fix(xmtp_mls): defer unknown task kinds instead of deleting them

[tylerhawkes]

Follow-up to #4018, addressing a Macroscope finding raised on #4011.

#4018 added a KpLiveness arm to run_task for the task variant the proto regen introduced. Nothing in this crate schedules that kind yet, so the arm deleted the row.

A row of an unknown kind can only appear one way: a newer client wrote it into a database this build also opens. Deleting it discards work that newer client still expects to run, and it is not recoverable — the scheduling decision lived in the row.

Reschedule instead. The row survives for whichever client understands it, and expires_at_ns (checked at the top of run_and_reschedule_task) still bounds how long an unclaimed row can sit there, so an old client running alone cannot accumulate them.

One hour was picked so an old client sharing the database wakes on it rarely, while a newer client still picks it up promptly after a restart.

Independent of the app-data callback stack — it touches only worker/tasks.rs and is based directly on main.

🤖 Generated with Claude Code

https://claude.ai/code/session_01AdY7WKkNbJmdzpmUWvW1my

Note

Defer unknown task kinds instead of deleting them in MLS worker

Previously, unrecognized task kinds (such as KpLiveness in older builds) were deleted when encountered. Now they are rescheduled 1 hour into the future using a new UNKNOWN_TASK_DEFER_NS constant, preserving tasks for future processing by a compatible build.

Behavioral Change: unknown tasks are no longer permanently deleted; they persist and retry every hour until a build that recognizes them is deployed.

^{Macroscope summarized 38ff2e2.}

[claude]

Claude finished @tylerhawkes's task —— View job

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Code Review Completed

Review Date: August 19, 2026 at 20:07 UTC

• Read the modified file
• Analyze the implementation
• Provide review feedback

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Review Feedback

The implementation is solid and well-reasoned. The approach of deferring unknown task kinds instead of deleting them correctly handles the multi-version database sharing scenario.

Strengths:

• Clear constant naming (UNKNOWN_TASK_DEFER_NS) with excellent inline documentation explaining the timing rationale (lines 22-25)
• The 1-hour deferral strikes a good balance between avoiding hot-loops on old clients and allowing prompt pickup by newer clients
• Leverages existing expires_at_ns bound to prevent unbounded accumulation
• Appropriate logging with tracing::warn! that includes context (task ID, task kind)

Minor observations:

1. Pattern consistency: The KpLiveness arm is the only one that returns early with a reschedule. Consider if a None case should also defer (lines 466-469) - though deletion makes sense there since truly empty tasks are malformed.

2. Generic applicability: This pattern will need to be repeated for each new proto variant. The code could benefit from a comment noting that future proto additions should follow this pattern if they're not immediately implemented. The current comment (lines 451-457) is excellent but focuses on KpLiveness specifically.

3. Test coverage: No test explicitly validates the defer behavior for unknown task kinds. Consider adding a test that:

   • Seeds a task with a recognizable-but-unhandled variant
   • Calls run_and_reschedule_task
   • Asserts the task survives with next_attempt_at_ns advanced by ~1 hour

No blocking issues. The change addresses the stated problem correctly and follows the codebase's patterns for task handling.

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[macroscopeapp]

Approvability

Verdict: Approved at 38ff2e2

Macroscope's review found this PR approvable — Small, self-contained bug fix by the designated code owner. Changes unknown task handling from deletion to 1-hour deferral for multi-version database compatibility, with clear rationale documented in comments.

^{You can add or adjust custom eligibility rules. Learn more.}

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.92%. Comparing base (e3f9c62) to head (38ff2e2).

Files with missing lines	Patch %	Lines
crates/xmtp_mls/src/worker/tasks.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4020      +/-   ##
==========================================
- Coverage   85.93%   85.92%   -0.01%     
==========================================
  Files         417      417              
  Lines       68170    68172       +2     
==========================================
- Hits        58579    58575       -4     
- Misses       9591     9597       +6     

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