net/http: quiet useless warning during shutdown

What was happening on Issue 7010 was handler intentionally took 30
milliseconds and the proxy's client timeout was 35 milliseconds. Then it
slammed the proxy with a bunch of requests.

Sometimes the server would be too slow to respond in its 5 millisecond
window and the client code would cancel the request, force-closing the
persistConn.  If this came at the right time, the server's reply was
already in flight, and one of the goroutines would report:

Unsolicited response received on idle HTTP channel starting with "H"; err=<nil>

... rightfully scaring the user.

But the error was already handled and returned to the user, and this
connection knows it's been shut down. So look at the closed flag after
acquiring the same mutex guarding another field we were checking, and
don't complain if it's a known shutdown.

Also move closed down below the mutex which guards it.

Fixes #7010

LGTM=dsymonds
R=golang-codereviews, dsymonds
CC=adg, golang-codereviews, rsc
https://golang.org/cl/86740044

src/pkg/net/http/transport.go

@@ -719,7 +719,6 @@ type persistConn struct {
 	cacheKey connectMethodKey
 	conn     net.Conn
 	tlsState *tls.ConnectionState
-	closed   bool                // whether conn has been closed
 	br       *bufio.Reader       // from conn
 	sawEOF   bool                // whether we've seen EOF from conn; owned by readLoop
 	bw       *bufio.Writer       // to conn
@@ -733,8 +732,9 @@ type persistConn struct {
 	// whether or not a connection can be reused. Issue 7569.
 	writeErrCh chan error
 
-	lk                   sync.Mutex // guards following 3 fields
+	lk                   sync.Mutex // guards following fields
 	numExpectedResponses int
+	closed               bool // whether conn has been closed
 	broken               bool // an error has happened on this connection; marked broken so it's not reused.
 	// mutateHeaderFunc is an optional func to modify extra
 	// headers on each outbound request before it's written. (the
@@ -774,12 +774,14 @@ func (pc *persistConn) readLoop() {
 
 		pc.lk.Lock()
 		if pc.numExpectedResponses == 0 {
+			if !pc.closed {
 				pc.closeLocked()
-			pc.lk.Unlock()
 				if len(pb) > 0 {
 					log.Printf("Unsolicited response received on idle HTTP channel starting with %q; err=%v",
 						string(pb), err)
 				}
+			}
+			pc.lk.Unlock()
 			return
 		}
 		pc.lk.Unlock()