net/http: improve errors in TestCancelRequestWhenSharingConnection

Provide more information about why this test might be hanging waiting
for PutIdleConn to be called (#56587): If the round trip that should
result in PutIdleConn being invoked completes, report that to the
goroutine waiting for PutIdleConn.

For #56587

Change-Id: Ie476ea0ce4a48d2bda6b9b109f89d675a10e7e45
Reviewed-on: https://go-review.googlesource.com/c/go/+/457775
Auto-Submit: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/net/http/transport_test.go

@@ -6565,7 +6565,8 @@ func testCancelRequestWhenSharingConnection(t *testing.T, mode testMode) {
 	var wg sync.WaitGroup
 
 	wg.Add(1)
-	putidlec := make(chan chan struct{})
+	putidlec := make(chan chan struct{}, 1)
+	reqerrc := make(chan error, 1)
 	go func() {
 		defer wg.Done()
 		ctx := httptrace.WithClientTrace(context.Background(), &httptrace.ClientTrace{
@@ -6574,24 +6575,31 @@ func testCancelRequestWhenSharingConnection(t *testing.T, mode testMode) {
 				// and wait for the order to proceed.
 				ch := make(chan struct{})
 				putidlec <- ch
+				close(putidlec) // panic if PutIdleConn runs twice for some reason
 				<-ch
 			},
 		})
 		req, _ := NewRequestWithContext(ctx, "GET", ts.URL, nil)
 		res, err := client.Do(req)
+		reqerrc <- err
 		if err == nil {
 			res.Body.Close()
 		}
-		if err != nil {
-			t.Errorf("request 1: got err %v, want nil", err)
-		}
 	}()
 
 	// Wait for the first request to receive a response and return the
 	// connection to the idle pool.
 	r1c := <-reqc
 	close(r1c)
-	idlec := <-putidlec
+	var idlec chan struct{}
+	select {
+	case err := <-reqerrc:
+		if err != nil {
+			t.Fatalf("request 1: got err %v, want nil", err)
+		}
+		idlec = <-putidlec
+	case idlec = <-putidlec:
+	}
 
 	wg.Add(1)
 	cancelctx, cancel := context.WithCancel(context.Background())