net/http: describe where multiple response.WriteHeader calls occur

[dsnet]

I see this in an HTTP server log:

server.go:2315: http: multiple response.WriteHeader calls

Written by the http package here.

It would be nice if it described file+line of the first non-http package function that caused WriteHeader to be called so that it's easier to find the offending code.

\cc @bradfitz

[siadat]

I can work on this if it is okay.

Regarding the implementation, would it be okay to use runtime.Callers here, and find the first function whose name does not start with "net/http."?

[bradfitz]

@siadat, sounds like it could work.

As long as you only do that work when we're already spamming logs, and not for all WriteHeader calls.

[gopherbot]

CL https://golang.org/cl/36096 mentions this issue.

[odeke-em]

Hello there @siadat, Go1.11 is active right now and @danp pinged you on the CL, please let us know if you'll be carrying on with the CL, if you are busy, no worries, we can work on it.

[siadat]

Hi @odeke-em, thanks for the reminder, I will have a look at it!

[gopherbot]

Change https://golang.org/cl/130997 mentions this issue: net/http: print caller which causes multiple header writes

[nightlyone]

I debugged quite a few of these in PHP in a previous life nearly a decade ago.

From that experience I would like to mention that you usually don't know whether the first or the second one is the caller that should have been written the header. So I would suggest reporting both: The first writer and the second one.

[bradfitz]

@nightlyone, that means making all users pay the cost of recording that first write's stack frame, even though the majority will only ever have one write.