libstore: make diff-hooks work for non-root users

it's kind of weird to not allow diff-hooks in single user mode unless
that user is root. maybe that's why we have no tests for them? we can
at least add a test that diff hooks are run at all when we expect it.

Change-Id: I54d623b5416acda1c205cc062b0f3a33c9f4aaa7

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

@@ -72,8 +72,8 @@
 namespace nix {
 
 static kj::Promise<Result<void>> handleDiffHook(
-    uid_t uid,
-    uid_t gid,
+    std::optional<uid_t> uid,
+    std::optional<uid_t> gid,
     const Path & tryA,
     const Path & tryB,
     const Path & drvPath,
@@ -1773,8 +1773,8 @@ try {
                     movePath(actualPath, dst);
 
                     TRY_AWAIT(handleDiffHook(
-                        buildUser ? buildUser->getUID() : getuid(),
-                        buildUser ? buildUser->getGID() : getgid(),
+                        buildUser ? std::optional(buildUser->getUID()) : std::nullopt,
+                        buildUser ? std::optional(buildUser->getGID()) : std::nullopt,
                         finalDestPath,
                         dst,
                         worker.store.printStorePath(drvPath),

tests/functional/check.sh

@@ -139,3 +139,11 @@ nix-store -q --references "$obtained" >/dev/null
 nix-build check.nix -A nondeterministic --no-out-link
 nix-build check.nix -A nondeterministic -A hashmismatch --no-out-link --check --keep-going || status=$?
 [ "$status" = "110" ]
+
+
+# basic check that diff hooks are started at all
+STDERR=$(expectStderr 104 nix-build check.nix -A nondeterministic \
+  --diff-hook "$PWD/diff-hook.sh" \
+  --run-diff-hook \
+  --check 2>&1)
+grep "diff-hook ran" <<<"$STDERR"

tests/functional/diff-hook.sh

@@ -0,0 +1,3 @@
+#!/usr/bin/env bash
+
+echo diff-hook ran