fix: don't warn on routine client-disconnect write/flush errors - #2589
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go_sapi_flush logged every ResponseController.Flush() error with the same "not a flusher" message, even when the writer clearly supports Flush but the call failed for another reason (e.g. a client disconnect). Only use that message for http.ErrNotSupported and log the real error otherwise.
A client disconnecting mid-flush is routine and not actionable, unlike the "not a flusher" misconfiguration case.
Same event as go_ub_write's "write error" (client disconnect), so log it at the same level for consistency. Flatten the nested if/else into early returns.
Caddy (streaming.go: "backConn write failed", "streaming error") and Mercure (subscribe.go: "Failed to write comment", "Connection closed by the client") both log repeated per-write streaming failures at debug, reserving warn for one-off actionable problems. Match that: a client disconnecting mid-response is routine and can repeat on every write of a request, so it doesn't belong at warn. Only the "not a flusher" misconfiguration case stays at warn. Also avoid computing the message/level before knowing whether the logger is even enabled for it.
dunglas
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dunglas
marked this pull request as ready for review
August 4, 2026 19:39
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Summary
While investigating #2588, found two related logging bugs around client disconnects.
go_sapi_flushlogged everyResponseController.Flush()error under the same message:That message is only accurate for
http.ErrNotSupported. A plain disconnect/write error (e.g. an aborted HTTP/2 stream) got mislabeled the same way, misleading and able to mask the real cause when triaging issues shaped like #2588.Also reconsidered the log level.
go_ub_write's equivalent "write error" was logged at warn, so the new "flush error" branch was first matched to it. But a client disconnecting mid-response is routine, and both writes and flushes happen in a loop, so a single request can log this dozens or hundreds of times, one per write/flush call after the disconnect. Not warn-worthy.Checked how the two other Caddy-family projects here handle this same class of error:
streaming.go): "backConn write failed", "response flush", "streaming error" - all logged at debug.subscribe.go): "Failed to write comment" (per-write, in a loop) and "Connection closed by the client" are debug; only one-off lifecycle events like "Subscriber disconnected" are info.Matched that convention:
go_ub_write's "write error" -> debug (was warn).go_sapi_flush's "flush error" -> debug (routine disconnect), correctly separated from "not a flusher".go_sapi_flush's "not a flusher" -> stays warn (real misconfiguration, not a per-request occurrence).This only fixes logging, not the segfault. I was not able to reproduce the crash reported in #2588 (see comment on the issue).
Test plan
go build ./...go vet ./...go test -run "TestConnectionAbort|TestLog" ./...