fix(profiler): Prevent buffer race condition during rapid start/stop cycles

[ericapisani]

Fixes #5621 and PY-2127

Fix a race condition in ContinuousScheduler where the profiler thread's cleanup could destroy a buffer created by a concurrent ensure_running() call.

The Problem

During rapid start/stop cycles, a race condition could occur:

1. stop_profiler() sets self.running = False
2. The profiler thread's run() loop exits, but hasn't reached cleanup yet
3. Another thread calls start_profiler() → ensure_running()
4. ensure_running() acquires the lock, sets self.running = True, and calls reset_buffer() to create a new buffer
5. The old profiler thread finally reaches cleanup and executes self.buffer = None
6. This destruction of the new buffer means the new profiler thread's sampler sees self.buffer is None and silently drops all samples

This race is a latent bug on all Python versions, but manifests as flaky tests on Python 3.6 with gevent due to timing and thread scheduling differences.

The Solution

Two changes prevent the race:

1. In profiler_id property: Add not self.running check to avoid accessing a buffer during shutdown
2. In run() cleanup: Capture self.buffer in a local variable for flushing and never set self.buffer = None. This ensures concurrent ensure_running() calls can safely create new buffers without them being destroyed.

Testing

Added a test that simulates rapid start/stop cycles and verifies run() doesn't null out self.buffer after exiting the sampling loop.

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Semver Impact of This PR

🟢 Patch (bug fixes)

📋 Changelog Preview

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Bug Fixes 🐛

• (celery) Propagate user-set headers by sentrivana in #5581

• (profiler) Prevent buffer race condition during rapid start/stop cycles by ericapisani in #5622

• (utils) Avoid double serialization of strings in safe_serialize by ericapisani in #5587

Documentation 📚

• (openai-agents) Remove inapplicable comment by alexander-alderman-webb in #5495
• Add AGENTS.md by sentrivana in #5579
• Add set_attribute example to changelog by sentrivana in #5578

Internal Changes 🔧

Openai Agents

• Set streaming header when library uses with_streaming_response() by alexander-alderman-webb in #5583
• Replace mocks with httpx for streamed responses by alexander-alderman-webb in #5580
• Replace mocks with httpx in non-MCP tool tests by alexander-alderman-webb in #5602
• Replace mocks with httpx in MCP tool tests by alexander-alderman-webb in #5605
• Replace mocks with httpx in handoff tests by alexander-alderman-webb in #5604
• Replace mocks with httpx in API error test by alexander-alderman-webb in #5601
• Replace mocks with httpx in non-error single-response tests by alexander-alderman-webb in #5600
• Remove test for unreachable state by alexander-alderman-webb in #5584
• Expect namespace tool field for new openai versions by alexander-alderman-webb in #5599

Other

• Do not run actions on potel-base by sentrivana in #5614

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Codecov Results 📊

✅ 13 passed | Total: 13 | Pass Rate: 100% | Execution Time: 8.15s

All tests are passing successfully.

❌ Patch coverage is 25.00%. Project has 13866 uncovered lines.

Files with missing lines (1)

File	Patch %	Lines
continuous_profiler.py	43.45%	⚠️ 177 Missing and 17 partials

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