[FrenchGithubUser]

same as element-hq/synapse#20090

[cursor]

PR Summary

Medium Risk
Touches core MultiWriterIdGenerator async entry used by presence and other streams; logic is narrow (cleanup on failed/cancelled __aenter__) but incorrect handling could advance tokens without persisted rows.

Overview
Fixes intermittent presence stream stalls where clients stop receiving presence updates until Synapse restarts.

When a cancellable /sync request is aborted while MultiWriterIdGenerator.get_next() is still in __aenter__, the DB sequence may already have allocated a stream ID and left it in _unfinished_ids, but Python never runs __aexit__ because __aenter__ did not return. That leaked ID pins the persisted stream position (including presence).

The change records allocated IDs on the context manager inside the load transaction, and on any exception from runInteraction (including CancelledError) calls _mark_ids_as_finished and notifies replication—the same cleanup as the failure path in __aexit__, without advancing stream_positions on an incomplete write.

A regression test simulates cancelled /sync during presence-like allocation and asserts a later write can advance the token past the wedged position.

^{Reviewed by Cursor Bugbot for commit d6706b8. Bugbot is set up for automated code reviews on this repo. Configure here.}

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.73%. Comparing base (013821e) to head (d6706b8).

Files with missing lines	Patch %	Lines
synapse/storage/util/id_generators.py	90.90%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #281      +/-   ##
==========================================
- Coverage   80.73%   80.73%   -0.01%     
==========================================
  Files         503      503              
  Lines       72286    72296      +10     
  Branches    10872    10873       +1     
==========================================
+ Hits        58363    58369       +6     
- Misses      10658    10660       +2     
- Partials     3265     3267       +2     

Files with missing lines	Coverage Δ
synapse/storage/util/id_generators.py	90.72% <90.90%> (-0.03%)	⬇️

... and 2 files with indirect coverage changes

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[FrenchGithubUser]

upstream PR got merged earlier, we'll cherry pick the commit in a follow up pr