[BUG] Wait on a completion state in the Elasticsearch exporter

[thc1006]

Fixes #4296

Draft while CI runs. Ready for review from my side.

The problem

bool waitForResponse()
{
  std::unique_lock<std::mutex> lk(mutex_);
  cv_.wait(lk);
  return response_received_;
}

OnResponse() set response_received_ under the mutex and called notify_all() after releasing it. If that landed before the exporting thread reached cv_.wait(lk), the notification was gone and the thread blocked with nothing left to wake it. The OnEvent() failure cases were worse: they called notify_all() without taking the mutex and without recording anything, so an early failure left the waiter with no state to observe even if it did wake.

A spurious wakeup also returned response_received_, still false, so a request that was still in flight was reported as a failure.

SessionState::Destroyed neither notified nor recorded anything, so a session that ended without a response or an error event left the waiter blocked indefinitely.

Two threads, one recording the outcome and one waiting, with the ordering flipped between runs:

=== notification arrives AFTER the waiter parks ===
  old: response after wait                 returned success
  new: response after wait                 returned success
=== notification arrives BEFORE the waiter parks ===
  new: response before wait                returned success
  old: response before wait                HUNG, no wakeup left to receive

The last run had to be killed by a timeout.

The fix

The handler records an explicit completion state under the mutex on both the success and the failure paths, and the wait is a predicate over that state:

cv_.wait(lk, [this] { return completion_ != CompletionState::Pending; });
return completion_ == CompletionState::Success;

Because the predicate is evaluated under the same mutex that publishes the state, an outcome recorded before the waiter arrives is simply observed rather than missed, and a spurious wakeup goes back to waiting instead of reporting a failure.

Recording is first writer wins. That matters for Destroyed, which now records a failure too: it fires at the end of every session, including successful ones, and must not overwrite a result that has already been recorded. Its purpose here is to release a waiter when a session ends without ever producing a response.

What this does not change

The wait still has no timeout of its own. It is bounded by the request timeout arriving as a TimedOut session event, plus the Destroyed fallback added here. Giving the wait its own deadline would be a behavior change beyond this fix, and I would rather do it separately if you want it.

Verification

• The comparison above was run, not reasoned about.
• clang-format 18 clean.
• I could not run the repo's clang-tidy against this file locally, since it needs the exporter's full include set. The enum carries an explicit std::uint8_t base type so it does not trip performance-enum-size, and <cstdint> is added for it. CI is the authority on warning_limit.

Scope

Synchronization only. The bulk response parsing is #4295 with its own PR, and the internal linkage of ResponseHandler belongs to the #4196 cleanup.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.30%. Comparing base (d3c7a47) to head (2129088).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4298      +/-   ##
==========================================
- Coverage   81.31%   81.30%   -0.01%     
==========================================
  Files         445      445              
  Lines       18859    18859              
==========================================
- Hits        15333    15331       -2     
- Misses       3526     3528       +2     

Files with missing lines	Coverage Δ
...orters/elasticsearch/src/es_log_record_exporter.cc	12.22% <ø> (ø)

... and 1 file with indirect coverage changes

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