net/http: add MaxBytesHandler

[earthboundkid]

Fixes #39567

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Message from Damien Neil:

Patch Set 1: Run-TryBot+1 Trust+1

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Patch Set 2: Patch Set 1 was rebased

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[inliquid]

Sorry, may I ask you what's the purpose of copying request here?
I already use my own implementation in middleware like this, for a while already, and didn't get into any issues so far:

func maxBodyMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		r.Body = http.MaxBytesReader(w, r.Body, int64(cfg.MaxBodySize))

		next.ServeHTTP(w, r)
	})
}

so would like to know if there are some edge cases which I missed.

[earthboundkid]

From the docs for http.Handler,

Except for reading the body, handlers should not modify the provided Request.

You will often be fine modifying a request, but you aren’t supposed to do it because it might be a race condition if some other middleware happens to read the request at the same time.

[inliquid]

I see, thank you!