fix(subscriptions): keep catching up when the checkpoint timer fires

[yordis]

• A subscription catching up would jump its checkpoint past every unread event between the last sent event and the head, silently losing them; upstream reports this hitting production twice on a replay from origin, skipping roughly 40 million events
• Carrying it here rather than waiting for upstream to merge, because a subscription that quietly skips events is indistinguishable from one that has caught up

Cherry-picked from commanded/eventstore#327 (cb21b09), authored by Doru Pirvu, including its regression test.

[cursor]

PR Summary

High Risk
Changes subscription checkpoint FSM transitions in a path that previously caused silent mass event loss in production replays; incorrect behavior would still be hard to detect without thorough catch-up testing.

Overview
Fixes a bug where a subscription catching up could persist its checkpoint and transition to subscribed while events were still in-flight or not yet delivered, effectively skipping everything between the last sent event and the stream head.

checkpoint handling in SubscriptionFsm no longer forces :subscribed from catch-up-related states. After persist_checkpoint, the FSM stays in request_catch_up, catching_up, or max_capacity (unchanged behavior when already subscribed). That keeps draining the backlog instead of looking “caught up” after the timer fires.

Adds a regression test that arms a short checkpoint_after, appends events mid catch-up, sleeps through the timer while acking one-by-one with buffer_size: 1, and asserts no events are missed.

^{Reviewed by Cursor Bugbot for commit f28afe5. Bugbot is set up for automated code reviews on this repo. Configure here.}

[coderabbitai]

Warning

Review limit reached

Next included review available in 58 seconds.

Check out review usage here.

View limit details

Limit details: You’ve used the included review currently available.

You've used all free OSS reviews for now. Wait for the free limit to reset to keep reviewing this public repository.

Learn how review limits work.

Review configuration:

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: d12d669c-ff8a-4972-8c9b-6458e2207231

📥 Commits

Reviewing files that changed from the base of the PR and between f6a1fc6 and f28afe5.

📒 Files selected for processing (2)

• lib/event_store/subscriptions/subscription_fsm.ex
• test/subscriptions/subscription_catch_up_test.exs

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}