avoid request stalls during proactive rebuilds

scripts/test_dependency_invalidation.sh

@@ -48,13 +48,37 @@ assert_marker parent dependency-marker-a
 sed -i 's/dependency-marker-a/dependency-marker-b/' "$source_dir/child.uce"
 assert_marker parent dependency-marker-b
 
+# A proactive rebuild owns these same per-unit locks. While it publishes fresh
+# artifacts, requests must use the last complete artifacts instead of waiting
+# across the transitive graph. Atomic publication keeps those artifacts safe.
+parent_wasm="$cache_dir/parent.uce.wasm"
+child_wasm="$cache_dir/child.uce.wasm"
+(
+	exec 8>"$parent_wasm.lock"
+	exec 9>"$child_wasm.lock"
+	flock 8
+	flock 9
+	sleep 3
+) &
+rebuild_lock_pid=$!
+sleep 0.2
+sed -i 's/dependency-marker-b/dependency-marker-d/' "$source_dir/child.uce"
+started_at=$(date +%s%N)
+assert_marker parent dependency-marker-b
+elapsed_ms=$(( ($(date +%s%N) - started_at) / 1000000 ))
+if (( elapsed_ms >= 2000 )); then
+	echo "request waited ${elapsed_ms}ms for an active transitive rebuild" >&2
+	exit 1
+fi
+wait "$rebuild_lock_pid"
+assert_marker parent dependency-marker-d
+
 # Warm every configured worker, then replace the artifact while retaining its
 # whole-second mtime. The worker cache must notice the nanosecond/ctime change.
-for _ in {1..16}; do assert_marker parent dependency-marker-b; done
-sed 's/dependency-marker-b/dependency-marker-c/' "$source_dir/child.uce" >"$source_dir/alternate-child.uce"
+for _ in {1..16}; do assert_marker parent dependency-marker-d; done
+sed 's/dependency-marker-d/dependency-marker-c/' "$source_dir/child.uce" >"$source_dir/alternate-child.uce"
 sed 's/child.uce/alternate-child.uce/' "$source_dir/parent.uce" >"$source_dir/alternate.uce"
 assert_marker alternate dependency-marker-c
-parent_wasm="$cache_dir/parent.uce.wasm"
 alternate_wasm="$cache_dir/alternate.uce.wasm"
 parent_mtime=$(stat -c %Y "$parent_wasm")
 cp "$alternate_wasm" "$parent_wasm"