libstore: don’t include derivation names in build directories

They have variable size, which is bad for #913.

Change-Id: I6a6a6964a84804dee281ade48e6517419a6143fa

lix/libstore/build/local-derivation-goal.cc

@@ -446,8 +446,7 @@ try {
 
         /* Create a temporary directory where the build will take
            place. */
-        tmpDirRoot =
-            createTempSubdir(buildDir, "nix-build-" + std::string(drvPath.name()), 0700);
+        tmpDirRoot = createTempSubdir(buildDir, std::nullopt, 0700);
     } catch (SysError & e) {
         /*
          * Fallback to the global tmpdir and create a safe space there
@@ -472,9 +471,7 @@ try {
             settings.buildDir.get(),
             nixBuildsTmp
         );
-        tmpDirRoot = createTempSubdir(
-            nixBuildsTmp, "nix-build-" + std::string(drvPath.name()), 0700
-        );
+        tmpDirRoot = createTempSubdir(nixBuildsTmp, std::nullopt, 0700);
         worker.buildDirOverride = nixBuildsTmp;
     }
     /* The TOCTOU between the previous mkdir call and this open call is unavoidable due to

tests/functional/check.sh

@@ -40,8 +40,8 @@ test_custom_build_dir() {
   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-"*
+  [[ 1 == "$(count "$customBuildDir/"*)" ]]
+  local buildDir="$customBuildDir/"*
   grep $checkBuildId $buildDir/b/checkBuildId
 }
 test_custom_build_dir