Close both HTTP/2 proxy channels after backend failures

[cragkhit]

Http2ProxyBackendHandler currently closes only one side of a proxied connection when a backend write or handler exception fails. The peer channel can remain open until another event closes it. The proposed change closes both sides on these failure paths. This issue accompanies a Matcha study recommendation derived from https://stackoverflow.com/a/42681543.

[RockteMQ-AI]

Issue Evaluation

Category: type/bug | Status: Confirmed

The reported issue has been verified against the current codebase on develop branch.

Root Cause:
In Http2ProxyBackendHandler, two failure paths close only the backend channel without closing the paired frontend (inbound) channel:

1. channelRead() — when future.isSuccess() is false, only future.channel().close() is called (backend side). The inboundChannel remains open.
2. exceptionCaught() — only calls closeOnFlush(ctx.channel()) (backend side). The inboundChannel is not closed.

The symmetric issue exists in Http2ProxyFrontendHandler.channelRead() — on write failure, only the outbound channel is closed while the backend channel stays open.

channelInactive() does close the peer channel, but it only fires after the channel becomes inactive — if the close is not propagated, the peer can remain open indefinitely, causing a resource leak.

Impact: Proxy layer — connection/resource leak under backend failure scenarios.
Severity: Medium

Proposed Fix: Close both channels on failure paths in both Http2ProxyBackendHandler and Http2ProxyFrontendHandler.

An automated fix proposal will be generated. Reply /approve to proceed with PR generation, or /reject to decline.

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