net: support TCP half-close when HTTP is upgraded in ReverseProxy

[sngchlko]

In order to support TCP half-close in ReverseProxy,
ReverseProxy.handleUpgradeResponse should not close all sockets
immediately when the one side gets an EOF. But we should close the
write stream on the socket to inform the transfer is complete.

Fixes #35892

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Message from Emmanuel Odeke:

Patch Set 1: Run-TryBot+1

(7 comments)

Thank you for this CL Seungcheol!

I have added some comments in my review, please take a look, but it would be
great to explicitly call out that we are adding an interface to net.

R=bradfitz

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[savely-krasovsky]

@sngchlko I know that it is another issue, but did you notice that after hijacking
http.ResponseWriter we cannot use it anymore?

From http/server.go docs:

	// Hijack lets the caller take over the connection.
	// After a call to Hijack the HTTP server library
	// will not do anything else with the connection.
	//
	// It becomes the caller's responsibility to manage
	// and close the connection.

But it's still used to write errors with p.getErrorHandler()(rw, req, ...). I guess we should add getHijackedErrorHandler() or kinda. It should create manually *http.Response and write it directly into hijacked connection.

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