From 9de81da9ce952fc52387414d4c82f282e9b3c8e0 Mon Sep 17 00:00:00 2001
From: Robert Hensing <robert@roberthensing.nl>
Date: Sun, 24 Mar 2024 15:07:00 +0100
Subject: [PATCH] Add build-dir setting

---
 src/libstore/build/local-derivation-goal.cc |  2 +-
 src/libstore/globals.hh                     | 30 +++++++++++++++++----
 tests/functional/check.sh                   | 15 +++++++++++
 3 files changed, 41 insertions(+), 6 deletions(-)

diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index c91b3c97533f..349aa75e9fd2 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -488,7 +488,7 @@ void LocalDerivationGoal::startBuilder()
 
     /* Create a temporary directory where the build will take
        place. */
-    tmpDir = createTempDir("", "nix-build-" + std::string(drvPath.name()), false, false, 0700);
+    tmpDir = createTempDir(settings.buildDir.get().value_or(""), "nix-build-" + std::string(drvPath.name()), false, false, 0700);
 
     chownToBuilder(tmpDir);
 
diff --git a/src/libstore/globals.hh b/src/libstore/globals.hh
index e6544976a5ac..e6acf0a4f038 100644
--- a/src/libstore/globals.hh
+++ b/src/libstore/globals.hh
@@ -687,16 +687,36 @@ public:
     Setting<std::string> sandboxShmSize{
         this, "50%", "sandbox-dev-shm-size",
         R"(
-          This option determines the maximum size of the `tmpfs` filesystem
-          mounted on `/dev/shm` in Linux sandboxes. For the format, see the
-          description of the `size` option of `tmpfs` in mount(8). The default
-          is `50%`.
+            *Linux only*
+
+            This option determines the maximum size of the `tmpfs` filesystem
+            mounted on `/dev/shm` in Linux sandboxes. For the format, see the
+            description of the `size` option of `tmpfs` in mount(8). The default
+            is `50%`.
         )"};
 
     Setting<Path> sandboxBuildDir{this, "/build", "sandbox-build-dir",
-        "The build directory inside the sandbox."};
+        R"(
+            *Linux only*
+
+            The build directory inside the sandbox.
+
+            This directory is backed by [`build-dir`](#conf-build-dir) on the host.
+        )"};
 #endif
 
+    Setting<std::optional<Path>> buildDir{this, std::nullopt, "build-dir",
+        R"(
+            The directory on the host, in which derivations' temporary build directories are created.
+
+            If not set, Nix will use the system temporary directory indicated by the `TMPDIR` environment variable.
+            Note that builds are often performed by the Nix daemon, so its `TMPDIR` is used, and not that of the Nix command line interface.
+
+            This is also the location where [`--keep-failed`](@docroot@/command-ref/opt-common.md#opt-keep-failed) leaves its files.
+
+            If Nix runs without sandbox, or if the platform does not support sandboxing with bind mounts (e.g. macOS), then the [`builder`](@docroot@/language/derivations.md#attr-builder)'s environment will contain this directory, instead of the virtual location [`sandbox-build-dir`](#conf-sandbox-build-dir).
+        )"};
+
     Setting<PathSet> allowedImpureHostPrefixes{this, {}, "allowed-impure-host-deps",
         "Which prefixes to allow derivations to ask for access to (primarily for Darwin)."};
 
diff --git a/tests/functional/check.sh b/tests/functional/check.sh
index e13abf7470ab..38883c5d71e7 100644
--- a/tests/functional/check.sh
+++ b/tests/functional/check.sh
@@ -34,6 +34,21 @@ nix-build check.nix -A failed --argstr checkBuildId $checkBuildId \
 [ "$status" = "100" ]
 if checkBuildTempDirRemoved $TEST_ROOT/log; then false; fi
 
+test_custom_build_dir() {
+  local customBuildDir="$TEST_ROOT/custom-build-dir"
+
+  # Nix does not create the parent directories, and perhaps it shouldn't try to
+  # decide the permissions of build-dir.
+  mkdir "$customBuildDir"
+  nix-build check.nix -A failed --argstr checkBuildId $checkBuildId \
+      --no-out-link --keep-failed --option build-dir "$TEST_ROOT/custom-build-dir" 2> $TEST_ROOT/log || status=$?
+  [ "$status" = "100" ]
+  [[ 1 == "$(count "$customBuildDir/nix-build-"*)" ]]
+  local buildDir="$customBuildDir/nix-build-"*
+  grep $checkBuildId $buildDir/checkBuildId
+}
+test_custom_build_dir
+
 nix-build check.nix -A deterministic --argstr checkBuildId $checkBuildId \
     --no-out-link 2> $TEST_ROOT/log
 checkBuildTempDirRemoved $TEST_ROOT/log