The PROXY protocol is supported by several proxy servers such as haproxy
and Amazon ELB.  This protocol allows services running behind a proxy to
learn the remote address of the actual client connecting to the proxy,
by including a single textual line at the beginning of the TCP
connection.
http://www.haproxy.org/download/1.5/doc/proxy-protocol.txt

There are several Go libraries for this protocol (such as
https://github.com/armon/go-proxyproto), which operate by wrapping a
net.Conn with an implementation whose RemoteAddr method reads the
protocol line before returning. This means that RemoteAddr is a blocking
call.

Before this change, http.Serve called RemoteAddr from the main Accepting
goroutine, not from the per-connection goroutine. This meant that it
would not Accept another connection until RemoteAddr returned, which is
not appropriate if RemoteAddr needs to do a blocking read from the
socket first.

Fixes #12943.

Change-Id: I1a242169e6e4aafd118b794e7c8ac45d0d573421
Reviewed-on: https://go-review.googlesource.com/15835Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>