Handle a race condition where onSubscribe may be emitted downstream after onError

[jrhee17]

Motivation:

While debugging #5880 (comment), I found that onSubscribe could be emitted after onError.
This is a clear violation of specifications https://github.com/reactive-streams/reactive-streams-jvm?tab=readme-ov-file#1.7 and https://github.com/reactive-streams/reactive-streams-jvm?tab=readme-ov-file#1.9.

I propose that before aborting, onSubscribe is called if it hasn't been called yet. To prevent onSubscribe being called multiple times, invokedOnSubscribe is used as a guard.

Modifications:

• Extracted maybeInvokeOnSubscribe and called it also from notifySubscriberOfCloseEvent0

Result:

• We no longer observe race conditions between onSubscribe and onError when aborting a StreamMessage.

[github-actions]

🔍 Build Scan® (commit: 8e37d48)

Job name	Status	Build Scan®
build-ubicloud-standard-16-jdk-8	✅	https://ge.armeria.dev/s/abgv7ztilylpq
build-ubicloud-standard-16-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/voh4u32gv47ka
build-ubicloud-standard-16-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/5ux65iq4z3xy4
build-ubicloud-standard-16-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/xza2jkj3sqwji
build-ubicloud-standard-16-jdk-17-leak	✅	https://ge.armeria.dev/s/e5nic65xwida4
build-ubicloud-standard-16-jdk-11	✅	https://ge.armeria.dev/s/6eihybd7fddii
build-macos-latest-jdk-21	✅	https://ge.armeria.dev/s/2j7wtnyc5ejc6

[minwoox]

Nice finding. 👍

[trustin]

👍