Make the location of the build directory in the sandbox configurable

This is mostly for use in the sandbox tests, since if the Nix store is
under /build, then we can't use /build as the build directory.

src/libstore/build.cc

@@ -1661,9 +1661,6 @@ int childEntry(void * arg)
 }
 
 
-const std::string buildDir = "/build";
-
-
 void DerivationGoal::startBuilder()
 {
     auto f = format(
@@ -1725,7 +1722,7 @@ void DerivationGoal::startBuilder()
     /* In a sandbox, for determinism, always use the same temporary
        directory. */
 #if __linux__
-    tmpDirInSandbox = useChroot ? buildDir : tmpDir;
+    tmpDirInSandbox = useChroot ? settings.sandboxBuildDir : tmpDir;
 #elif __APPLE__
     // On Darwin, we canonize /tmp because its probably a symlink to /private/tmp.
     tmpDirInSandbox = useChroot ? canonPath("/tmp", true) + "/nix-build-" + drvName + "-0" : tmpDir;
@@ -1843,7 +1840,7 @@ void DerivationGoal::startBuilder()
                 "root:x:0:0:Nix build user:%3%:/noshell\n"
                 "nixbld:x:%1%:%2%:Nix build user:%3%:/noshell\n"
                 "nobody:x:65534:65534:Nobody:/:/noshell\n",
-                sandboxUid, sandboxGid, buildDir));
+                sandboxUid, sandboxGid, settings.sandboxBuildDir));
 
         /* Declare the build user's group so that programs get a consistent
            view of the system (e.g., "id -gn"). */

src/libstore/globals.hh

@@ -224,6 +224,9 @@ public:
 #if __linux__
     Setting<std::string> sandboxShmSize{this, "50%", "sandbox-dev-shm-size",
         "The size of /dev/shm in the build sandbox."};
+
+    Setting<Path> sandboxBuildDir{this, "/build", "sandbox-build-dir",
+        "The build directory inside the sandbox."};
 #endif
 
     Setting<PathSet> allowedImpureHostPrefixes{this, {}, "allowed-impure-host-deps",

src/libstore/machines.cc

@@ -55,7 +55,7 @@ void parseMachines(const std::string & s, Machines & machines)
         if (sz < 1)
             throw FormatError("bad machine specification ‘%s’", line);
 
-        auto isSet = [&](int n) {
+        auto isSet = [&](size_t n) {
             return tokens.size() > n && tokens[n] != "" && tokens[n] != "-";
         };
 

tests/build-remote.sh

@@ -12,9 +12,13 @@ rm -rf $TEST_ROOT/store0 $TEST_ROOT/store1
 # FIXME: --option is not passed to build-remote, so have to create a config file.
 export NIX_CONF_DIR=$TEST_ROOT/etc2
 mkdir -p $NIX_CONF_DIR
-echo "build-sandbox-paths = /nix/store" > $NIX_CONF_DIR/nix.conf
+echo "
+build-sandbox-paths = /nix/store
+sandbox-build-dir = /build-tmp
+" > $NIX_CONF_DIR/nix.conf
 
-outPath=$(nix-build build-hook.nix --no-out-link -j0 --option builders "local?root=$TEST_ROOT/store0; local?root=$TEST_ROOT/store1 - - 1 1 foo" --option build-sandbox-paths /nix/store)
+outPath=$(nix-build build-hook.nix --no-out-link -j0 \
+  --option builders "local?root=$TEST_ROOT/store0; local?root=$TEST_ROOT/store1 - - 1 1 foo")
 
 cat $outPath/foobar | grep FOOBAR