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promhttp: Ignore informational response status codes when handling response #1773

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promhttp: Ignore informational response status codes when handling response #1773

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21 changes: 15 additions & 6 deletions prometheus/promhttp/delegator.go
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Original file line number Diff line number Diff line change
Expand Up @@ -53,12 +53,21 @@ func (r *responseWriterDelegator) Written() int64 {
}

func (r *responseWriterDelegator) WriteHeader(code int) {
if r.observeWriteHeader != nil && !r.wroteHeader {
// Only call observeWriteHeader for the 1st time. It's a bug if
// WriteHeader is called more than once, but we want to protect
// against it here. Note that we still delegate the WriteHeader
// to the original ResponseWriter to not mask the bug from it.
r.observeWriteHeader(code)
if !r.wroteHeader {
// Ignore informational response status codes.
// Based on https://github.com/golang/go/blob/go1.24.1/src/net/http/server.go#L1216
if code >= 100 && code <= 199 && code != http.StatusSwitchingProtocols {
r.ResponseWriter.WriteHeader(code)
return
}

if r.observeWriteHeader != nil {
// Only call observeWriteHeader for the 1st time. It's a bug if
// WriteHeader is called more than once, but we want to protect
// against it here. Note that we still delegate the WriteHeader
// to the original ResponseWriter to not mask the bug from it.
r.observeWriteHeader(code)
}
}
r.status = code
r.wroteHeader = true
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