HTTP request is aborted after response write in Golang - file

Question

I have a server in Go that handles the files upload. It is a legacy code so I can't touch it so much.
The server should interrupt the upload if it detects some errors in the request header and it should return a message to the client that something is gone wrong.
The handler function is something like the following:
func(w http.ResponseWriter, r *http.Request) {
// Check the header. Could be more than one.
if r.Header.Get("key") == "not as expected" {
w.WriteHeader(500)
w.Write("key is wrong")
return
}
//handle the file upload
}
The header check is only an example to show the problem
The server closes the connection after Write and return from the function even if the request is not completed (file received).
On the client-side (Java) when I make a request with the key with a wrong value and the file to upload as a body, I get a broken pipe exception and It can't handle the response correctly.
Actually I can't touch the client-side code.
There is way on server side to wait until the request ends before closing the connection?

Answer 1

The "broken pipe" error¹ seen on the Java client suggests the client insists on sending the payload (body) of its request before attempting to read the response from the server.
In HTTP/1.1 (and 1.0), the client is correct: nothing in the spec says the client has to expect the server to respond before the whole request — that is, the header and the body, if any, — gets submitted.
In your particular case, the simplest approach is to pipe the clien's body to nowhere and after that respond with an error. One idiomatic approach is using io/ioutil.Discard type:
func(w http.ResponseWriter, r *http.Request) {
// Check the checksum header
if r.Header.Get("key") == "not as expected" {
_, err := io.Copy(ioutil.Discard, r.Body)
if err != nil {
// The client went away.
// May be log something, then bail out.
panic(http.ErrAbortHandler)
}
w.WriteHeader(500)
w.Write("key is wrong")
return
}
//handle the file upload
}
net/http.ErrAbortHandler may be used to tell the HTTP server library code the request should not be carried out the normal way.
By the way, responding with 5xx to an ill-formed client request is incorrect, you should have been using 4xx instead. But it's a legacy code, so just take this as a hint for future developments.
¹ See EPIPE in the send(2) manual page.