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net/http: ResponseWriter doesn't overwrite content-type after WriteHeader is called. #17083

GordonJiang opened this issue Sep 13, 2016 · 2 comments


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@GordonJiang GordonJiang commented Sep 13, 2016

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?


What operating system and processor architecture are you using (go env)?

ubuntu, amd64

What did you do?

in a http server handler,
first call w.WriteHeader(http.StatusOK)
then call w.Header().Set("Content-Type", "application/json")
then w.Write(somedata compressed by gzip)

What did you expect to see?

The Content-Type in response message captured by troubleshooting tool of chrome/firefox becomes application/x-gzip

What did you see instead?

The Content-Type should be application/json as I set.

On the other hand, if I change the order. call w.Header().Set("Content-Type", "application/json") ahead of the w.WriteHeader(http.StatusOK), the Content-Type keeps what it is. Seems setting status ahead of write content-type trigger the logic to DetectContentType automatically. It's overwrite whatever content-type I set later.
Because of that, gzip content can't be parsed in firefox. Of course, I worked around it by changing the code to set content-type first. But it apparently a trap since there is no doc saying you should always set content-type before set status by WriteHeader.

@odeke-em odeke-em changed the title golang http server overwrite content-type I set if WriteHeader(http.StatusOK) is called first. net/http: ResponseWriter doesn't overwrite content-type after WriteHeader is called. Sep 13, 2016
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@odeke-em odeke-em commented Sep 13, 2016

@GordonJiang this behavior is documented in
screen shot 2016-09-12 at 6 11 46 pm

as the documentation for property Header().

Closing this issue unless someone else thinks we should do more.
/cc @bradfitz.

@odeke-em odeke-em closed this Sep 13, 2016
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@minux minux commented Sep 13, 2016

@golang golang locked and limited conversation to collaborators Sep 13, 2017
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