EC
http.ErrHandlerTimeout
GoERRORNotableHTTP

http: Handler timeout

Quick Answer

Wrap slow handlers with http.TimeoutHandler and check for this error to avoid writing after the deadline.

What this means

Returned by ResponseWriter.Write calls after the timeout set by http.TimeoutHandler has elapsed. The response has already been sent to the client.

Why it happens
  1. 1Handler took longer than the timeout duration configured in http.TimeoutHandler
  2. 2Slow database or external API call blocking the handler past its deadline

Fix

Check before writing

Check before writing
_, err := w.Write(data)
if errors.Is(err, http.ErrHandlerTimeout) {
    return // response already sent
}

Why this works

Returning immediately prevents superfluous write attempts after the timeout response.

Code examples
Wrap with TimeoutHandlergo
h := http.TimeoutHandler(myHandler,
    2*time.Second, "timeout")
http.ListenAndServe(":8080", h)
Detect in handlergo
_, err := fmt.Fprintln(w, "hello")
if errors.Is(err, http.ErrHandlerTimeout) {
    return
}
Context deadline checkgo
ctx := r.Context()
select {
case <-ctx.Done():
    return
}
Sources
Official documentation ↗

Go standard library

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