-compat.follows = "";
+
+  # hide nix-eval-jobs dev tooling from our lock file
+  inputs.nix-eval-jobs.inputs.nix-github-actions.follows = "";

  outputs = { self, nixpkgs, nix, nix-eval-jobs, ... }:
    let
@@ -30,9 +30,9 @@

      # A Nixpkgs overlay that provides a 'hydra' package.
      overlays.default = final: prev: {
-        nix-eval-jobs = final.callPackage nix-eval-jobs {};
        hydra = final.callPackage ./package.nix {
          inherit (nixpkgs.lib) fileset;
+          nix-eval-jobs = nix-eval-jobs.packages.${final.system}.default;
          rawSrc = self;
          nix-perl-bindings = final.nixComponents.nix-perl-bindings;
        };
@@ -74,19 +74,11 @@
      });

      packages = forEachSystem (system: {
-        nix-eval-jobs = nixpkgs.legacyPackages.${system}.callPackage nix-eval-jobs {
-          nix = nix.packages.${system}.nix;
-        };
        hydra = nixpkgs.legacyPackages.${system}.callPackage ./package.nix {
          inherit (nixpkgs.lib) fileset;
-          inherit (self.packages.${system}) nix-eval-jobs;
+          nix-eval-jobs = nix-eval-jobs.packages.${system}.default;
          rawSrc = self;
-          inherit (nix.packages.${system})
-            nix-util
-            nix-store
-            nix-main
-            nix-cli
-            ;
+          nix = nix.packages.${system}.nix;
          nix-perl-bindings = nix.hydraJobs.perlBindings.${system};
        };
        default = self.packages.${system}.hydra;
--- a/meson.build
+++ b/meson.build
@@ -8,22 +8,22 @@ project('hydra', 'cpp',
  ],
 )

-nix_util_dep = dependency('nix-util', required: true)
 nix_store_dep = dependency('nix-store', required: true)
 nix_main_dep = dependency('nix-main', required: true)
+nix_expr_dep = dependency('nix-expr', required: true)
+nix_flake_dep = dependency('nix-flake', required: true)
+nix_cmd_dep = dependency('nix-cmd', required: true)

 # Nix need extra flags not provided in its pkg-config files.
 nix_dep = declare_dependency(
  dependencies: [
-    nix_util_dep,
    nix_store_dep,
    nix_main_dep,
+    nix_expr_dep,
+    nix_flake_dep,
+    nix_cmd_dep,
  ],
-  compile_args: [
-    '-include', 'nix/config-util.hh',
-    '-include', 'nix/config-store.hh',
-    '-include', 'nix/config-main.hh',
-  ],
+  compile_args: ['-include', 'nix/config.h'],
 )

 pqxx_dep = dependency('libpqxx', required: true)
--- a/package.nix
+++ b/package.nix
@@ -8,10 +8,7 @@

 , perlPackages

-, nix-util
-, nix-store
-, nix-main
-, nix-cli
+, nix
 , nix-perl-bindings
 , git

@@ -93,6 +90,7 @@ let
        DateTime
        DBDPg
        DBDSQLite
+        DBIxClassHelpers
        DigestSHA1
        EmailMIME
        EmailSender
@@ -165,7 +163,7 @@ stdenv.mkDerivation (finalAttrs: {
    nukeReferences
    pkg-config
    mdbook
-    nix-cli
+    nix
    perlDeps
    perl
    unzip
@@ -175,9 +173,7 @@ stdenv.mkDerivation (finalAttrs: {
    libpqxx
    openssl
    libxslt
-    nix-util
-    nix-store
-    nix-main
+    nix
    perlDeps
    perl
    boost
@@ -204,14 +200,13 @@ stdenv.mkDerivation (finalAttrs: {
    glibcLocales
    libressl.nc
    python3
-    nix-cli
  ];

  hydraPath = lib.makeBinPath (
    [
      subversion
      openssh
-      nix-cli
+      nix
      coreutils
      findutils
      pixz
@@ -272,7 +267,7 @@ stdenv.mkDerivation (finalAttrs: {
            --prefix PATH ':' $out/bin:$hydraPath \
            --set HYDRA_RELEASE ${version} \
            --set HYDRA_HOME $out/libexec/hydra \
-            --set NIX_RELEASE ${nix-cli.name or "unknown"} \
+            --set NIX_RELEASE ${nix.name or "unknown"} \
            --set NIX_EVAL_JOBS_RELEASE ${nix-eval-jobs.name or "unknown"}
    done
  '';
@@ -280,5 +275,5 @@ stdenv.mkDerivation (finalAttrs: {
  dontStrip = true;

  meta.description = "Build of Hydra on ${stdenv.system}";
-  passthru = { inherit perlDeps; };
+  passthru = { inherit perlDeps nix; };
 })
--- a/src/hydra-queue-runner/build-remote.cc
+++ b/src/hydra-queue-runner/build-remote.cc
@@ -7,7 +7,6 @@

 #include "build-result.hh"
 #include "path.hh"
-#include "legacy-ssh-store.hh"
 #include "serve-protocol.hh"
 #include "serve-protocol-impl.hh"
 #include "state.hh"
@@ -22,23 +21,28 @@

 using namespace nix;

-bool ::Machine::isLocalhost() const
-{
-    return storeUri.params.empty() && std::visit(overloaded {
-        [](const StoreReference::Auto &) {
-            return true;
-        },
-        [](const StoreReference::Specified & s) {
-            return
-               (s.scheme == "local" || s.scheme == "unix") ||
-               ((s.scheme == "ssh" || s.scheme == "ssh-ng") &&
-                s.authority == "localhost");
-        },
-    }, storeUri.variant);
-}
-
 namespace nix::build_remote {

+static Strings extraStoreArgs(std::string & machine)
+{
+    Strings result;
+    try {
+        auto parsed = parseURL(machine);
+        if (parsed.scheme != "ssh") {
+            throw SysError("Currently, only (legacy-)ssh stores are supported!");
+        }
+        machine = parsed.authority.value_or("");
+        auto remoteStore = parsed.query.find("remote-store");
+        if (remoteStore != parsed.query.end()) {
+            result = {"--store", shellEscape(remoteStore->second)};
+        }
+    } catch (BadURL &) {
+        // We just try to continue with `machine->sshName` here for backwards compat.
+    }
+
+    return result;
+}
+
 static std::unique_ptr<SSHMaster::Connection> openConnection(
    ::Machine::ptr machine, SSHMaster & master)
 {
@@ -47,11 +51,7 @@ static std::unique_ptr<SSHMaster::Connection> openConnection(
        command.push_back("--builders");
        command.push_back("");
    } else {
-        auto remoteStore = machine->storeUri.params.find("remote-store");
-        if (remoteStore != machine->storeUri.params.end()) {
-            command.push_back("--store");
-            command.push_back(shellEscape(remoteStore->second));
-        }
+        command.splice(command.end(), extraStoreArgs(machine->sshName));
    }

    auto ret = master.startCommand(std::move(command), {
@@ -198,7 +198,7 @@ static BasicDerivation sendInputs(
        MaintainCount<counter> mc2(nrStepsCopyingTo);

        printMsg(lvlDebug, "sending closure of ‘%s’ to ‘%s’",
-            localStore.printStorePath(step.drvPath), conn.machine->storeUri.render());
+            localStore.printStorePath(step.drvPath), conn.machine->sshName);

        auto now1 = std::chrono::steady_clock::now();

@@ -278,6 +278,32 @@ static BuildResult performBuild(
    return result;
 }

+static std::map<StorePath, UnkeyedValidPathInfo> queryPathInfos(
+    ::Machine::Connection & conn,
+    Store & localStore,
+    StorePathSet & outputs,
+    size_t & totalNarSize
+)
+{
+
+    /* Get info about each output path. */
+    std::map<StorePath, UnkeyedValidPathInfo> infos;
+    conn.to << ServeProto::Command::QueryPathInfos;
+    ServeProto::write(localStore, conn, outputs);
+    conn.to.flush();
+    while (true) {
+        auto storePathS = readString(conn.from);
+        if (storePathS == "") break;
+
+        auto storePath = localStore.parseStorePath(storePathS);
+        auto info = ServeProto::Serialise<UnkeyedValidPathInfo>::read(localStore, conn);
+        totalNarSize += info.narSize;
+        infos.insert_or_assign(std::move(storePath), std::move(info));
+    }
+
+    return infos;
+}
+
 static void copyPathFromRemote(
    ::Machine::Connection & conn,
    NarMemberDatas & narMembers,
@@ -386,8 +412,19 @@ void RemoteResult::updateWithBuildResult(const nix::BuildResult & buildResult)

 }

+/* Utility guard object to auto-release a semaphore on destruction. */
+template <typename T>
+class SemaphoreReleaser {
+public:
+    SemaphoreReleaser(T* s) : sem(s) {}
+    ~SemaphoreReleaser() { sem->release(); }
+
+private:
+    T* sem;
+};

 void State::buildRemote(ref<Store> destStore,
+    MachineReservation::ptr & reservation,
    ::Machine::ptr machine, Step::ptr step,
    const ServeProto::BuildOptions & buildOptions,
    RemoteResult & result, std::shared_ptr<ActiveStep> activeStep,
@@ -404,22 +441,14 @@ void State::buildRemote(ref<Store> destStore,

        updateStep(ssConnecting);

-        auto storeRef = machine->completeStoreReference();
-
-        auto * pSpecified = std::get_if<StoreReference::Specified>(&storeRef.variant);
-        if (!pSpecified || pSpecified->scheme != "ssh") {
-            throw Error("Currently, only (legacy-)ssh stores are supported!");
-        }
-
-        LegacySSHStoreConfig storeConfig {
-            pSpecified->scheme,
-            pSpecified->authority,
-            storeRef.params
-        };
-
-        auto master = storeConfig.createSSHMaster(
+        SSHMaster master {
+            machine->sshName,
+            machine->sshKey,
+            machine->sshPublicHostKey,
            false, // no SSH master yet
-            logFD.get());
+            false, // no compression yet
+            logFD.get(),
+        };

        // FIXME: rewrite to use Store.
        auto child = build_remote::openConnection(machine, master);
@@ -464,13 +493,19 @@ void State::buildRemote(ref<Store> destStore,
                conn.to,
                conn.from,
                our_version,
-                machine->storeUri.render());
+                machine->sshName);
        } catch (EndOfFile & e) {
            child->sshPid.wait();
            std::string s = chomp(readFile(result.logFile));
-            throw Error("cannot connect to ‘%1%’: %2%", machine->storeUri.render(), s);
+            throw Error("cannot connect to ‘%1%’: %2%", machine->sshName, s);
        }

+        // Do not attempt to speak a newer version of the protocol.
+        //
+        // Per https://github.com/NixOS/nix/issues/9584 should be handled as
+        // part of `handshake` in upstream nix.
+        conn.remoteVersion = std::min(conn.remoteVersion, our_version);
+
        {
            auto info(machine->state->connectInfo.lock());
            info->consecutiveFailures = 0;
@@ -499,7 +534,7 @@ void State::buildRemote(ref<Store> destStore,
        /* Do the build. */
        printMsg(lvlDebug, "building ‘%s’ on ‘%s’",
            localStore->printStorePath(step->drvPath),
-            machine->storeUri.render());
+            machine->sshName);

        updateStep(ssBuilding);

@@ -522,11 +557,29 @@ void State::buildRemote(ref<Store> destStore,
           get a build log. */
        if (result.isCached) {
            printMsg(lvlInfo, "outputs of ‘%s’ substituted or already valid on ‘%s’",
-                localStore->printStorePath(step->drvPath), machine->storeUri.render());
+                localStore->printStorePath(step->drvPath), machine->sshName);
            unlink(result.logFile.c_str());
            result.logFile = "";
        }

+        /* Throttle CPU-bound work. Opportunistically skip updating the current
+         * step, since this requires a DB roundtrip. */
+        if (!localWorkThrottler.try_acquire()) {
+            MaintainCount<counter> mc(nrStepsWaitingForDownloadSlot);
+            updateStep(ssWaitingForLocalSlot);
+            localWorkThrottler.acquire();
+        }
+        SemaphoreReleaser releaser(&localWorkThrottler);
+
+        /* Once we've started copying outputs, release the machine reservation
+         * so further builds can happen. We do not release the machine earlier
+         * to avoid situations where the queue runner is bottlenecked on
+         * copying outputs and we end up building too many things that we
+         * haven't been able to allow copy slots for. */
+        assert(reservation.unique());
+        reservation = 0;
+        wakeDispatcher();
+
        StorePathSet outputs;
        for (auto & [_, realisation] : buildResult.builtOutputs)
            outputs.insert(realisation.outPath);
@@ -539,10 +592,8 @@ void State::buildRemote(ref<Store> destStore,

            auto now1 = std::chrono::steady_clock::now();

-            auto infos = conn.queryPathInfos(*localStore, outputs);
-
            size_t totalNarSize = 0;
-            for (auto & [_, info] : infos) totalNarSize += info.narSize;
+            auto infos = build_remote::queryPathInfos(conn, *localStore, outputs, totalNarSize);

            if (totalNarSize > maxOutputSize) {
                result.stepStatus = bsNarSizeLimitExceeded;
@@ -551,7 +602,7 @@ void State::buildRemote(ref<Store> destStore,

            /* Copy each path. */
            printMsg(lvlDebug, "copying outputs of ‘%s’ from ‘%s’ (%d bytes)",
-                localStore->printStorePath(step->drvPath), machine->storeUri.render(), totalNarSize);
+                localStore->printStorePath(step->drvPath), machine->sshName, totalNarSize);

            build_remote::copyPathsFromRemote(conn, narMembers, *localStore, *destStore, infos);
            auto now2 = std::chrono::steady_clock::now();
@@ -590,7 +641,7 @@ void State::buildRemote(ref<Store> destStore,
            info->consecutiveFailures = std::min(info->consecutiveFailures + 1, (unsigned int) 4);
            info->lastFailure = now;
            int delta = retryInterval * std::pow(retryBackoff, info->consecutiveFailures - 1) + (rand() % 30);
-            printMsg(lvlInfo, "will disable machine ‘%1%’ for %2%s", machine->storeUri.render(), delta);
+            printMsg(lvlInfo, "will disable machine ‘%1%’ for %2%s", machine->sshName, delta);
            info->disabledUntil = now + std::chrono::seconds(delta);
        }
        throw;
--- a/src/hydra-queue-runner/builder.cc
+++ b/src/hydra-queue-runner/builder.cc
@@ -37,19 +37,22 @@ void State::builder(MachineReservation::ptr reservation)

        try {
            auto destStore = getDestStore();
+            // Might release the reservation.
            res = doBuildStep(destStore, reservation, activeStep);
        } catch (std::exception & e) {
            printMsg(lvlError, "uncaught exception building ‘%s’ on ‘%s’: %s",
-                localStore->printStorePath(reservation->step->drvPath),
-                reservation->machine->storeUri.render(),
+                localStore->printStorePath(activeStep->step->drvPath),
+                reservation ? reservation->machine->sshName : std::string("(no machine)"),
                e.what());
        }
    }

-    /* Release the machine and wake up the dispatcher. */
-    assert(reservation.unique());
-    reservation = 0;
-    wakeDispatcher();
+    /* If the machine hasn't been released yet, release and wake up the dispatcher. */
+    if (reservation) {
+        assert(reservation.unique());
+        reservation = 0;
+        wakeDispatcher();
+    }

    /* If there was a temporary failure, retry the step after an
       exponentially increasing interval. */
@@ -72,11 +75,11 @@ void State::builder(MachineReservation::ptr reservation)


 State::StepResult State::doBuildStep(nix::ref<Store> destStore,
-    MachineReservation::ptr reservation,
+    MachineReservation::ptr & reservation,
    std::shared_ptr<ActiveStep> activeStep)
 {
-    auto & step(reservation->step);
-    auto & machine(reservation->machine);
+    auto step(reservation->step);
+    auto machine(reservation->machine);

    {
        auto step_(step->state.lock());
@@ -150,7 +153,7 @@ State::StepResult State::doBuildStep(nix::ref<Store> destStore,
        buildOptions.buildTimeout = build->buildTimeout;

        printInfo("performing step ‘%s’ %d times on ‘%s’ (needed by build %d and %d others)",
-            localStore->printStorePath(step->drvPath), buildOptions.nrRepeats + 1, machine->storeUri.render(), buildId, (dependents.size() - 1));
+            localStore->printStorePath(step->drvPath), buildOptions.nrRepeats + 1, machine->sshName, buildId, (dependents.size() - 1));
    }

    if (!buildOneDone)
@@ -178,7 +181,7 @@ State::StepResult State::doBuildStep(nix::ref<Store> destStore,
                    unlink(result.logFile.c_str());
                }
            } catch (...) {
-                ignoreExceptionInDestructor();
+                ignoreException();
            }
        }
    });
@@ -196,7 +199,7 @@ State::StepResult State::doBuildStep(nix::ref<Store> destStore,
        {
            auto mc = startDbUpdate();
            pqxx::work txn(*conn);
-            stepNr = createBuildStep(txn, result.startTime, buildId, step, machine->storeUri.render(), bsBusy);
+            stepNr = createBuildStep(txn, result.startTime, buildId, step, machine->sshName, bsBusy);
            txn.commit();
        }

@@ -211,7 +214,7 @@ State::StepResult State::doBuildStep(nix::ref<Store> destStore,

        try {
            /* FIXME: referring builds may have conflicting timeouts. */
-            buildRemote(destStore, machine, step, buildOptions, result, activeStep, updateStep, narMembers);
+            buildRemote(destStore, reservation, machine, step, buildOptions, result, activeStep, updateStep, narMembers);
        } catch (Error & e) {
            if (activeStep->state_.lock()->cancelled) {
                printInfo("marking step %d of build %d as cancelled", stepNr, buildId);
@@ -253,7 +256,7 @@ State::StepResult State::doBuildStep(nix::ref<Store> destStore,
    /* Finish the step in the database. */
    if (stepNr) {
        pqxx::work txn(*conn);
-        finishBuildStep(txn, result, buildId, stepNr, machine->storeUri.render());
+        finishBuildStep(txn, result, buildId, stepNr, machine->sshName);
        txn.commit();
    }

@@ -261,7 +264,7 @@ State::StepResult State::doBuildStep(nix::ref<Store> destStore,
       issue). Retry a number of times. */
    if (result.canRetry) {
        printMsg(lvlError, "possibly transient failure building ‘%s’ on ‘%s’: %s",
-            localStore->printStorePath(step->drvPath), machine->storeUri.render(), result.errorMsg);
+            localStore->printStorePath(step->drvPath), machine->sshName, result.errorMsg);
        assert(stepNr);
        bool retry;
        {
@@ -452,7 +455,7 @@ void State::failStep(
                    build->finishedInDB)
                    continue;
                createBuildStep(txn,
-                    0, build->id, step, machine ? machine->storeUri.render() : "",
+                    0, build->id, step, machine ? machine->sshName : "",
                    result.stepStatus, result.errorMsg, buildId == build->id ? 0 : buildId);
            }

--- a/src/hydra-queue-runner/dispatcher.cc
+++ b/src/hydra-queue-runner/dispatcher.cc
@@ -40,13 +40,15 @@ void State::dispatcher()
            printMsg(lvlDebug, "dispatcher woken up");
            nrDispatcherWakeups++;

-            auto now1 = std::chrono::steady_clock::now();
+            auto t_before_work = std::chrono::steady_clock::now();

            auto sleepUntil = doDispatch();

-            auto now2 = std::chrono::steady_clock::now();
+            auto t_after_work = std::chrono::steady_clock::now();

-            dispatchTimeMs += std::chrono::duration_cast<std::chrono::milliseconds>(now2 - now1).count();
+            prom.dispatcher_time_spent_running.Increment(
+                std::chrono::duration_cast<std::chrono::microseconds>(t_after_work - t_before_work).count());
+            dispatchTimeMs += std::chrono::duration_cast<std::chrono::milliseconds>(t_after_work - t_before_work).count();

            /* Sleep until we're woken up (either because a runnable build
               is added, or because a build finishes). */
@@ -60,6 +62,10 @@ void State::dispatcher()
                *dispatcherWakeup_ = false;
            }

+            auto t_after_sleep = std::chrono::steady_clock::now();
+            prom.dispatcher_time_spent_waiting.Increment(
+                std::chrono::duration_cast<std::chrono::microseconds>(t_after_sleep - t_after_work).count());
+
        } catch (std::exception & e) {
            printError("dispatcher: %s", e.what());
            sleep(1);
@@ -256,7 +262,7 @@ system_time State::doDispatch()
                /* Can this machine do this step? */
                if (!mi.machine->supportsStep(step)) {
                    debug("machine '%s' does not support step '%s' (system type '%s')",
-                        mi.machine->storeUri.render(), localStore->printStorePath(step->drvPath), step->drv->platform);
+                        mi.machine->sshName, localStore->printStorePath(step->drvPath), step->drv->platform);
                    continue;
                }

--- a/src/hydra-queue-runner/hydra-queue-runner.cc
+++ b/src/hydra-queue-runner/hydra-queue-runner.cc
@@ -70,10 +70,31 @@ State::PromMetrics::PromMetrics()
            .Register(*registry)
            .Add({})
    )
-    , queue_max_id(
-        prometheus::BuildGauge()
-            .Name("hydraqueuerunner_queue_max_build_id_info")
-            .Help("Maximum build record ID in the queue")
+    , dispatcher_time_spent_running(
+        prometheus::BuildCounter()
+            .Name("hydraqueuerunner_dispatcher_time_spent_running")
+            .Help("Time (in micros) spent running the dispatcher")
+            .Register(*registry)
+            .Add({})
+    )
+    , dispatcher_time_spent_waiting(
+        prometheus::BuildCounter()
+            .Name("hydraqueuerunner_dispatcher_time_spent_waiting")
+            .Help("Time (in micros) spent waiting for the dispatcher to obtain work")
+            .Register(*registry)
+            .Add({})
+    )
+    , queue_monitor_time_spent_running(
+        prometheus::BuildCounter()
+            .Name("hydraqueuerunner_queue_monitor_time_spent_running")
+            .Help("Time (in micros) spent running the queue monitor")
+            .Register(*registry)
+            .Add({})
+    )
+    , queue_monitor_time_spent_waiting(
+        prometheus::BuildCounter()
+            .Name("hydraqueuerunner_queue_monitor_time_spent_waiting")
+            .Help("Time (in micros) spent waiting for the queue monitor to obtain work")
            .Register(*registry)
            .Add({})
    )
@@ -85,6 +106,7 @@ State::State(std::optional<std::string> metricsAddrOpt)
    : config(std::make_unique<HydraConfig>())
    , maxUnsupportedTime(config->getIntOption("max_unsupported_time", 0))
    , dbPool(config->getIntOption("max_db_connections", 128))
+    , localWorkThrottler(config->getIntOption("max_local_worker_threads", std::min(maxSupportedLocalWorkers, std::max(4u, std::thread::hardware_concurrency()) - 2)))
    , maxOutputSize(config->getIntOption("max_output_size", 2ULL << 30))
    , maxLogSize(config->getIntOption("max_log_size", 64ULL << 20))
    , uploadLogsToBinaryCache(config->getBoolOption("upload_logs_to_binary_cache", false))
@@ -135,26 +157,65 @@ void State::parseMachines(const std::string & contents)
        oldMachines = *machines_;
    }

-    for (auto && machine_ : nix::Machine::parseConfig({}, contents)) {
-        auto machine = std::make_shared<::Machine>(std::move(machine_));
+    for (auto line : tokenizeString<Strings>(contents, "\n")) {
+        line = trim(std::string(line, 0, line.find('#')));
+        auto tokens = tokenizeString<std::vector<std::string>>(line);
+        if (tokens.size() < 3) continue;
+        tokens.resize(8);
+
+        if (tokens[5] == "-") tokens[5] = "";
+        auto supportedFeatures = tokenizeString<StringSet>(tokens[5], ",");
+
+        if (tokens[6] == "-") tokens[6] = "";
+        auto mandatoryFeatures = tokenizeString<StringSet>(tokens[6], ",");
+
+        for (auto & f : mandatoryFeatures)
+            supportedFeatures.insert(f);
+
+        using MaxJobs = std::remove_const<decltype(nix::Machine::maxJobs)>::type;
+
+        auto machine = std::make_shared<::Machine>(nix::Machine {
+            // `storeUri`, not yet used
+            "",
+            // `systemTypes`
+            tokenizeString<StringSet>(tokens[1], ","),
+            // `sshKey`
+            tokens[2] == "-" ? "" : tokens[2],
+            // `maxJobs`
+            tokens[3] != ""
+                ? string2Int<MaxJobs>(tokens[3]).value()
+                : 1,
+            // `speedFactor`
+            std::stof(tokens[4].c_str()),
+            // `supportedFeatures`
+            std::move(supportedFeatures),
+            // `mandatoryFeatures`
+            std::move(mandatoryFeatures),
+            // `sshPublicHostKey`
+            tokens[7] != "" && tokens[7] != "-"
+                ? tokens[7]
+                : "",
+        });
+
+        machine->sshName = tokens[0];

        /* Re-use the State object of the previous machine with the
           same name. */
-        auto i = oldMachines.find(machine->storeUri.variant);
+        auto i = oldMachines.find(machine->sshName);
        if (i == oldMachines.end())
-            printMsg(lvlChatty, "adding new machine ‘%1%’", machine->storeUri.render());
+            printMsg(lvlChatty, "adding new machine ‘%1%’", machine->sshName);
        else
-            printMsg(lvlChatty, "updating machine ‘%1%’", machine->storeUri.render());
+            printMsg(lvlChatty, "updating machine ‘%1%’", machine->sshName);
        machine->state = i == oldMachines.end()
            ? std::make_shared<::Machine::State>()
            : i->second->state;
-        newMachines[machine->storeUri.variant] = machine;
+        newMachines[machine->sshName] = machine;
    }

    for (auto & m : oldMachines)
        if (newMachines.find(m.first) == newMachines.end()) {
            if (m.second->enabled)
-                printInfo("removing machine ‘%1%’", m.second->storeUri.render());
+                printInfo("removing machine ‘%1%’", m.first);
            /* Add a disabled ::Machine object to make sure stats are
               maintained. */
            auto machine = std::make_shared<::Machine>(*(m.second));
@@ -551,6 +612,7 @@ void State::dumpStatus(Connection & conn)
        {"nrActiveSteps", activeSteps_.lock()->size()},
        {"nrStepsBuilding", nrStepsBuilding.load()},
        {"nrStepsCopyingTo", nrStepsCopyingTo.load()},
+        {"nrStepsWaitingForDownloadSlot", nrStepsWaitingForDownloadSlot.load()},
        {"nrStepsCopyingFrom", nrStepsCopyingFrom.load()},
        {"nrStepsWaiting", nrStepsWaiting.load()},
        {"nrUnsupportedSteps", nrUnsupportedSteps.load()},
@@ -618,7 +680,7 @@ void State::dumpStatus(Connection & conn)
                    machine["avgStepTime"] = (float) s->totalStepTime / s->nrStepsDone;
                    machine["avgStepBuildTime"] = (float) s->totalStepBuildTime / s->nrStepsDone;
                }
-                statusJson["machines"][m->storeUri.render()] = machine;
+                statusJson["machines"][m->sshName] = machine;
            }
        }

--- a/src/hydra-queue-runner/queue-monitor.cc
+++ b/src/hydra-queue-runner/queue-monitor.cc
@@ -1,6 +1,7 @@
 #include "state.hh"
 #include "hydra-build-result.hh"
 #include "globals.hh"
+#include "thread-pool.hh"

 #include <cstring>

@@ -37,16 +38,21 @@ void State::queueMonitorLoop(Connection & conn)

    auto destStore = getDestStore();

-    unsigned int lastBuildId = 0;
-
    bool quit = false;
    while (!quit) {
+        auto t_before_work = std::chrono::steady_clock::now();
+
        localStore->clearPathInfoCache();

-        bool done = getQueuedBuilds(conn, destStore, lastBuildId);
+        bool done = getQueuedBuilds(conn, destStore);

        if (buildOne && buildOneDone) quit = true;

+        auto t_after_work = std::chrono::steady_clock::now();
+
+        prom.queue_monitor_time_spent_running.Increment(
+            std::chrono::duration_cast<std::chrono::microseconds>(t_after_work - t_before_work).count());
+
        /* Sleep until we get notification from the database about an
           event. */
        if (done && !quit) {
@@ -56,12 +62,10 @@ void State::queueMonitorLoop(Connection & conn)
            conn.get_notifs();

        if (auto lowestId = buildsAdded.get()) {
-            lastBuildId = std::min(lastBuildId, static_cast<unsigned>(std::stoul(*lowestId) - 1));
            printMsg(lvlTalkative, "got notification: new builds added to the queue");
        }
        if (buildsRestarted.get()) {
            printMsg(lvlTalkative, "got notification: builds restarted");
-            lastBuildId = 0; // check all builds
        }
        if (buildsCancelled.get() || buildsDeleted.get() || buildsBumped.get()) {
            printMsg(lvlTalkative, "got notification: builds cancelled or bumped");
@@ -71,6 +75,10 @@ void State::queueMonitorLoop(Connection & conn)
            printMsg(lvlTalkative, "got notification: jobset shares changed");
            processJobsetSharesChange(conn);
        }
+
+        auto t_after_sleep = std::chrono::steady_clock::now();
+        prom.queue_monitor_time_spent_waiting.Increment(
+            std::chrono::duration_cast<std::chrono::microseconds>(t_after_sleep - t_after_work).count());
    }

    exit(0);
@@ -84,20 +92,18 @@ struct PreviousFailure : public std::exception {


 bool State::getQueuedBuilds(Connection & conn,
-    ref<Store> destStore, unsigned int & lastBuildId)
+    ref<Store> destStore)
 {
    prom.queue_checks_started.Increment();

-    printInfo("checking the queue for builds > %d...", lastBuildId);
+    printInfo("checking the queue for builds...");

    /* Grab the queued builds from the database, but don't process
       them yet (since we don't want a long-running transaction). */
    std::vector<BuildID> newIDs;
-    std::map<BuildID, Build::ptr> newBuildsByID;
+    std::unordered_map<BuildID, Build::ptr> newBuildsByID;
    std::multimap<StorePath, BuildID> newBuildsByPath;

-    unsigned int newLastBuildId = lastBuildId;
-
    {
        pqxx::work txn(conn);

@@ -106,17 +112,12 @@ bool State::getQueuedBuilds(Connection & conn,
             "jobsets.name as jobset, job, drvPath, maxsilent, timeout, timestamp, "
             "globalPriority, priority from Builds "
             "inner join jobsets on builds.jobset_id = jobsets.id "
-             "where builds.id > $1 and finished = 0 order by globalPriority desc, builds.id",
-            lastBuildId);
+             "where finished = 0 order by globalPriority desc, random()");

        for (auto const & row : res) {
            auto builds_(builds.lock());
            BuildID id = row["id"].as<BuildID>();
            if (buildOne && id != buildOne) continue;
-            if (id > newLastBuildId) {
-                newLastBuildId = id;
-                prom.queue_max_id.Set(id);
-            }
            if (builds_->count(id)) continue;

            auto build = std::make_shared<Build>(
@@ -318,15 +319,13 @@ bool State::getQueuedBuilds(Connection & conn,

        /* Stop after a certain time to allow priority bumps to be
           processed. */
-        if (std::chrono::system_clock::now() > start + std::chrono::seconds(600)) {
+        if (std::chrono::system_clock::now() > start + std::chrono::seconds(60)) {
            prom.queue_checks_early_exits.Increment();
            break;
        }
    }

    prom.queue_checks_finished.Increment();
-
-    lastBuildId = newBuildsByID.empty() ? newLastBuildId : newBuildsByID.begin()->first - 1;
    return newBuildsByID.empty();
 }

@@ -405,6 +404,34 @@ void State::processQueueChange(Connection & conn)
 }


+std::map<DrvOutput, std::optional<StorePath>> State::getMissingRemotePaths(
+    ref<Store> destStore,
+    const std::map<DrvOutput, std::optional<StorePath>> & paths)
+{
+    Sync<std::map<DrvOutput, std::optional<StorePath>>> missing_;
+    ThreadPool tp;
+
+    for (auto & [output, maybeOutputPath] : paths) {
+        if (!maybeOutputPath) {
+            auto missing(missing_.lock());
+            missing->insert({output, maybeOutputPath});
+        } else {
+            tp.enqueue([&] {
+                if (!destStore->isValidPath(*maybeOutputPath)) {
+                    auto missing(missing_.lock());
+                    missing->insert({output, maybeOutputPath});
+                }
+            });
+        }
+    }
+
+    tp.process();
+
+    auto missing(missing_.lock());
+    return *missing;
+}
+
+
 Step::ptr State::createStep(ref<Store> destStore,
    Connection & conn, Build::ptr build, const StorePath & drvPath,
    Build::ptr referringBuild, Step::ptr referringStep, std::set<StorePath> & finishedDrvs,
@@ -485,16 +512,15 @@ Step::ptr State::createStep(ref<Store> destStore,

    /* Are all outputs valid? */
    auto outputHashes = staticOutputHashes(*localStore, *(step->drv));
-    bool valid = true;
-    std::map<DrvOutput, std::optional<StorePath>> missing;
+    std::map<DrvOutput, std::optional<StorePath>> paths;
    for (auto & [outputName, maybeOutputPath] : destStore->queryPartialDerivationOutputMap(drvPath, &*localStore)) {
        auto outputHash = outputHashes.at(outputName);
-        if (maybeOutputPath && destStore->isValidPath(*maybeOutputPath))
-            continue;
-        valid = false;
-        missing.insert({{outputHash, outputName}, maybeOutputPath});
+        paths.insert({{outputHash, outputName}, maybeOutputPath});
    }

+    auto missing = getMissingRemotePaths(destStore, paths);
+    bool valid = missing.empty();
+
    /* Try to copy the missing paths from the local store or from
       substitutes. */
    if (!missing.empty()) {
--- a/src/hydra-queue-runner/state.hh
+++ b/src/hydra-queue-runner/state.hh
@@ -6,6 +6,8 @@
 #include <map>
 #include <memory>
 #include <queue>
+#include <regex>
+#include <semaphore>

 #include <prometheus/counter.h>
 #include <prometheus/gauge.h>
@@ -57,6 +59,7 @@ typedef enum {
    ssConnecting = 10,
    ssSendingInputs = 20,
    ssBuilding = 30,
+    ssWaitingForLocalSlot = 35,
    ssReceivingOutputs = 40,
    ssPostProcessing = 50,
 } StepState;
@@ -240,6 +243,10 @@ struct Machine : nix::Machine
 {
    typedef std::shared_ptr<Machine> ptr;

+    /* TODO Get rid of: `nix::Machine::storeUri` is normalized in a way
+       we are not yet used to, but once we are, we don't need this. */
+    std::string sshName;
+
    struct State {
        typedef std::shared_ptr<State> ptr;
        counter currentJobs{0};
@@ -289,7 +296,11 @@ struct Machine : nix::Machine
        return true;
    }

-    bool isLocalhost() const;
+    bool isLocalhost()
+    {
+        std::regex r("^(ssh://|ssh-ng://)?localhost$");
+        return std::regex_search(sshName, r);
+    }

    // A connection to a machine
    struct Connection : nix::ServeProto::BasicClientConnection {
@@ -349,9 +360,13 @@ private:

    /* The build machines. */
    std::mutex machinesReadyLock;
-    typedef std::map<nix::StoreReference::Variant, Machine::ptr> Machines;
+    typedef std::map<std::string, Machine::ptr> Machines;
    nix::Sync<Machines> machines; // FIXME: use atomic_shared_ptr

+    /* Throttler for CPU-bound local work. */
+    static constexpr unsigned int maxSupportedLocalWorkers = 1024;
+    std::counting_semaphore<maxSupportedLocalWorkers> localWorkThrottler;
+
    /* Various stats. */
    time_t startedAt;
    counter nrBuildsRead{0};
@@ -361,6 +376,7 @@ private:
    counter nrStepsDone{0};
    counter nrStepsBuilding{0};
    counter nrStepsCopyingTo{0};
+    counter nrStepsWaitingForDownloadSlot{0};
    counter nrStepsCopyingFrom{0};
    counter nrStepsWaiting{0};
    counter nrUnsupportedSteps{0};
@@ -449,7 +465,12 @@ private:
        prometheus::Counter& queue_steps_created;
        prometheus::Counter& queue_checks_early_exits;
        prometheus::Counter& queue_checks_finished;
-        prometheus::Gauge& queue_max_id;
+
+        prometheus::Counter& dispatcher_time_spent_running;
+        prometheus::Counter& dispatcher_time_spent_waiting;
+
+        prometheus::Counter& queue_monitor_time_spent_running;
+        prometheus::Counter& queue_monitor_time_spent_waiting;

        PromMetrics();
    };
@@ -493,8 +514,7 @@ private:
    void queueMonitorLoop(Connection & conn);

    /* Check the queue for new builds. */
-    bool getQueuedBuilds(Connection & conn,
-        nix::ref<nix::Store> destStore, unsigned int & lastBuildId);
+    bool getQueuedBuilds(Connection & conn, nix::ref<nix::Store> destStore);

    /* Handle cancellation, deletion and priority bumps. */
    void processQueueChange(Connection & conn);
@@ -502,6 +522,12 @@ private:
    BuildOutput getBuildOutputCached(Connection & conn, nix::ref<nix::Store> destStore,
        const nix::StorePath & drvPath);

+    /* Returns paths missing from the remote store. Paths are processed in
+     * parallel to work around the possible latency of remote stores. */
+    std::map<nix::DrvOutput, std::optional<nix::StorePath>> getMissingRemotePaths(
+        nix::ref<nix::Store> destStore,
+        const std::map<nix::DrvOutput, std::optional<nix::StorePath>> & paths);
+
    Step::ptr createStep(nix::ref<nix::Store> store,
        Connection & conn, Build::ptr build, const nix::StorePath & drvPath,
        Build::ptr referringBuild, Step::ptr referringStep, std::set<nix::StorePath> & finishedDrvs,
@@ -537,10 +563,11 @@ private:
       retried. */
    enum StepResult { sDone, sRetry, sMaybeCancelled };
    StepResult doBuildStep(nix::ref<nix::Store> destStore,
-        MachineReservation::ptr reservation,
+        MachineReservation::ptr & reservation,
        std::shared_ptr<ActiveStep> activeStep);

    void buildRemote(nix::ref<nix::Store> destStore,
+        MachineReservation::ptr & reservation,
        Machine::ptr machine, Step::ptr step,
        const nix::ServeProto::BuildOptions & buildOptions,
        RemoteResult & result, std::shared_ptr<ActiveStep> activeStep,
--- a/src/lib/Hydra/Controller/Build.pm
+++ b/src/lib/Hydra/Controller/Build.pm
@@ -238,7 +238,7 @@ sub serveFile {
        # XSS hole.
        $c->response->header('Content-Security-Policy' => 'sandbox allow-scripts');

-        $c->stash->{'plain'} = { data => grab(cmd => ["nix", "--experimental-features", "nix-command",
+        $c->stash->{'plain'} = { data => readIntoSocket(cmd => ["nix", "--experimental-features", "nix-command",
                                                      "store", "cat", "--store", getStoreUri(), "$path"]) };

        # Detect MIME type.
--- a/src/lib/Hydra/Controller/Jobset.pm
+++ b/src/lib/Hydra/Controller/Jobset.pm
@@ -364,6 +364,21 @@ sub evals_GET {
    );
 }

+sub errors :Chained('jobsetChain') :PathPart('errors') :Args(0) :ActionClass('REST') { }
+
+sub errors_GET {
+    my ($self, $c) = @_;
+
+    $c->stash->{template} = 'eval-error.tt';
+
+    my $jobsetName = $c->stash->{params}->{name};
+    $c->stash->{jobset} = $c->stash->{project}->jobsets->find(
+        { name => $jobsetName },
+        { '+columns' => { 'errormsg' => 'errormsg' } }
+    );
+
+    $self->status_ok($c, entity => $c->stash->{jobset});
+}

 # Redirect to the latest finished evaluation of this jobset.
 sub latest_eval : Chained('jobsetChain') PathPart('latest-eval') {
--- a/src/lib/Hydra/Controller/JobsetEval.pm
+++ b/src/lib/Hydra/Controller/JobsetEval.pm
@@ -86,6 +86,17 @@ sub view_GET {
    );
 }

+sub errors :Chained('evalChain') :PathPart('errors') :Args(0) :ActionClass('REST') { }
+
+sub errors_GET {
+    my ($self, $c) = @_;
+
+    $c->stash->{template} = 'eval-error.tt';
+
+    $c->stash->{eval} = $c->model('DB::JobsetEvals')->find($c->stash->{eval}->id, { prefetch => 'evaluationerror' });
+
+    $self->status_ok($c, entity => $c->stash->{eval});
+}

 sub create_jobset : Chained('evalChain') PathPart('create-jobset') Args(0) {
    my ($self, $c) = @_;
--- a/src/lib/Hydra/Controller/Root.pm
+++ b/src/lib/Hydra/Controller/Root.pm
@@ -162,7 +162,7 @@ sub status_GET {
            { "buildsteps.busy" => { '!=', 0 } },
            { order_by => ["globalpriority DESC", "id"],
              join => "buildsteps",
-              columns => [@buildListColumns]
+              columns => [@buildListColumns, 'buildsteps.drvpath', 'buildsteps.type']
            })]
    );
 }
--- a/src/lib/Hydra/Helper/BuildDiff.pm
+++ b/src/lib/Hydra/Helper/BuildDiff.pm
@@ -37,7 +37,16 @@ sub buildDiff {

    my $n = 0;
    foreach my $build (@{$builds}) {
-        my $aborted = $build->finished != 0 && ($build->buildstatus == 3 || $build->buildstatus == 4);
+        my $aborted = $build->finished != 0 && (
+            # aborted
+            $build->buildstatus == 3
+            # cancelled
+            || $build->buildstatus == 4
+            # timeout
+            || $build->buildstatus == 7
+            # log limit exceeded
+            || $build->buildstatus == 10
+        );
        my $d;
        my $found = 0;
        while ($n < scalar(@{$builds2})) {
@@ -79,4 +88,4 @@ sub buildDiff {
    return $ret;
 }

-1;
+1;
--- a/src/lib/Hydra/Helper/Nix.pm
+++ b/src/lib/Hydra/Helper/Nix.pm
@@ -36,6 +36,7 @@ our @EXPORT = qw(
    jobsetOverview
    jobsetOverview_
    pathIsInsidePrefix
+    readIntoSocket
    readNixFile
    registerRoot
    restartBuilds
@@ -296,8 +297,7 @@ sub getEvals {

    my @evals = $evals_result_set->search(
        { hasnewbuilds => 1 },
-        { order_by => "$me.id DESC", rows => $rows, offset => $offset
-        , prefetch => { evaluationerror => [ ] } });
+        { order_by => "$me.id DESC", rows => $rows, offset => $offset });
    my @res = ();
    my $cache = {};

@@ -417,6 +417,16 @@ sub pathIsInsidePrefix {
    return $cur;
 }

+sub readIntoSocket{
+    my (%args) = @_;
+    my $sock;
+
+    eval {
+        open($sock, "-|", @{$args{cmd}}) or die q(failed to open socket from command:\n $x);
+    };
+
+    return $sock;
+}



--- a/src/lib/Hydra/Schema/Result/EvaluationErrors.pm
+++ b/src/lib/Hydra/Schema/Result/EvaluationErrors.pm
@@ -105,4 +105,6 @@ __PACKAGE__->add_column(
    "+id" => { retrieve_on_insert => 1 }
 );

+__PACKAGE__->mk_group_accessors('column' => 'has_error');
+
 1;
--- a/src/lib/Hydra/Schema/Result/Jobsets.pm
+++ b/src/lib/Hydra/Schema/Result/Jobsets.pm
@@ -386,6 +386,8 @@ __PACKAGE__->add_column(
    "+id" => { retrieve_on_insert => 1 }
 );

+__PACKAGE__->mk_group_accessors('column' => 'has_error');
+
 sub supportsDynamicRunCommand {
  my ($self) = @_;

--- a/src/lib/Hydra/Schema/ResultSet/EvaluationErrors.pm
+++ b/src/lib/Hydra/Schema/ResultSet/EvaluationErrors.pm
@@ -0,0 +1,30 @@
+package Hydra::Schema::ResultSet::EvaluationErrors;
+
+use strict;
+use utf8;
+use warnings;
+
+use parent 'DBIx::Class::ResultSet';
+
+use Storable qw(dclone);
+
+__PACKAGE__->load_components('Helper::ResultSet::RemoveColumns');
+
+# Exclude expensive error message values unless explicitly requested, and
+# replace them with a summary field describing their presence/absence.
+sub search_rs {
+  my ( $class, $query, $attrs ) = @_;
+
+  if ($attrs) {
+    $attrs = dclone($attrs);
+  }
+
+  unless (exists $attrs->{'select'} || exists $attrs->{'columns'}) {
+    $attrs->{'+columns'}->{'has_error'} = "errormsg != ''";
+  }
+  unless (exists $attrs->{'+columns'}->{'errormsg'}) {
+    push @{ $attrs->{'remove_columns'} }, 'errormsg';
+  }
+
+  return $class->next::method($query, $attrs);
+}
--- a/src/lib/Hydra/Schema/ResultSet/Jobsets.pm
+++ b/src/lib/Hydra/Schema/ResultSet/Jobsets.pm
@@ -0,0 +1,30 @@
+package Hydra::Schema::ResultSet::Jobsets;
+
+use strict;
+use utf8;
+use warnings;
+
+use parent 'DBIx::Class::ResultSet';
+
+use Storable qw(dclone);
+
+__PACKAGE__->load_components('Helper::ResultSet::RemoveColumns');
+
+# Exclude expensive error message values unless explicitly requested, and
+# replace them with a summary field describing their presence/absence.
+sub search_rs {
+  my ( $class, $query, $attrs ) = @_;
+
+  if ($attrs) {
+    $attrs = dclone($attrs);
+  }
+
+  unless (exists $attrs->{'select'} || exists $attrs->{'columns'}) {
+    $attrs->{'+columns'}->{'has_error'} = "errormsg != ''";
+  }
+  unless (exists $attrs->{'+columns'}->{'errormsg'}) {
+    push @{ $attrs->{'remove_columns'} }, 'errormsg';
+  }
+
+  return $class->next::method($query, $attrs);
+}
--- a/src/root/build.tt
+++ b/src/root/build.tt
@@ -61,21 +61,7 @@ END;
            <td>[% IF step.busy != 0 || ((step.machine || step.starttime) && (step.status == 0 || step.status == 1 || step.status == 3 || step.status == 4 || step.status == 7)); INCLUDE renderMachineName machine=step.machine; ELSE; "<em>n/a</em>"; END %]</td>
            <td class="step-status">
              [% IF step.busy != 0 %]
-                [% IF step.busy == 1 %]
-                  <strong>Preparing</strong>
-                [% ELSIF step.busy == 10 %]
-                  <strong>Connecting</strong>
-                [% ELSIF step.busy == 20 %]
-                  <strong>Sending inputs</strong>
-                [% ELSIF step.busy == 30 %]
-                  <strong>Building</strong>
-                [% ELSIF step.busy == 40 %]
-                  <strong>Receiving outputs</strong>
-                [% ELSIF step.busy == 50 %]
-                  <strong>Post-processing</strong>
-                [% ELSE %]
-                  <strong>Unknown state</strong>
-                [% END %]
+                [% INCLUDE renderBusyStatus %]
              [% ELSIF step.status == 0 %]
                [% IF step.isnondeterministic %]
                  <span class="warn">Succeeded with non-determistic result</span>
--- a/src/root/common.tt
+++ b/src/root/common.tt
@@ -91,6 +91,17 @@ BLOCK renderDuration;
  duration % 60 %]s[%
 END;

+BLOCK renderDrvInfo;
+  drvname = step.drvpath
+    .substr(11) # strip `/nix/store/`
+    .split('-').slice(1).join("-") # strip hash part
+    .substr(0, -4); # strip `.drv`
+  IF drvname != releasename;
+    IF step.type == 0; action = "Build"; ELSE; action = "Substitution"; END;
+    IF drvname; %]<em> ([% action %] of [% drvname %])</em>[% END;
+  END;
+END;
+

 BLOCK renderBuildListHeader %]
  <table class="table table-striped table-condensed clickable-rows">
@@ -131,7 +142,12 @@ BLOCK renderBuildListBody;
      [% END %]
      <td><a class="row-link" href="[% link %]">[% build.id %]</a></td>
      [% IF !hideJobName %]
-        <td><a href="[%link%]">[% IF !hideJobsetName %][%build.jobset.get_column("project")%]:[%build.jobset.get_column("name")%]:[% END %][%build.get_column("job")%]</td>
+        <td>
+          <a href="[%link%]">[% IF !hideJobsetName %][%build.jobset.get_column("project")%]:[%build.jobset.get_column("name")%]:[% END %][%build.get_column("job")%]</a>
+          [% IF showStepName %]
+            [% INCLUDE renderDrvInfo step=build.buildsteps releasename=build.nixname %]
+          [% END %]
+        </td>
      [% END %]
      <td class="nowrap">[% t = showSchedulingInfo ? build.timestamp : build.stoptime; IF t; INCLUDE renderRelativeDate timestamp=(showSchedulingInfo ? build.timestamp : build.stoptime); ELSE; "-"; END %]</td>
      <td>[% !showSchedulingInfo and build.get_column('releasename') ? build.get_column('releasename') : build.nixname %]</td>
@@ -245,6 +261,27 @@ BLOCK renderBuildStatusIcon;
 END;


+BLOCK renderBusyStatus;
+  IF step.busy == 1 %]
+    <strong>Preparing</strong>
+  [% ELSIF step.busy == 10 %]
+    <strong>Connecting</strong>
+  [% ELSIF step.busy == 20 %]
+    <strong>Sending inputs</strong>
+  [% ELSIF step.busy == 30 %]
+    <strong>Building</strong>
+  [% ELSIF step.busy == 35 %]
+    <strong>Waiting to receive outputs</strong>
+  [% ELSIF step.busy == 40 %]
+    <strong>Receiving outputs</strong>
+  [% ELSIF step.busy == 50 %]
+    <strong>Post-processing</strong>
+  [% ELSE %]
+    <strong>Unknown state</strong>
+  [% END;
+END;
+
+
 BLOCK renderStatus;
  IF build.finished;
    buildstatus = build.buildstatus;
@@ -476,7 +513,7 @@ BLOCK renderEvals %]
            ELSE %]
             -
            [% END %]
-            [% IF eval.evaluationerror.errormsg %]
+            [% IF eval.evaluationerror.has_error %]
              <span class="badge badge-warning">Eval Errors</span>
            [% END %]
          </td>
@@ -602,7 +639,7 @@ BLOCK renderJobsetOverview %]
        <td>[% HTML.escape(j.description) %]</td>
        <td>[% IF j.lastcheckedtime;
                 INCLUDE renderDateTime timestamp = j.lastcheckedtime;
-                 IF j.errormsg || j.fetcherrormsg; %]&nbsp;<span class = 'badge badge-warning'>Error</span>[% END;
+                 IF j.has_error || j.fetcherrormsg; %]&nbsp;<span class = 'badge badge-warning'>Error</span>[% END;
                 ELSE; "-";
               END %]</td>
        [% IF j.get_column('nrtotal') > 0 %]
--- a/src/root/eval-error.tt
+++ b/src/root/eval-error.tt
@@ -0,0 +1,26 @@
+[% PROCESS common.tt %]
+<!DOCTYPE html>
+
+<html lang="en">
+
+  <head>
+    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
+    <meta http-equiv="X-UA-Compatible" content="IE=Edge" />
+    [% INCLUDE style.tt %]
+  </head>
+
+  <body>
+
+    <div class="tab-content tab-pane">
+        <div id="tabs-errors" class="">
+          [% IF jobset %]
+              <p>Errors occurred at [% INCLUDE renderDateTime timestamp=(jobset.errortime || jobset.lastcheckedtime) %].</p>
+              <div class="card bg-light"><div class="card-body"><pre>[% HTML.escape(jobset.fetcherrormsg || jobset.errormsg) %]</pre></div></div>
+          [% ELSIF eval %]
+              <p>Errors occurred at [% INCLUDE renderDateTime timestamp=(eval.evaluationerror.errortime || eval.timestamp) %].</p>
+              <div class="card bg-light"><div class="card-body"><pre>[% HTML.escape(eval.evaluationerror.errormsg) %]</pre></div></div>
+          [% END %]
+        </div>
+    </div>
+  </body>
+</html>
--- a/src/root/jobset-eval.tt
+++ b/src/root/jobset-eval.tt
@@ -65,7 +65,7 @@ c.uri_for(c.controller('JobsetEval').action_for('view'),
  [% END %]

  [% IF aborted.size > 0 %]
-    <li class="nav-item"><a class="nav-link" href="#tabs-aborted" data-toggle="tab"><span class="text-warning">Aborted Jobs ([% aborted.size %])</span></a></li>
+    <li class="nav-item"><a class="nav-link" href="#tabs-aborted" data-toggle="tab"><span class="text-warning">Aborted / Timed out Jobs ([% aborted.size %])</span></a></li>
  [% END %]
  [% IF nowFail.size > 0 %]
    <li class="nav-item"><a class="nav-link" href="#tabs-now-fail" data-toggle="tab"><span class="text-warning">Newly Failing Jobs ([% nowFail.size %])</span></a></li>
@@ -90,7 +90,7 @@ c.uri_for(c.controller('JobsetEval').action_for('view'),
  [% END %]
  <li class="nav-item"><a class="nav-link" href="#tabs-inputs" data-toggle="tab">Inputs</a></li>

-  [% IF eval.evaluationerror.errormsg %]
+  [% IF eval.evaluationerror.has_error %]
    <li class="nav-item"><a class="nav-link" href="#tabs-errors" data-toggle="tab"><span class="text-warning">Evaluation Errors</span></a></li>
  [% END %]
 </ul>
@@ -108,13 +108,6 @@ c.uri_for(c.controller('JobsetEval').action_for('view'),

 <div class="tab-content">

-  [% IF eval.evaluationerror.errormsg %]
-    <div id="tabs-errors" class="tab-pane">
-      <p>Errors occurred at [% INCLUDE renderDateTime timestamp=(eval.evaluationerror.errortime || eval.timestamp) %].</p>
-      <div class="card bg-light"><div class="card-body"><pre>[% HTML.escape(eval.evaluationerror.errormsg) %]</pre></div></div>
-    </div>
-  [% END %]
-
  <div id="tabs-aborted" class="tab-pane">
    [% INCLUDE renderSome builds=aborted tabname="#tabs-aborted" %]
  </div>
@@ -172,10 +165,9 @@ c.uri_for(c.controller('JobsetEval').action_for('view'),
    [% END %]
  </div>

-  [% IF eval.evaluationerror.errormsg %]
+  [% IF eval.evaluationerror.has_error %]
    <div id="tabs-errors" class="tab-pane">
-      <p>Errors occurred at [% INCLUDE renderDateTime timestamp=(eval.evaluationerror.errortime || eval.timestamp) %].</p>
-      <div class="card bg-light"><div class="card-body"><pre>[% HTML.escape(eval.evaluationerror.errormsg) %]</pre></div></div>
+      <iframe src="[% c.uri_for(c.controller('JobsetEval').action_for('errors'), [eval.id], params) %]" loading="lazy" frameBorder="0" width="100%"></iframe>
    </div>
  [% END %]
 </div>
--- a/src/root/jobset.tt
+++ b/src/root/jobset.tt
@@ -61,7 +61,7 @@
  [% END %]

  <li class="nav-item"><a class="nav-link active" href="#tabs-evaluations" data-toggle="tab">Evaluations</a></li>
-  [% IF jobset.errormsg || jobset.fetcherrormsg %]
+  [% IF jobset.has_error || jobset.fetcherrormsg %]
    <li class="nav-item"><a class="nav-link" href="#tabs-errors" data-toggle="tab"><span class="text-warning">Evaluation Errors</span></a></li>
  [% END %]
  <li class="nav-item"><a class="nav-link" href="#tabs-jobs" data-toggle="tab">Jobs</a></li>
@@ -79,7 +79,7 @@
        <th>Last checked:</th>
        <td>
          [% IF jobset.lastcheckedtime %]
-            [% INCLUDE renderDateTime timestamp = jobset.lastcheckedtime %], [% IF jobset.errormsg || jobset.fetcherrormsg %]<em class="text-warning">with errors!</em>[% ELSE %]<em>no errors</em>[% END %]
+            [% INCLUDE renderDateTime timestamp = jobset.lastcheckedtime %], [% IF jobset.has_error || jobset.fetcherrormsg %]<em class="text-warning">with errors!</em>[% ELSE %]<em>no errors</em>[% END %]
          [% ELSE %]
            <em>never</em>
          [% END %]
@@ -117,10 +117,9 @@

  </div>

-  [% IF jobset.errormsg || jobset.fetcherrormsg %]
+  [% IF jobset.has_error || jobset.fetcherrormsg %]
    <div id="tabs-errors" class="tab-pane">
-      <p>Errors occurred at [% INCLUDE renderDateTime timestamp=(jobset.errortime || jobset.lastcheckedtime) %].</p>
-      <div class="card bg-light"><div class="card-body"><pre>[% HTML.escape(jobset.fetcherrormsg || jobset.errormsg) %]</pre></div></div>
+      <iframe src="[% c.uri_for('/jobset' project.name jobset.name "errors") %]" loading="lazy" frameBorder="0" width="100%"></iframe>
    </div>
  [% END %]

--- a/src/root/layout.tt
+++ b/src/root/layout.tt
@@ -10,31 +10,7 @@

    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=Edge" />
-
-    <script type="text/javascript" src="[% c.uri_for("/static/js/jquery/jquery-3.4.1.min.js") %]"></script>
-    <script type="text/javascript" src="[% c.uri_for("/static/js/jquery/jquery-ui-1.10.4.min.js") %]"></script>
-    <script type="text/javascript" src="[% c.uri_for("/static/js/moment/moment-2.24.0.min.js") %]"></script>
-
-    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
-
-    <link href="[% c.uri_for("/static/fontawesome/css/all.css") %]" rel="stylesheet" />
-    <script type="text/javascript" src="[% c.uri_for("/static/js/popper.min.js") %]"></script>
-    <script type="text/javascript" src="[% c.uri_for("/static/bootstrap/js/bootstrap.min.js") %]"></script>
-    <link href="[% c.uri_for("/static/bootstrap/css/bootstrap.min.css") %]" rel="stylesheet" />
-
-    <!-- hydra.css may need to be moved to before boostrap to make the @media rule work. -->
-    <link rel="stylesheet" href="[% c.uri_for("/static/css/hydra.css") %]" type="text/css" />
-    <link rel="stylesheet" href="[% c.uri_for("/static/css/rotated-th.css") %]" type="text/css" />
-
-    <style>
-      .popover { max-width: 40%; }
-    </style>
-
-    <script type="text/javascript" src="[% c.uri_for("/static/js/bootbox.min.js") %]"></script>
-
-    <link rel="stylesheet" href="[% c.uri_for("/static/css/tree.css") %]" type="text/css" />
-
-    <script type="text/javascript" src="[% c.uri_for("/static/js/common.js") %]"></script>
+    [% INCLUDE style.tt %]

    [% IF c.config.enable_google_login %]
      <meta name="google-signin-client_id" content="[% c.config.google_client_id %]">
--- a/src/root/machine-status.tt
+++ b/src/root/machine-status.tt
@@ -6,10 +6,10 @@
  <thead>
    <tr>
      <th>Job</th>
-      <th>System</th>
      <th>Build</th>
      <th>Step</th>
      <th>What</th>
+      <th>Status</th>
      <th>Since</th>
    </tr>
  </thead>
@@ -40,10 +40,10 @@
          [% idle = 0 %]
          <tr>
            <td><tt>[% INCLUDE renderFullJobName project=step.project jobset=step.jobset job=step.job %]</tt></td>
-            <td><tt>[% step.system %]</tt></td>
            <td><a href="[% c.uri_for('/build' step.build) %]">[% step.build %]</a></td>
            <td>[% IF step.busy >= 30 %]<a class="row-link" href="[% c.uri_for('/build' step.build 'nixlog' step.stepnr 'tail') %]">[% step.stepnr %]</a>[% ELSE; step.stepnr; END %]</td>
            <td><tt>[% step.drvpath.match('-(.*)').0 %]</tt></td>
+            <td>[% INCLUDE renderBusyStatus %]</td>
            <td style="width: 10em">[% INCLUDE renderDuration duration = curTime - step.starttime %] </td>
          </tr>
        [% END %]
--- a/src/root/static/js/common.js
+++ b/src/root/static/js/common.js
@@ -129,6 +129,12 @@ $(document).ready(function() {
            el.addClass("is-local");
        }
    });
+
+    [...document.getElementsByTagName("iframe")].forEach((element) => {
+        element.contentWindow.addEventListener("DOMContentLoaded", (_) => {
+            element.style.height = element.contentWindow.document.body.scrollHeight + 'px';
+        })
+    })
 });

 var tabsLoaded = {};
--- a/src/root/status.tt
+++ b/src/root/status.tt
@@ -7,7 +7,7 @@

 [% ELSE %]

-  [% INCLUDE renderBuildList builds=resource showSchedulingInfo=1 hideResultInfo=1 busy=1 %]
+  [% INCLUDE renderBuildList builds=resource showSchedulingInfo=1 hideResultInfo=1 busy=1 showStepName=1 %]

 [% END %]

--- a/src/root/style.tt
+++ b/src/root/style.tt
@@ -0,0 +1,24 @@
+<script type="text/javascript" src="[% c.uri_for("/static/js/jquery/jquery-3.4.1.min.js") %]"></script>
+<script type="text/javascript" src="[% c.uri_for("/static/js/jquery/jquery-ui-1.10.4.min.js") %]"></script>
+<script type="text/javascript" src="[% c.uri_for("/static/js/moment/moment-2.24.0.min.js") %]"></script>
+
+<meta name="viewport" content="width=device-width, initial-scale=1.0" />
+
+<link href="[% c.uri_for("/static/fontawesome/css/all.css") %]" rel="stylesheet" />
+<script type="text/javascript" src="[% c.uri_for("/static/js/popper.min.js") %]"></script>
+<script type="text/javascript" src="[% c.uri_for("/static/bootstrap/js/bootstrap.min.js") %]"></script>
+<link href="[% c.uri_for("/static/bootstrap/css/bootstrap.min.css") %]" rel="stylesheet" />
+
+<!-- hydra.css may need to be moved to before boostrap to make the @media rule work. -->
+<link rel="stylesheet" href="[% c.uri_for("/static/css/hydra.css") %]" type="text/css" />
+<link rel="stylesheet" href="[% c.uri_for("/static/css/rotated-th.css") %]" type="text/css" />
+
+<style>
+  .popover { max-width: 40%; }
+</style>
+
+<script type="text/javascript" src="[% c.uri_for("/static/js/bootbox.min.js") %]"></script>
+
+<link rel="stylesheet" href="[% c.uri_for("/static/css/tree.css") %]" type="text/css" />
+
+<script type="text/javascript" src="[% c.uri_for("/static/js/common.js") %]"></script>
--- a/t/Hydra/Controller/Jobset/evals.t
+++ b/t/Hydra/Controller/Jobset/evals.t
@@ -32,4 +32,9 @@ subtest "/jobset/PROJECT/JOBSET/evals" => sub {
    ok($jobsetevals->is_success, "The page showing the jobset evals returns 200.");
 };

+subtest "/jobset/PROJECT/JOBSET/errors" => sub {
+    my $jobsetevals = request(GET '/jobset/' . $project->name . '/' . $jobset->name . '/errors');
+    ok($jobsetevals->is_success, "The page showing the jobset eval errors returns 200.");
+};
+
 done_testing;
--- a/t/Hydra/Controller/JobsetEval/fetch.t
+++ b/t/Hydra/Controller/JobsetEval/fetch.t
@@ -35,6 +35,10 @@ subtest "Fetching the eval's overview" => sub {
    is($fetch->code, 200, "channel page is 200");
 };

+subtest "Fetching the eval's overview" => sub {
+    my $fetch = request(GET '/eval/' . $eval->id, '/errors');
+    is($fetch->code, 200, "errors page is 200");
+};


 done_testing;
--- a/t/lib/CliRunners.pm
+++ b/t/lib/CliRunners.pm
@@ -14,7 +14,7 @@ our @EXPORT = qw(
 sub evalSucceeds {
    my ($jobset) = @_;
    my ($res, $stdout, $stderr) = captureStdoutStderr(60, ("hydra-eval-jobset", $jobset->project->name, $jobset->name));
-    $jobset->discard_changes;  # refresh from DB
+    $jobset->discard_changes({ '+columns' => {'errormsg' => 'errormsg'} });  # refresh from DB
    if ($res) {
        chomp $stdout; chomp $stderr;
        utf8::decode($stdout) or die "Invalid unicode in stdout.";
@@ -29,7 +29,7 @@ sub evalSucceeds {
 sub evalFails {
    my ($jobset) = @_;
    my ($res, $stdout, $stderr) = captureStdoutStderr(60, ("hydra-eval-jobset", $jobset->project->name, $jobset->name));
-    $jobset->discard_changes;  # refresh from DB
+    $jobset->discard_changes({ '+columns' => {'errormsg' => 'errormsg'} });  # refresh from DB
    if (!$res) {
        chomp $stdout; chomp $stderr;
        utf8::decode($stdout) or die "Invalid unicode in stdout.";
--- a/t/meson.build
+++ b/t/meson.build
@@ -27,7 +27,6 @@ testenv.prepend('PERL5LIB',
  separator: ':'
 )
 testenv.prepend('PATH',
-  fs.parent(find_program('nix').full_path()),
  fs.parent(hydra_evaluator.full_path()),
  fs.parent(hydra_queue_runner.full_path()),
  meson.project_source_root() / 'src/script',
--- a/t/queue-runner/direct-indirect-constituents.t
+++ b/t/queue-runner/direct-indirect-constituents.t
@@ -13,7 +13,7 @@ my $constituentBuildA = $builds->{"constituentA"};
 my $constituentBuildB = $builds->{"constituentB"};

 my $eval = $constituentBuildA->jobsetevals->first();
-is($eval->evaluationerror->errormsg, "");
+is($eval->evaluationerror->has_error, 0);

 subtest "Verifying the direct aggregate" => sub {
    my $aggBuild = $builds->{"direct_aggregate"};
Author	SHA1	Message	Date
Pierre Bourdon	c3b6e7b425	queue runner: fix nullptr deref on build exception after releasing a machine reservation	2025-02-16 13:27:26 +01:00
K900	c60e7955bf	Add metric for builds waiting for download slot (cherry picked from commit f23ec71227911891807706b6b978836e4d80edde)	2025-02-12 10:35:17 +01:00
Maximilian Bosch	90399cb674	readIntoSocket: fix with store URIs containing an `&` The third argument to `open()` in `-\|` mode is passed to a shell if it's a string. In my case the store URI contains `?secret-key=${signingKey.directory}/secret&compression=zstd` For the `nix store cat` case this means that * until `&` the process will be started in the background. This fails immediately because no path to cat is specified. * `compression=zstd` is a variable assignment * the `$path` argument to `store cat` is attempted to be executed as another command Passing just the list solves the problem. (cherry picked from commit 3ee51dbe589458cc54ff753317bbc6db530bddc0)	2025-02-12 10:35:17 +01:00
git@71rd.net	d6a3ef484c	Stream files from store instead of buffering them When an artifact is requested from hydra the output is first copied from the nix store into memory and then sent as a response, delaying the download and taking up significant amounts of memory. As reported in https://github.com/NixOS/hydra/issues/1357 Instead of calling a command and blocking while reading in the entire output, this adds read_into_socket(). the function takes a command, starting a subprocess with that command, returning a file descriptor attached to stdout. This file descriptor is then by responsebuilder of Catalyst to steam the output directly (cherry picked from commit 459aa0a5983a0bd546399c08231468d6e9282f54)	2025-02-12 10:35:17 +01:00
Pierre Bourdon	685857df2e	web: replace 'errormsg' with 'errormsg IS NULL' in most cases This is implement in an extremely hacky way due to poor DBIx feature support. Ideally, what we'd need is a way to tell DBIx to ignore the errormsg column unless explicitly requested, and to automatically add a computed 'errormsg IS NULL' column in others. Since it does not support that, this commit instead hacks some support via method overrides while taking care to not break anything obvious.	2025-02-12 10:35:17 +01:00
ajs124	f2b6e9d8ab	lazy-load evaluation errors Closes #1362	2025-02-12 10:35:17 +01:00
Maximilian Bosch	b04847335f	Only show stepname if it doesn't equal the name of the drv When building e.g. nixpkgs, the "Running builds" view will mostly look like this hello.x86_64-linux (Build of hello-X.Y) exa.x86_64-linux (Build of exa-X.Y) ... This doesn't provide any useful information. Showing the step name only makes sense if it's not a child of the job's derivation. With this patch, that information will only be shown if the drv name (i.e. w/o `/nix/store/` prefix, .drv ext & hash) is not equal to the drv name of the job itself (build.nixname).	2025-02-12 10:35:17 +01:00
Maximilian Bosch	7e61000172	Running builds view: show build step names When using Hydra to build machine configurations, you'll often see "nixosConfigurations.foo" five times, i.e. for each build step being run. This isn't very helpful I think because in such a case, a single build step can also be compiling the Linux kernel. This change also fetches the `drvpath` and `type` from the `buildsteps` relation. We're already joining it, so this doesn't make much difference (confirmed via query logging that this doesn't cause extra SQL queries). Unfortunately build steps don't have a human readable name, so I'm deriving it from the drvpath by stripping away the hash (assuming that it'll never contain a `-` and that `/nix/store/` is used as prefix). I decided against using the Nix bindings for that to avoid too much overhead due to store operations for each build step.	2025-02-12 10:35:17 +01:00
Maximilian Bosch	6d04e824d5	Make "timed out" and "log limit exceeded" builds aborted In `73694087a0` I gave builds that failed because of a timeout or exceeded log limit a stop sign and I stand by that reasoning: with that it's possible to distinguish between actual build failures and rather transient things such as timeouts. Back then I considered it a feature that these are shown in a different tab, but I don't think that's a good idea anymore. When using a jobset to e.g. track the regressions from a mass rebuild (like a compiler or gcc update), "Newly failed builds" should exclusively display regressions (and flaky builds of course, not much I can do about that). Also, when a bunch of builds fail in such a jobset because of e.g. a broken connection to a builder that results in a timeout, I want to be able to restart them all w/o rebuilding actual regressions. To make it clear that we not only have "Aborted" builds in the tab, I renamed the label to "Aborted / Timed out".	2025-02-12 10:35:16 +01:00
Pierre Bourdon	36e25d8fd2	queue-runner: release machine reservation while copying outputs This allows for better builder usage when the queue runner is busy. To avoid running into uncontrollable imbalances between builder/queue runner, we only release the machine reservation after the local throttler has found a slot to start copying the outputs for that build.	2025-02-12 10:35:16 +01:00
Pierre Bourdon	d4e273f7b1	queue-runner: switch to pseudorandom ordering of builds processing We don't rely on sequential / monotonic build IDs processing anymore, so randomizing actually has the advantage of mixing builds for different systems together, to avoid only one chunk of builds for a single system getting processed while builders for other systems are starved.	2025-02-12 10:35:16 +01:00
Pierre Bourdon	23366ec10d	queue runner: introduce some parallelism for remote paths lookup Each output for a given step being ingested is looked up in parallel, which should basically multiply the speed of builds ingestion by the average number of outputs per derivation.	2025-02-12 10:35:16 +01:00
Pierre Bourdon	98759e4ff9	queue-runner: reduce the time between queue monitor restarts This will induce more DB queries (though these are fairly cheap), but at the benefit of processing bumps within 1m instead of within 10m.	2025-02-12 10:35:16 +01:00
Pierre Bourdon	d850c99883	queue-runner: remove id > X from new builds query Running the query with/without it shows that it makes no difference to postgres, since there's an index on finished=0 already. This allows a few simplifications, but also paves the way towards running multiple parallel monitor threads in the future.	2025-02-12 10:35:16 +01:00
Pierre Bourdon	1b76eec4e8	queue-runner: add prom metrics to allow detecting internal bottlenecks By looking at the ratio of running vs. waiting for the dispatcher and the queue monitor, we should get better visibility into what hydra is currently bottlenecked on. There are other side effects we can try to measure to get to the same result, but having a simple way doesn't cost us much.	2025-02-12 10:35:15 +01:00
Pierre Bourdon	3bb1a61a7d	web: include current step status on /machines	2025-02-12 10:35:15 +01:00
Pierre Bourdon	3b9045f60d	queue-runner: limit parallelism of CPU intensive operations My current theory is that running more parallel xz than available CPU cores is reducing our overall throughput by requiring more scheduling overhead and more cache thrashing.	2025-02-12 10:35:15 +01:00