fix(mcp): reset background capture after fork

[marandaneto]

💡 Motivation and Context

The MCP synchronous capture path can inherit background event-loop state across fork(), but the daemon thread that drives that loop does not survive in the child. An inherited locked mutex or pending parent work can then deadlock or prevent later child captures from completing.

Reset the MCP background capture loop, lock, and pending-work references in forked children without touching the parent's state or trying to close the defunct inherited loop.

💚 How did you test it?

• /Users/marandaneto/Github/posthog-python/.venv/bin/python -m pytest posthog/test/mcp/test_instrumentation_fork.py -q
• /Users/marandaneto/Github/posthog-python/.venv/bin/ruff check posthog/mcp/_instrumentation.py posthog/test/mcp/test_instrumentation_fork.py
• /Users/marandaneto/Github/posthog-python/.venv/bin/ruff format --check posthog/mcp/_instrumentation.py posthog/test/mcp/test_instrumentation_fork.py
• /Users/marandaneto/Github/posthog-python/.venv/bin/python -W error -c "import posthog.mcp._instrumentation"
• git diff --check

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Pi implemented the targeted fix under human direction. Pi's autoreview report guided the final submission and found no actionable issues. The chosen approach uses os.register_at_fork to replace only child-process background capture state, including the potentially locked mutex, while preserving parent-process behavior. A real-fork regression verifies that inherited work is cleared and a fresh child loop completes capture.

[github-actions]

posthog-python Compliance Report

Date: 2026-07-30 14:50:09 UTC
Duration: 338839ms

✅ All Tests Passed!

111/111 tests passed

---

Capture_V1 Tests

✅ 94/94 tests passed

View Details

Test	Status	Duration
Endpoint And Method.Targets V1 Endpoint	✅	517ms
Endpoint And Method.Does Not Use Legacy Endpoints	✅	1010ms
Required Headers.Has Authorization Bearer Header	✅	1009ms
Required Headers.Has Content Type Json	✅	1009ms
Required Headers.Has Posthog Sdk Info Format	✅	1009ms
Required Headers.Has Posthog Attempt Header	✅	1008ms
Required Headers.Has Posthog Request Id	✅	1008ms
Required Headers.Has Posthog Request Timestamp	✅	1009ms
Required Headers.Has User Agent	✅	1009ms
Body Format.Body Has Created At And Batch	✅	1008ms
Body Format.No Api Key In Body	✅	1009ms
Body Format.No Sent At In Body	✅	1009ms
Event Format.Event Has Required Root Fields	✅	1008ms
Event Format.Event Uuid Is Valid	✅	1009ms
Event Format.Event Timestamp Is Rfc3339	✅	1008ms
Event Format.Distinct Id Is String	✅	1008ms
Event Format.Distinct Id At Root Not Properties	✅	1008ms
Event Format.Custom Properties Preserved	✅	1010ms
Event Format.Set Properties Preserved	✅	1009ms
Event Format.Set Once Properties Preserved	✅	1008ms
Event Format.Groups Properties Preserved	✅	1009ms
Event Format.Sdk Generates Uuid If Not Provided	✅	1009ms
Event Format.Event Has Required Root Fields Batch	✅	1012ms
Event Format.Event Uuid Is Valid Batch	✅	1014ms
Event Format.Event Timestamp Is Rfc3339 Batch	✅	1012ms
Event Format.Distinct Id Is String Batch	✅	1012ms
Event Format.Distinct Id At Root Not Properties Batch	✅	1012ms
Event Format.Custom Properties Preserved Batch	✅	1012ms
Event Format.Set Properties Preserved Batch	✅	1013ms
Event Format.Set Once Properties Preserved Batch	✅	1013ms
Event Format.Groups Properties Preserved Batch	✅	1013ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	1013ms
Batch Behavior.Multiple Events In Single Batch	✅	1507ms
Batch Behavior.Batch Envelope Smoke	✅	1013ms
Batch Behavior.Flush With No Events Sends Nothing	✅	1005ms
Batch Behavior.Flush At Triggers Batch	✅	1509ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	1010ms
Deduplication.Generates Unique Uuids	✅	1507ms
Deduplication.Different Events Same Content Different Uuids	✅	1506ms
Deduplication.Preserves Uuid On Retry	✅	7519ms
Deduplication.Preserves Timestamp On Retry	✅	7511ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	7512ms
Deduplication.No Duplicate Events In Batch	✅	1504ms
Header Behavior On Retry.Attempt Header Starts At One	✅	1010ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	14524ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	7506ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	3517ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	7514ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	1006ms
Response Format Validation.Success Response Has Ok For Each Event	✅	1507ms
Response Format Validation.Success No Retry After When All Ok	✅	1507ms
Response Format Validation.Success Retry After Present When Retry Events	✅	2510ms
Response Format Validation.Success No Retry After When Drop Only	✅	1507ms
Response Format Validation.Response Echoes Request Id	✅	1009ms
Retry Behavior.Retries On 408	✅	7510ms
Retry Behavior.Retries On 500	✅	7516ms
Retry Behavior.Retries On 503	✅	9514ms
Retry Behavior.Retries On 504	✅	7510ms
Retry Behavior.Retryable Errors Have Retry After	✅	4512ms
Retry Behavior.Respects Retry After On Retryable Error	✅	12519ms
Retry Behavior.Does Not Retry On 400	✅	3502ms
Retry Behavior.Does Not Retry On 401	✅	3509ms
Retry Behavior.Does Not Retry On 402	✅	3507ms
Retry Behavior.Does Not Retry On 413	✅	3508ms
Retry Behavior.Does Not Retry On 415	✅	3508ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	3508ms
Retry Behavior.Implements Backoff	✅	23521ms
Retry Behavior.Max Retries Respected	✅	23521ms
Partial Batch Handling.Handles 200 Full Success	✅	3011ms
Partial Batch Handling.Handles 200 With All Ok	✅	4508ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	4508ms
Partial Batch Handling.Does Not Retry Limited Events	✅	4508ms
Partial Batch Handling.Prunes Ok Events On Partial Retry	✅	7513ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry	✅	7511ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	7513ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	7511ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	7513ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	7511ms
Partial Batch Handling.Respects Retry After On Partial	✅	9512ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	4504ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	4511ms
Compression.Sends Gzip Content Encoding	✅	1007ms
Compression.No Content Encoding When Disabled	✅	1009ms
Compression.Compressed Body Is Decompressible	✅	1008ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	3509ms
Event Options.Cookieless Mode Override	✅	1007ms
Event Options.Disable Skew Correction Override	✅	1008ms
Event Options.Process Person Profile Override	✅	1009ms
Event Options.Product Tour Id Override	✅	1008ms
Event Options.Unset Options Omitted	✅	1009ms
Event Options.Options Override In Batch	✅	1011ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	1009ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	1008ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	1009ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	1007ms
Request Payload.Flags Request Uses V2 Query Param	✅	1007ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	1006ms
Request Payload.Flags Request Omits Authorization Header	✅	1006ms
Request Payload.Token In Flags Body Matches Init	✅	1007ms
Request Payload.Groups Round Trip	✅	1006ms
Request Payload.Groups Default To Empty Object	✅	1006ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	1007ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	1007ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	1006ms
Request Lifecycle.No Flags Request On Init Alone	✅	503ms
Request Lifecycle.No Flags Request On Normal Capture	✅	1507ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	1010ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	1003ms
Retry Behavior.Retries Flags On 502	✅	1006ms
Retry Behavior.Retries Flags On 504	✅	1007ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	1508ms

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(mcp): reset background capture after..." | Re-trigger Greptile}

[gesh]

Thanks! 🙌