bug: file watcher native callback loses ALS context

packages/opencode/test/file/watcher-als-bug.test.ts

@@ -0,0 +1,88 @@
+/**
+ * Repro for: @parcel/watcher native callback loses AsyncLocalStorage context
+ *
+ * Background:
+ *   opencode uses AsyncLocalStorage (ALS) to track which project directory
+ *   is active. Bus.publish reads Instance.directory from ALS to route events
+ *   to the right instance. This works for normal JS async code (setTimeout,
+ *   Promises, etc.) because Node propagates ALS through those.
+ *
+ *   But @parcel/watcher is a native C++ addon. Its callback re-enters JS
+ *   from C++ via libuv, bypassing Node's async hooks — so ALS is empty.
+ *   Bus.publish silently throws Context.NotFound, and the event vanishes.
+ *
+ * What this breaks:
+ *   The git HEAD watcher (always active, no experimental flag) should detect
+ *   branch switches and update the TUI. But because events never arrive,
+ *   the branch indicator never live-updates.
+ *
+ * This test:
+ *   1. Creates a tmp git repo and boots an instance with the file watcher
+ *   2. Listens on GlobalBus for watcher events (Bus.publish emits to GlobalBus)
+ *   3. Runs `git checkout -b` to change .git/HEAD — the watcher WILL detect
+ *      this change and fire the callback, but Bus.publish will fail silently
+ *   4. Times out after 5s because the event never reaches GlobalBus
+ *
+ * Fix: Instance.bind(fn) captures ALS context at subscription time and
+ * restores it in the callback. See #17601.
+ */
+import { $ } from "bun"
+import { afterEach, expect, test } from "bun:test"
+import { tmpdir } from "../fixture/fixture"
+
+async function load() {
+  const { FileWatcher } = await import("../../src/file/watcher")
+  const { GlobalBus } = await import("../../src/bus/global")
+  const { Instance } = await import("../../src/project/instance")
+  return { FileWatcher, GlobalBus, Instance }
+}
+
+afterEach(async () => {
+  const { Instance } = await load()
+  await Instance.disposeAll()
+})
+
+test("git HEAD watcher publishes events via Bus (ALS context test)", async () => {
+  const { FileWatcher, GlobalBus, Instance } = await load()
+
+  // 1. Create a temp git repo and start the file watcher inside an instance.
+  //    The watcher subscribes to .git/HEAD changes via @parcel/watcher.
+  //    At this point we're inside Instance.provide, so ALS is active.
+  await using tmp = await tmpdir({ git: true })
+  await Instance.provide({
+    directory: tmp.path,
+    fn: async () => {
+      FileWatcher.init()
+      await Bun.sleep(200) // wait for native watcher to finish subscribing
+    },
+  })
+
+  // 2. Listen on GlobalBus and trigger a branch switch.
+  //    When .git/HEAD changes, @parcel/watcher fires our callback from C++.
+  //    The callback calls Bus.publish, which needs ALS to read Instance.directory.
+  //    Without Instance.bind, ALS is empty → Bus.publish throws → event never arrives.
+  const got = await new Promise<any>((resolve, reject) => {
+    const timeout = setTimeout(() => {
+      GlobalBus.off("event", on)
+      reject(new Error("timed out — native callback likely lost ALS context"))
+    }, 5000)
+
+    function on(evt: any) {
+      if (evt.directory !== tmp.path) return
+      if (evt.payload?.type !== FileWatcher.Event.Updated.type) return
+      clearTimeout(timeout)
+      GlobalBus.off("event", on)
+      resolve(evt.payload.properties)
+    }
+
+    GlobalBus.on("event", on)
+
+    // This changes .git/HEAD, which the native watcher will detect
+    $`git checkout -b test-branch`.cwd(tmp.path).quiet().nothrow()
+  })
+
+  // 3. If we get here, the event arrived — ALS context was preserved.
+  //    On the unfixed code, we never get here (the promise rejects with timeout).
+  expect(got).toBeDefined()
+  expect(got.event).toBe("change")
+})