perf(run-events): unblock bus dispatch with runFork in subscriber callbacks

Bus.subscribeCallback wraps the callback's return value in
Effect.tryPromise(() => Promise.resolve().then(() => callback(msg)))
inside a runForEach loop, so a Promise-returning callback (such as
Effect.runPromise) serializes handler completion per subscription —
each descendant question/permission event waits for the previous
handler's reject/reply round trip before dispatch advances.

Replace Effect.runPromise with Effect.runFork. runFork returns a
Fiber synchronously (non-thenable), so the bus's tryPromise resolves
immediately and the next event dispatches without waiting. Handler
defects no longer surface through Bus.on's tryPromise.catch, so wrap
the forked effect with Effect.tapCause + log.error.

Drops the per-event Promise wrapper allocation and unblocks
concurrent dispatch for long-running subagent loops with many
simultaneous descendants.

Addresses audit finding F7 (Opus diamond review, 2026-04-22).

Author: tesdal

packages/opencode/src/cli/cmd/run-events.ts

@@ -1,4 +1,4 @@
-import { Effect, Option } from "effect"
+import { Cause, Effect, Fiber, Option } from "effect"
 import { Bus } from "@/bus"
 import { Permission } from "@/permission"
 import { Question } from "@/question"
@@ -86,8 +86,44 @@ export const make = Effect.fn("RunEvents.make")(function* (config: Config) {
     }
   }
 
+  // bus.subscribeCallback wraps the callback in an Effect.tryPromise-based
+  // subscription handler, so a Promise-returning callback (like Effect.runPromise)
+  // serializes handler completion per subscription. runFork returns a Fiber
+  // synchronously (non-thenable), unblocking dispatch so descendant question/
+  // permission events are processed concurrently — important for long-running
+  // subagent loops with many simultaneous descendants. Defects inside the forked
+  // fiber do not surface through that subscription callback wrapper, so log them
+  // here instead. Track in-flight fibers so unsubscribe() can interrupt them and
+  // bound handler work to the RunEvents lifecycle.
+  const inflight = new Set<Fiber.Fiber<void>>()
+  let closed = false
+  const fork = (effect: Effect.Effect<void>) => {
+    if (closed) {
+      // unsubscribe() already ran but bus subscription teardown is async, so
+      // a late callback can still reach fork(). Skip starting the handler
+      // entirely so no side effects (bump, reject, reply) leak past teardown.
+      // Returning undefined (not a Promise) still unblocks the bus dispatch
+      // wrapper without spawning a no-op fiber.
+      return
+    }
+    const fiber = Effect.runFork(
+      effect.pipe(
+        Effect.tapCause((cause) =>
+          Cause.hasInterruptsOnly(cause)
+            ? Effect.void
+            : Effect.sync(() => log.error("handler failed", { cause })),
+        ),
+      ),
+    )
+    inflight.add(fiber)
+    // Register cleanup outside the forked effect to avoid a TDZ/race between
+    // synchronous fiber completion and inflight.add — Fiber.await observes
+    // completion regardless of how fast the fiber runs.
+    Effect.runFork(Fiber.await(fiber).pipe(Effect.ensuring(Effect.sync(() => inflight.delete(fiber)))))
+  }
+
   const unsubQuestion = yield* bus.subscribeCallback(Question.Event.Asked, (evt) =>
-    Effect.runPromise(
+    fork(
       Effect.gen(function* () {
         const mine = yield* isDescendant(evt.properties.sessionID)
         if (!mine) return
@@ -98,7 +134,7 @@ export const make = Effect.fn("RunEvents.make")(function* (config: Config) {
   )
 
   const unsubPermission = yield* bus.subscribeCallback(Permission.Event.Asked, (evt) =>
-    Effect.runPromise(
+    fork(
       Effect.gen(function* () {
         const mine = yield* isDescendant(evt.properties.sessionID)
         if (!mine) return
@@ -113,8 +149,13 @@ export const make = Effect.fn("RunEvents.make")(function* (config: Config) {
   )
 
   const unsubscribe = () => {
+    closed = true
     unsubQuestion()
     unsubPermission()
+    inflight.forEach((fiber) => Effect.runFork(Fiber.interrupt(fiber)))
+    // Don't clear() — let the per-fiber Fiber.await observers remove entries
+    // as their interrupts settle, so any stragglers caught by the closed-flag
+    // branch above still get cleaned up correctly.
   }
 
   return { stats, unsubscribe } satisfies Handle

packages/opencode/test/cli/run-events.test.ts

@@ -523,4 +523,50 @@ describe("cli/run-events", () => {
       }),
     ),
   )
+
+  // Lifecycle test for F7 fiber-tracking: after unsubscribe(), late-arriving
+  // bus callbacks must not produce new auto-rejects. This exercises both the
+  // bus unsubscription path and the `closed` flag in fork() that interrupts
+  // any handler reaching it after teardown has begun.
+  it.live("does not auto-reject question.asked after unsubscribe()", () =>
+    provideTmpdirInstance(() =>
+      Effect.gen(function* () {
+        const question = yield* Question.Service
+        const rootSessionID = SessionID.make("ses_root_post_unsub_000000000000")
+        const handler = yield* RunEvents.make({
+          rootSessionID,
+          skipPermissions: false,
+          jsonMode: false,
+        })
+        yield* Effect.sync(() => handler.unsubscribe())
+
+        // Ask after unsubscribe — without the bus subscription, no auto-reject
+        // handler runs and the question stays pending.
+        const fiber = yield* question
+          .ask({
+            sessionID: rootSessionID,
+            questions: [{ question: "post?", header: "h", options: [{ label: "x", description: "x" }] }],
+          })
+          .pipe(Effect.forkScoped)
+
+        const pending = yield* pollForLength(() => question.list(), 1)
+        expect(pending[0].sessionID).toBe(rootSessionID)
+        expect(handler.stats.autoRejectedQuestions).toBe(0)
+
+        // Give any late async bus callbacks a chance to run, then verify the
+        // question is still pending and no auto-reject occurred.
+        yield* Effect.sleep("50 millis")
+        const stillPending = yield* question.list()
+        expect(stillPending).toHaveLength(1)
+        expect(stillPending[0].id).toBe(pending[0].id)
+        expect(stillPending[0].sessionID).toBe(rootSessionID)
+        expect(handler.stats.autoRejectedQuestions).toBe(0)
+
+        // Manually clear the pending question so the test can finish.
+        yield* question.reject(pending[0].id)
+        const exit = yield* Fiber.await(fiber)
+        expect(Exit.isFailure(exit)).toBe(true)
+      }),
+    ),
+  )
 })