Fix disposal race condition in AsyncBatchHandler

[haacked]

Summary

• Await background tasks during disposal — DisposeAsync now stores and awaits both the timer and flush-signal Task references before attempting the final flush. Previously, cancellation fired without waiting for in-flight flushes, so the final FlushBatchesAsync could be silently skipped when _flushing was still 1.
• Reorder resource disposal — CTS is now disposed after awaiting the background tasks (which need the token during cancellation), rather than immediately after CancelAsync.
• Add race condition regression test — FlushesItemWhenDisposedDuringInFlightFlush blocks a flush handler, starts disposal concurrently, then unblocks and verifies the item was sent.

Fixes #146

Test plan

• All existing AsyncBatchHandler tests pass (FlushesBatchOnTimer, FlushesBatchWhenDisposed, DoesNotDisposeTwice, etc.)
• New FlushesItemWhenDisposedDuringInFlightFlush test passes on both net8.0 and netcoreapp3.1
• Verify in an IIS-hosted app that queued events are reliably flushed on AppDomain shutdown

[github-actions]

posthog-dotnet Compliance Report

Date: 2026-02-25 23:23:33 UTC
Duration: 120001ms

✅ All Tests Passed!

29/29 tests passed

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Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	155ms
Format Validation.Event Has Uuid	✅	128ms
Format Validation.Event Has Lib Properties	✅	111ms
Format Validation.Distinct Id Is String	✅	111ms
Format Validation.Token Is Present	✅	110ms
Format Validation.Custom Properties Preserved	✅	118ms
Format Validation.Event Has Timestamp	✅	110ms
Retry Behavior.Retries On 503	✅	8124ms
Retry Behavior.Does Not Retry On 400	✅	2021ms
Retry Behavior.Does Not Retry On 401	✅	2013ms
Retry Behavior.Respects Retry After Header	✅	8119ms
Retry Behavior.Implements Backoff	✅	22133ms
Retry Behavior.Retries On 500	✅	6119ms
Retry Behavior.Retries On 502	✅	6119ms
Retry Behavior.Retries On 504	✅	6119ms
Retry Behavior.Max Retries Respected	✅	22023ms
Deduplication.Generates Unique Uuids	✅	116ms
Deduplication.Preserves Uuid On Retry	✅	6117ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	13121ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	6121ms
Deduplication.No Duplicate Events In Batch	✅	118ms
Deduplication.Different Events Have Different Uuids	✅	111ms
Compression.Sends Gzip When Enabled	✅	110ms
Batch Format.Uses Proper Batch Structure	✅	109ms
Batch Format.Flush With No Events Sends Nothing	✅	106ms
Batch Format.Multiple Events Batched Together	✅	115ms
Error Handling.Does Not Retry On 403	✅	2010ms
Error Handling.Does Not Retry On 413	✅	2012ms
Error Handling.Retries On 408	✅	6117ms

[greptile-apps]

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