io.Copy error handling

[RS]

Hi All, 

we copy response body from a call into responseWriter w.

my question is:

a) if err!=nil, it is possible that w is also not nil? Or no: if err!=nil then w is nil. 

b)  the following order is correct?

I would send response body (which is in src (resp.Body type io.ReadCloser)) as xml content type with OK status. 

And if error happens by io.Copy, then send a json body with status code 599. 

w.Header().Set("Content-Type", "application/xml")

_, err:=io.Copy(w,src) 

if err!=nil{

re := &res{ Message: "Error happened",}

w.Header().Set("Content-Type", "application/json")
w.WriteHeader(599) //sample status code
err = json.NewEncoder(w).Encode(re)

}//if

[Axel Wagner]

Your code is incorrect. You call `w.Header().Set` after you already called `w.Write` (implicitly, in the `io.Copy`). You can't change headers after you already started writing the body.

My personal recommendation is to a) only write the body at the very end and b) ignore any errors returned from w.Write - it's too late to do anything about them (the headers are already written) and it's not even that unusual, as a client might close the connection in the middle of you writing the body.

If reading from `src` (in your code) could create an error (for example, it does some encoding which could fail, or you're reading from a file), copy it to a *bytes.Buffer first - that way, you can first check that the encoding worked and then do the io.Copy(w, buf) without an error check. Reading from the buffer can't fail, and errors from writing to the ResponseWriter you can ignore. Or, if you don't want to buffer the result, live with the fact that the client might get a partial response.

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[RS]

w.Header().Set("Content-Type", "application/xml")

w.WriteHeader(300)

these lines above before io.Copy are correct? 

_, err = io.Copy(w, res.Body)

[RS]

You call `w.Header().Set` after you already called `w.Write` (implicitly, in the `io.Copy`). You can't change headers after you already started writing the body.

-> You mean the w.header().Set in err part?

axel.wa...@googlemail.com schrieb am Mittwoch, 6. Oktober 2021 um 15:23:31 UTC+2: