net/http: deflake TestClientRedirectContext

The test was checking for 1 of 2 possible error values. But based on
goroutine scheduling and the randomness of select statement receive
cases, it was possible for a 3rd type of error to be returned.

This modifies the code (not the test) to make that third type of error
actually the second type of error, which is a nicer error message.

The test is no longer flaky. The flake was very reproducible with a
5ms sleep before the select at the end of Transport.getConn.

Thanks to Github user @jaredborner for debugging.

Fixes #16049

Change-Id: I0d2a036c9555a8d2618b07bab01f28558d2b0b2c
Reviewed-on: https://go-review.googlesource.com/24748Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/transport.go

@@ -845,10 +845,26 @@ func (t *Transport) getConn(treq *transportRequest, cm connectMethod) (*persistC
 	select {
 	case v := <-dialc:
 		// Our dial finished.
-		if trace != nil && trace.GotConn != nil && v.pc != nil && v.pc.alt == nil {
-			trace.GotConn(httptrace.GotConnInfo{Conn: v.pc.conn})
+		if v.pc != nil {
+			if trace != nil && trace.GotConn != nil && v.pc.alt == nil {
+				trace.GotConn(httptrace.GotConnInfo{Conn: v.pc.conn})
+			}
+			return v.pc, nil
 		}
-		return v.pc, v.err
+		// Our dial failed. See why to return a nicer error
+		// value.
+		select {
+		case <-req.Cancel:
+		case <-req.Context().Done():
+		case <-cancelc:
+		default:
+			// It wasn't an error due to cancelation, so
+			// return the original error message:
+			return nil, v.err
+		}
+		// It was an error due to cancelation, so prioritize that
+		// error value. (Issue 16049)
+		return nil, errRequestCanceledConn
 	case pc := <-idleConnCh:
 		// Another request finished first and its net.Conn
 		// became available before our dial. Or somebody