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Viktor Liu
582c8b1ae8
ForceAttemptHTTP2 only puts h2 in the ALPN offer — the upstream still picks — so "auto" already meant "whatever the upstream chose". What ALPN cannot express is an upstream that selects h2 and then fails to speak it, which is the case the setting was added for: today that leaves the operator pinning every upstream to 1.1 to work around one broken backend. Auto now completes itself. The first h2-level failure for a host pins that host to an HTTP/1.1-only clone of its transport for 10 minutes and retries the request there when the body can be replayed, so a broken backend costs one failed h2 attempt instead of a configuration change. The pin is per upstream host, so one broken backend does not drop the others, and it expires so a fixed backend returns to h2 on its own. Only h2 framing errors trigger it: a dial, TLS or context error says nothing about the protocol and retrying it over HTTP/1.1 would fix nothing. The explicit values stay absolute — "2" never downgrades. Pinning HTTP/1.1 now also strips h2 from the ALPN offer. Configuring h2 makes net/http append it to the transport's TLSClientConfig, so a clone taken from a transport that already served a request would otherwise advertise a protocol the clone refuses to speak, and the reply would come back as h2 frames parsed as an HTTP/1.1 message.