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[core] Ensure open stream flush is await-able in pendingOps - #1446
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WorkflowServerWritableStream buffers writes and flushes via a 10ms
setTimeout for batching. The write() callback returned immediately
after buffering, causing flushablePipe's pendingOps counter to reach 0
before data actually reached the server via the deferred HTTP flush.
Fix: write() now returns a promise that resolves only after the
scheduled flush completes. Multiple writes within the 10ms window
still share a single batched flush — the batching optimization is
preserved. Each write's promise resolves when the batch's HTTP
round-trip finishes, so pendingOps accurately reflects server state.
This is implemented via a flushWaiters array: each write() pushes
a {resolve, reject} pair. When the setTimeout fires and flush()
completes, all waiters are resolved (or rejected on error).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>Signed-off-by: Peter Wielander <mittgfu@gmail.com>
Signed-off-by: Peter Wielander <mittgfu@gmail.com>
🦋 Changeset detectedLatest commit: 3b5d85b The changes in this PR will be included in the next version bump. This PR includes changesets to release 16 packages
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🧪 E2E Test Results❌ Some tests failed Summary
❌ Failed Tests🌍 Community Worlds (56 failed)mongodb (3 failed):
redis (2 failed):
turso (51 failed):
Details by Category✅ ▲ Vercel Production
✅ 💻 Local Development
✅ 📦 Local Production
✅ 🐘 Local Postgres
✅ 🪟 Windows
❌ 🌍 Community Worlds
✅ 📋 Other
|
Contributor
📊 Benchmark Results
workflow with no steps💻 Local Development
▲ Production (Vercel)
🔍 Observability: Nitro | Next.js (Turbopack) | Express workflow with 1 step💻 Local Development
▲ Production (Vercel)
🔍 Observability: Next.js (Turbopack) | Nitro | Express workflow with 10 sequential steps💻 Local Development
▲ Production (Vercel)
🔍 Observability: Express | Next.js (Turbopack) | Nitro workflow with 25 sequential steps💻 Local Development
▲ Production (Vercel)
🔍 Observability: Nitro | Express | Next.js (Turbopack) workflow with 50 sequential steps💻 Local Development
▲ Production (Vercel)
🔍 Observability: Next.js (Turbopack) | Express | Nitro Promise.all with 10 concurrent steps💻 Local Development
▲ Production (Vercel)
🔍 Observability: Next.js (Turbopack) | Express | Nitro Promise.all with 25 concurrent steps💻 Local Development
▲ Production (Vercel)
🔍 Observability: Nitro | Express | Next.js (Turbopack) Promise.all with 50 concurrent steps💻 Local Development
▲ Production (Vercel)
🔍 Observability: Express | Nitro | Next.js (Turbopack) Promise.race with 10 concurrent steps💻 Local Development
▲ Production (Vercel)
🔍 Observability: Next.js (Turbopack) | Nitro | Express Promise.race with 25 concurrent steps💻 Local Development
▲ Production (Vercel)
🔍 Observability: Nitro | Express | Next.js (Turbopack) Promise.race with 50 concurrent steps💻 Local Development
▲ Production (Vercel)
🔍 Observability: Express | Nitro | Next.js (Turbopack) Stream Benchmarks(includes TTFB metrics)workflow with stream💻 Local Development
▲ Production (Vercel)
🔍 Observability: Nitro | Express | Next.js (Turbopack) SummaryFastest Framework by WorldWinner determined by most benchmark wins
Fastest World by FrameworkWinner determined by most benchmark wins
Column Definitions
Worlds:
|
Restore and update test coverage that was reduced in the initial change. The test suite now covers 15 scenarios (up from 9): Flush-on-write behavior: - write() resolves only after data reaches server - Single chunk uses writeToStream (not writeToStreamMulti) - Falls back to sequential writes when writeToStreamMulti unavailable - Multiple sequential writes trigger separate flush cycles - Concurrent writes wait for in-progress flush before buffering Close behavior: - closeStream called on close - Remaining buffer flushed on close - Empty buffer on close skips write methods Abort & error handling: - Abort discards buffer and skips closeStream - Write errors propagate to caller - Close errors propagate to caller - Flush errors during write propagate to caller Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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| title: Changelog | |||
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This is a stub so we can add changelog entries for bigger changes later
| * @vercel/workflow | ||
| packages/next/ @ijjk @vercel/workflow | ||
| packages/next/src @ijjk @vercel/workflow |
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This ensures JJ doesn't get accidentally tagged on every release PR when only changesets change
VaguelySerious
marked this pull request as ready for review
March 18, 2026 21:17
…sh timer but never settles the `flushWaiters` promises, causing the internal `write()` async function to hang forever on an unsettled promise.
This commit fixes the issue reported at packages/core/src/serialization.ts:560
**Bug explanation:**
In `BufferedWritableStream`, the `write()` method (line 530) pushes chunks to a buffer, schedules a flush via a timer, and then awaits a promise that is added to `flushWaiters` (line 545):
```js
await new Promise<void>((resolve, reject) => {
flushWaiters.push({ resolve, reject });
});
```
Normally, when the flush timer fires (line 514), it captures `flushWaiters`, replaces it with a fresh array, calls `flush()`, and then resolves or rejects the captured waiters based on the flush result.
When `abort()` is called (line 560), it:
1. Clears the flush timer (preventing the timer callback from ever running)
2. Discards the buffer
But it does NOT settle the promises in `flushWaiters`. Since the timer that would have resolved/rejected those waiters has been cleared, and nothing else references them, those promises will **never settle**. The internal async function of `write()` is suspended at the `await` on line 545 and will remain suspended forever - it becomes a memory leak. The closures captured by those async functions (referencing `buffer`, `flushWaiters`, the world object, etc.) will also be retained in memory.
While the WritableStream infrastructure will reject the external write promise returned to consumers, the *internal* async function body doesn't get cancelled - JavaScript has no mechanism to cancel a suspended async function. It will hang indefinitely on the unsettled promise.
**Fix explanation:**
The fix adds code to the `abort()` handler to reject all pending `flushWaiters` before clearing the array. It:
1. Captures the current `flushWaiters` into a local variable
2. Replaces `flushWaiters` with an empty array (same pattern used in the timer callback)
3. Rejects each waiter with the abort reason (or a default "Stream aborted" error)
This ensures the `write()` async functions' awaited promises settle (with rejection), allowing those async functions to complete and their closures to be garbage collected. The rejection from `w.reject()` will propagate up through the `write()` function, but since the stream is already being aborted, this is the correct behavior.
Co-authored-by: Vercel <vercel[bot]@users.noreply.github.com>
Co-authored-by: VaguelySerious <mittgfu@gmail.com>pranaygp
approved these changes
Mar 20, 2026
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Mar 20, 2026
…hods Merge main into peter/step-optional, resolving conflicts in the Streamer interface, world-local streamer, world-postgres streamer, and writable stream tests. Incorporates negative startIndex support from main (#1460) and pendingOps flush behavior (#1446) into the namespaced streams API. Additionally swaps argument order so runId is always the first parameter for all stream methods: write, writeMulti, close, get, and list. Updates all implementations (local, postgres, vercel), callers (serialization, runtime, CLI, web), tests, and documentation. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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Stream tests seemed slightly flaky, and I suspect it might be partially because the 10ms buffer on streams wasn't await-able.