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🐛 Only call _subscribe once for concurrent subscribes to a channel - #753
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Fixes #739 At the moment, `PubSub.subscribe()` only marks a channel as subscribed once `_subscribe()` calls back, so any other subscribe to that channel in the meantime calls `_subscribe()` again. `sharedb-redis-pubsub` adds a listener per call, so every op is delivered once per listener. This change queues concurrent subscribers behind the in-flight `_subscribe()`. We create all of their streams before calling any callback, since `Agent` can destroy a presence stream from inside its callback, which would otherwise unsubscribe the channel from under the subscribers still waiting. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>
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coverage: 97.855% (+0.004%) from 97.851% — coalesce-concurrent-subscribes into main |
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🟢 Approval recommended
The implementation resolves duplicate adapter subscriptions and covers the relevant concurrency edge cases.
0 open findings
What changed in this PR
Queues concurrent channel subscriptions so only one adapter-level _subscribe call occurs.
Changes:
- Tracks in-flight subscriptions per channel.
- Creates all queued streams before invoking callbacks.
- Adds success, failure, teardown, and retry coverage.
| File | Description |
|---|---|
lib/pubsub/index.js |
Coalesces concurrent subscriptions. |
test/pubsub.js |
Tests delivery and stream destruction behavior. |
test/pubsub-memory.js |
Tests errors, retries, and resubscription. |
🧠 Review effort: Balanced
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alecgibson
marked this pull request as ready for review
October 9, 2026 09:46
dawidreedsy
approved these changes
Oct 9, 2026
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Fixes #739
At the moment,
PubSub.subscribe()only marks a channel as subscribed once_subscribe()calls back, so any other subscribe to that channel in the meantime calls_subscribe()again.sharedb-redis-pubsubadds a listener per call, so every op is delivered once per listener.This change queues concurrent subscribers behind the in-flight
_subscribe(). We create all of their streams before calling any callback, sinceAgentcan destroy a presence stream from inside its callback, which would otherwise unsubscribe the channel from under the subscribers still waiting.🤖 Generated with Claude Code
Co-Authored-By: Claude noreply@anthropic.com