Potential use-after-free during HTTP cleanup/completion overlap

[jhugard]

Summary

HCCleanupAsync can overlap completion of an in-flight HTTP request. If the HTTP completion callback frees its XAsyncBlock, cleanup can subsequently attempt to XAsyncCancel that same block, creating a potential use-after-free. This hazard was introduced with #931.

Impact

• Potential use-after-free on the client-owned XAsyncBlock
• Nondeterministic cleanup/completion ordering for HTTP performs
• Possible follow-on failures in later operations once the race is hit

Expected behavior

Once the HTTP completion callback begins running, library cleanup should no longer treat that request as cancelable through the client-owned XAsyncBlock.

Actual behavior

During cleanup, the request can still be observed as active and cancelable while the client completion callback is already executing.

Repro

At a high level:

1. Initialize libHttpClient.
2. Create a mocked HTTP call and perform it asynchronously.
3. Let the completion callback run on the client-owned XAsyncBlock.
4. Observe that cleanup can still treat that request as cancelable from within the callback window.

Note

A PR fix will be inbound after internal review.