net/http/pprof: return error when requested profile duration exceeds WriteTimeout

[vcabbage]

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

go version go1.8rc2 darwin/amd64

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As observed in #18746, if a profile duration >= to the server's WriteTimeout is requested, the request will always fail with an EOF once the WriteTimeout elapses. Handlers which accept a duration parameters should check the WriteTimeout and return an error before starting the profile.

(A handler can retrieve the server which started it from the request's context.)

[bradfitz]

I suppose we could've just reused that other bug, but this one is nice & succinct, so this works too.

/cc @ardan-bkennedy

[gopherbot]

CL https://golang.org/cl/35564 mentions this issue.

[ibrasho]

Will updating the handler to return a server error be enough?
The pprof tool will still need to be updated if we want it to print a clear error message. Otherwise, it'll just prints server response: 500. Is it safe to print the response body in addition to the HTTP status code?

[vcabbage]

@ibrasho I'm working on it in the CL linked above. It does modify go tool pprof to print the body under limited circumstances.