fix(contexts): reset context after fork

[marandaneto]

💡 Motivation and Context

PostHog context stored in contextvars is inherited when a process forks. This can leak a parent's distinct ID, session ID, device ID, and tags into child processes.

Reset the active context in forked children while preserving the parent's context.

💚 How did you test it?

• /Users/marandaneto/Github/posthog-python/.venv/bin/python -m pytest -q posthog/test/test_contexts.py
• /Users/marandaneto/Github/posthog-python/.venv/bin/ruff format --check posthog/contexts.py posthog/test/test_contexts.py
• /Users/marandaneto/Github/posthog-python/.venv/bin/ruff check posthog/contexts.py posthog/test/test_contexts.py
• 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. The autoreview report was used to confirm formatting, lint, compilation, and the absence of staged changes before submission. The implementation uses an os.register_at_fork child hook plus a generation marker so inherited and copied contexts remain invalid after a fork, including when inherited context managers unwind in the child.

[github-actions]

posthog-python Compliance Report

Date: 2026-07-31 07:58:42 UTC
Duration: 338883ms

✅ All Tests Passed!

111/111 tests passed

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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	✅	1008ms
Required Headers.Has Content Type Json	✅	1009ms
Required Headers.Has Posthog Sdk Info Format	✅	1009ms
Required Headers.Has Posthog Attempt Header	✅	1009ms
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	✅	1008ms
Body Format.No Sent At In Body	✅	1009ms
Event Format.Event Has Required Root Fields	✅	1008ms
Event Format.Event Uuid Is Valid	✅	1010ms
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	✅	1009ms
Event Format.Set Properties Preserved	✅	1009ms
Event Format.Set Once Properties Preserved	✅	1010ms
Event Format.Groups Properties Preserved	✅	1009ms
Event Format.Sdk Generates Uuid If Not Provided	✅	1008ms
Event Format.Event Has Required Root Fields Batch	✅	1013ms
Event Format.Event Uuid Is Valid Batch	✅	1013ms
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	✅	1013ms
Event Format.Set Properties Preserved Batch	✅	1013ms
Event Format.Set Once Properties Preserved Batch	✅	1013ms
Event Format.Groups Properties Preserved Batch	✅	1012ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	1012ms
Batch Behavior.Multiple Events In Single Batch	✅	1507ms
Batch Behavior.Batch Envelope Smoke	✅	1014ms
Batch Behavior.Flush With No Events Sends Nothing	✅	1005ms
Batch Behavior.Flush At Triggers Batch	✅	1508ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	1010ms
Deduplication.Generates Unique Uuids	✅	1507ms
Deduplication.Different Events Same Content Different Uuids	✅	1507ms
Deduplication.Preserves Uuid On Retry	✅	7516ms
Deduplication.Preserves Timestamp On Retry	✅	7514ms
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	✅	14519ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	7507ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	3519ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	7510ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	1011ms
Response Format Validation.Success Response Has Ok For Each Event	✅	1506ms
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	✅	1010ms
Retry Behavior.Retries On 408	✅	7511ms
Retry Behavior.Retries On 500	✅	7514ms
Retry Behavior.Retries On 503	✅	9517ms
Retry Behavior.Retries On 504	✅	7512ms
Retry Behavior.Retryable Errors Have Retry After	✅	4509ms
Retry Behavior.Respects Retry After On Retryable Error	✅	12518ms
Retry Behavior.Does Not Retry On 400	✅	3504ms
Retry Behavior.Does Not Retry On 401	✅	3508ms
Retry Behavior.Does Not Retry On 402	✅	3508ms
Retry Behavior.Does Not Retry On 413	✅	3508ms
Retry Behavior.Does Not Retry On 415	✅	3507ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	3509ms
Retry Behavior.Implements Backoff	✅	23530ms
Retry Behavior.Max Retries Respected	✅	23522ms
Partial Batch Handling.Handles 200 Full Success	✅	3000ms
Partial Batch Handling.Handles 200 With All Ok	✅	4509ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	4510ms
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	✅	7512ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	7513ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	7513ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	7512ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	7508ms
Partial Batch Handling.Respects Retry After On Partial	✅	9516ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	4505ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	4509ms
Compression.Sends Gzip Content Encoding	✅	1008ms
Compression.No Content Encoding When Disabled	✅	1009ms
Compression.Compressed Body Is Decompressible	✅	1009ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	3509ms
Event Options.Cookieless Mode Override	✅	1008ms
Event Options.Disable Skew Correction Override	✅	1009ms
Event Options.Process Person Profile Override	✅	1008ms
Event Options.Product Tour Id Override	✅	1008ms
Event Options.Unset Options Omitted	✅	1008ms
Event Options.Options Override In Batch	✅	1012ms
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	✅	1006ms
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	✅	1007ms
Request Payload.Token In Flags Body Matches Init	✅	1006ms
Request Payload.Groups Round Trip	✅	1007ms
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	✅	1006ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	1007ms
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	✅	1509ms

[marandaneto]

@PostHog/team-error-tracking I think the context bits were added by error tracking (Olly?), and a process fork may leak some data. I am not sure this was intentional; if not, we should merge this. What do you think?
If we want to leak to the fork's child, this should be intentional, e.g. passing a method param instead of by default

[greptile-apps]

Prompt To Fix All With AI

### Issue 1
posthog/test/test_contexts.py:305-306
**Child probe lacks guaranteed cleanup**

When the child raises before writing its result, it bypasses descriptor cleanup and `os._exit`, while the parent remains blocked in `os.read`; wrap the child probe so every path reports or closes the pipe and terminates safely.

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (1): Last reviewed commit: "test(contexts): document fork context po..." | Re-trigger Greptile}

[ablaszkiewicz]

lgtm