From b62b4b0e30c5d55ef0bb279d0f87fa3a79b3be19 Mon Sep 17 00:00:00 2001
From: Phil Larson <hello@phillarson.xyz>
Date: Thu, 25 Jun 2026 11:16:16 -0700
Subject: [PATCH] fix(FN-798): scope engine vitest fork pool

Fusion-Task-Id: FN-798
---
 packages/engine/vitest.config.ts              | 13 ++++++----
 .../engine-vitest-gate-policy.test.mjs        | 25 ++++++++++++++++---
 2 files changed, 30 insertions(+), 8 deletions(-)

diff --git a/packages/engine/vitest.config.ts b/packages/engine/vitest.config.ts
index 6210d621af..c084507b47 100644
--- a/packages/engine/vitest.config.ts
+++ b/packages/engine/vitest.config.ts
@@ -19,11 +19,9 @@ export default defineConfig({
       resolve(__dirname, "../core/src/__test-utils__/vitest-setup.ts"),
     ],
     globalSetup: [resolve(__dirname, "../core/src/__test-utils__/vitest-teardown.ts")],
-    // Node 24.15.0 on macOS aborts in libuv kqueue when Vitest's
-    // worker-thread pool closes unmanaged file descriptors during this gate.
-    // Fork workers preserve real failure semantics while avoiding the raw
-    // SIGABRT/warning flood; keep worker counts bounded below.
-    pool: "forks",
+    // Keep the broad engine lanes on worker threads; engine-core overrides this
+    // below because only the curated merge gate has hit the Node/macOS abort.
+    pool: "threads",
     maxWorkers,
     minWorkers: 1,
     fileParallelism: true,
@@ -56,6 +54,11 @@ export default defineConfig({
         extends: true,
         test: {
           name: "engine-core",
+          /*
+          FNXC:EngineTests 2026-06-25-11:11:
+          The curated engine-core merge gate hits a Node 24.15.0/macOS libuv kqueue SIGABRT when Vitest thread workers close unmanaged file descriptors. Scope fork workers to this gate so the broad default engine suite keeps its explicit worker-thread behavior.
+          */
+          pool: "forks",
           // The curated merge-gate suite (see docs/testing.md "Merge gate").
           // Membership is an explicit allow-list, NOT a glob: tests earn their
           // way in with evidence of value, and a flaky gate test is evicted by
diff --git a/scripts/__tests__/engine-vitest-gate-policy.test.mjs b/scripts/__tests__/engine-vitest-gate-policy.test.mjs
index 6e7e3a792d..9a544da03c 100644
--- a/scripts/__tests__/engine-vitest-gate-policy.test.mjs
+++ b/scripts/__tests__/engine-vitest-gate-policy.test.mjs
@@ -16,13 +16,32 @@ function readJson(relativePath) {
   return JSON.parse(read(relativePath));
 }
 
-test("engine-core gate keeps a Node 24/macOS-safe Vitest pool without hiding warnings", () => {
+test("engine-core gate keeps a Node 24/macOS-safe Vitest pool without changing broad engine lanes", () => {
   const config = read("packages/engine/vitest.config.ts");
+  const projectsIndex = config.indexOf("projects:");
+  const rootTestConfig = projectsIndex === -1 ? config : config.slice(0, projectsIndex);
+  const engineCoreBlock = config.match(/name:\s*"engine-core"[\s\S]*?include:\s*\[/)?.[0] ?? "";
+  const engineDefaultBlock = config.match(/name:\s*"engine-default"[\s\S]*?include:\s*\[/)?.[0] ?? "";
 
   assert.match(
-    config,
+    engineCoreBlock,
     /pool:\s*"forks"/,
-    "engine Vitest must use fork workers; thread workers abort with Node 24/macOS libuv kqueue",
+    "engine-core must use fork workers; thread workers abort with Node 24/macOS libuv kqueue",
+  );
+  assert.doesNotMatch(
+    rootTestConfig,
+    /pool:\s*"forks"/,
+    "fork workers must not be configured at root scope because that slows the broad engine-default lane",
+  );
+  assert.match(
+    rootTestConfig,
+    /pool:\s*"threads"/,
+    "root engine config must explicitly keep broad lanes on threads because Vitest 4 defaults to forks",
+  );
+  assert.doesNotMatch(
+    engineDefaultBlock,
+    /pool:\s*"forks"/,
+    "engine-default must keep inheriting Vitest's default thread pool for broad src/**/*.test.ts runs",
   );
   assert.doesNotMatch(
     config,