fix(subscriptions): stop a deleted subscription from silently skipping its successor's events

[yordis]

• A subscription row is identified by its stream and name, both of which a later subscription can reuse, so a checkpoint written after that row was deleted could be credited to a subscription that never acknowledged those events. Every event between the two positions is then skipped, and nothing anywhere reports it.
• A checkpoint that reached no row was reported as success, so a subscription whose row had been deleted underneath it kept acknowledging into nothing for as long as it ran.
• Deleting the row belongs to the subscription itself: only the subscription knows whether a subscriber is still connected to it, and only it holds the registered name that keeps a replacement from claiming the row while it is being removed.
• The prevention that buys is bounded, and worth naming rather than overclaiming: a caller whose last subscriber unsubscribes first finds no process left to ask, so it still deletes the row from outside. Noticing the vanished row is what covers that case, which is the common one.

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Reviewing files that changed from the base of the PR and between 5478e1b and 20a541e.

📒 Files selected for processing (6)

• lib/event_store/subscriptions/subscription_fsm.ex
• lib/event_store/subscriptions/supervisor.ex
• test/storage/subscription_persistence_test.exs
• test/subscriptions/subscribe_to_stream_test.exs
• test/subscriptions/subscription_fsm_test.exs
• test/subscriptions/support/slow_leaver.ex

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Walkthrough

Subscription acknowledgments now identify rows by subscription ID. Deletion requests go through the subscription supervisor, which handles registered and unregistered subscriptions. Tests cover row deletion and checkpoint behavior when a subscription row is missing or replaced.

Changes

Subscription lifecycle

Layer / File(s)	Summary
ID-based checkpoint acknowledgments lib/event_store/sql/statements/subscription_ack.sql.eex, lib/event_store/storage.ex, lib/event_store/storage/subscription.ex, lib/event_store/subscriptions/subscription_fsm.ex, lib/event_store/subscriptions/subscription.ex, test/storage/subscription_persistence_test.exs, test/subscriptions/subscription_locking_test.exs, test/subscriptions/subscribe_to_stream_test.exs	Acknowledgment calls and queries now use subscription_id. When the row is missing, the subscription receives a checkpoint failure and stops. Tests cover missing rows and replacement rows.
Supervisor-managed subscription deletion lib/event_store/subscriptions/subscription_fsm.ex, lib/event_store/subscriptions/subscription.ex, lib/event_store/subscriptions/supervisor.ex, lib/event_store/subscriptions.ex, lib/event_store.ex, test/subscriptions/subscribe_to_stream_test.exs	Deletion requests go through the supervisor. A registered subscription deletes its storage row before stopping; when no subscription is registered, the supervisor deletes the row directly. The event-store entry point calls this deletion flow. Tests verify direct deletion removes the row.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Sequence Diagram(s)

sequenceDiagram
  participant EventStore
  participant SubscriptionsSupervisor
  participant Subscription
  participant Storage
  EventStore->>SubscriptionsSupervisor: delete_subscription(event_store, conn, stream_uuid, name, opts)
  alt Subscription is registered
    SubscriptionsSupervisor->>Subscription: delete(subscription, opts)
    Subscription->>Storage: delete_subscription(conn, stream_uuid, name, opts)
    Storage-->>Subscription: deletion result
    Subscription-->>SubscriptionsSupervisor: reply and process termination
    SubscriptionsSupervisor-->>EventStore: deletion reply after process termination
  else Subscription is not registered
    SubscriptionsSupervisor->>Storage: delete_subscription(conn, stream_uuid, name, opts)
    Storage-->>SubscriptionsSupervisor: deletion result
    SubscriptionsSupervisor-->>EventStore: deletion result
  end

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Merge Risk: 🟡 Moderate · up to 5478e

Deleting a subscription can remove a newer subscription that reused the same name, which silently drops that successor's progress. A failed delete can also lose an already-acknowledged checkpoint. Delete by the subscription's own ID and preserve pending checkpoints on failure before merging.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 15.79% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 19 functions across 10 files. (1 skipped:…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the main fix: preventing deleted subscriptions from silently skipping events for successor subscriptions.
Description check	✅ Passed	The description directly explains the checkpoint reuse problem, the deletion changes, and the remaining edge case addressed by the pull request.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 15.79% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 19 functions across 10 files. (1 skipped: 1 unsupported.)

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_{A rabbit checks the checkpoint trail
By ID, it finds the row
If the row is gone, the process stops
The supervisor completes deletion
Then hops away through fields of green}

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Actionable comments posted: 3

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
In `@lib/event_store/subscriptions/subscription_fsm.ex`:
- Line 231: Update the subscription FSM delete-result handling so
checkpoints_pending is reset to zero only after a successful delete; preserve
its existing count when deletion is rejected or times out.

In `@lib/event_store/subscriptions/subscription.ex`:
- Line 280: Update the delete path around SubscriptionFsm.delete/1 to delete by
subscription_id rather than only stream_uuid and subscription_name. Skip
deletion when the process has not claimed a subscription row, so an old process
cannot remove a replacement row.

In `@lib/event_store/subscriptions/supervisor.ex`:
- Around line 90-93: Update delete_subscription/5 to pass its monitor ref into
delete/2, and update delete to accept that ref; when Subscription.delete/2 exits
with :timeout, demonitor with flush before re-raising the exit, preserving the
existing handling of other exits.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: 31ac19ba-9047-45fe-9c3c-2c0ee0a75b1c

📥 Commits

Reviewing files that changed from the base of the PR and between 43ac0b7 and 5478e1b.

📒 Files selected for processing (11)

• lib/event_store.ex
• lib/event_store/sql/statements/subscription_ack.sql.eex
• lib/event_store/storage.ex
• lib/event_store/storage/subscription.ex
• lib/event_store/subscriptions.ex
• lib/event_store/subscriptions/subscription.ex
• lib/event_store/subscriptions/subscription_fsm.ex
• lib/event_store/subscriptions/supervisor.ex
• test/storage/subscription_persistence_test.exs
• test/subscriptions/subscribe_to_stream_test.exs
• test/subscriptions/subscription_locking_test.exs

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