[release/9.0] Fix h3 connection-level and stream-level abort locking

[BrennanConroy]

Backport of #68102 to release/9.0

/cc @BrennanConroy @cincuranet

Fix h3 connection-level and stream-level abort locking

Description

Improve server reliability when using HTTP/3.

Fixes #68101

Customer Impact

Threads can get blocked resulting in more threads being created.

Regression?

• Yes
• No

Risk

• High
• Medium
• Low

Follows similar pattern already implemented in HTTP/2

Verification

• Manual (required)
• Automated

Packaging changes reviewed?

• Yes
• No
• N/A

[dotnet-policy-service]

Hi @BrennanConroy. If this is not a tell-mode PR, please make sure to follow the instructions laid out in the servicing process document.
Otherwise, please add tell-mode label.

[Copilot]

Copilot review overview

🔵 Needs a closer look

The change alters HTTP/3 concurrency/locking and stream lifecycle ordering in a release backport, which warrants final human review despite appearing correct.

Review tier: Lite
Findings: None

What changed in this PR

This PR backports the HTTP/3 fix from #68102 to release/9.0 to prevent deadlocks between connection-level and stream-level abort paths by reordering locking and ensuring abort side-effects complete before stream reuse/pooling.

Changes:

• Moves HTTP/3 abort side-effects (notably _http3Output.Stop() / frame-writer abort) outside _completionLock to avoid lock-order inversion with the output producer’s writer lock.
• Introduces an abort-completion TaskCompletionSource and awaits it during request finalization to prevent late abort side-effects from affecting a pooled/reused stream.

File	Description
src/​Servers/​Kestrel/​Core/​src/​Internal/​Http3/​Http3Stream.cs	Reworks abort locking and adds abort-completion synchronization before draining/disposing/pooling the underlying transport.

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[dotnet-policy-service]

Hi @BrennanConroy. This PR was just approved to be included in the upcoming servicing release. Somebody from the @dotnet/aspnet-build team will get it merged when the branches are open. Until then, please make sure all the CI checks pass and the PR is reviewed.