net: add more timing slop for TestDialParallel on Windows

Fixes #35616

Change-Id: I51b2490100cfe0e902da09eee8d027e0ec86ed53
Reviewed-on: https://go-review.googlesource.com/c/go/+/207466
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/dial_test.go

@@ -155,7 +155,7 @@ func slowDialTCP(ctx context.Context, network string, laddr, raddr *TCPAddr) (*T
 	return c, err
 }
 
-func dialClosedPort() (actual, expected time.Duration) {
+func dialClosedPort(t *testing.T) (actual, expected time.Duration) {
 	// Estimate the expected time for this platform.
 	// On Windows, dialing a closed port takes roughly 1 second,
 	// but other platforms should be instantaneous.
@@ -169,6 +169,7 @@ func dialClosedPort() (actual, expected time.Duration) {
 
 	l, err := Listen("tcp", "127.0.0.1:0")
 	if err != nil {
+		t.Logf("dialClosedPort: Listen failed: %v", err)
 		return 999 * time.Hour, expected
 	}
 	addr := l.Addr().String()
@@ -184,6 +185,7 @@ func dialClosedPort() (actual, expected time.Duration) {
 		}
 		elapsed := time.Now().Sub(startTime)
 		if i == 2 {
+			t.Logf("dialClosedPort: measured delay %v", elapsed)
 			return elapsed, expected
 		}
 	}
@@ -196,7 +198,7 @@ func TestDialParallel(t *testing.T) {
 		t.Skip("both IPv4 and IPv6 are required")
 	}
 
-	closedPortDelay, expectClosedPortDelay := dialClosedPort()
+	closedPortDelay, expectClosedPortDelay := dialClosedPort(t)
 	if closedPortDelay > expectClosedPortDelay {
 		t.Errorf("got %v; want <= %v", closedPortDelay, expectClosedPortDelay)
 	}
@@ -317,8 +319,14 @@ func TestDialParallel(t *testing.T) {
 			t.Errorf("#%d: got nil; want non-nil", i)
 		}
 
-		expectElapsedMin := tt.expectElapsed - 95*time.Millisecond
-		expectElapsedMax := tt.expectElapsed + 95*time.Millisecond
+		// We used to always use 95 milliseconds as the slop,
+		// but that was flaky on Windows.  See issue 35616.
+		slop := 95 * time.Millisecond
+		if fifth := tt.expectElapsed / 5; fifth > slop {
+			slop = fifth
+		}
+		expectElapsedMin := tt.expectElapsed - slop
+		expectElapsedMax := tt.expectElapsed + slop
 		if elapsed < expectElapsedMin {
 			t.Errorf("#%d: got %v; want >= %v", i, elapsed, expectElapsedMin)
 		} else if elapsed > expectElapsedMax {
@@ -667,7 +675,7 @@ func TestDialerDualStack(t *testing.T) {
 		t.Skip("both IPv4 and IPv6 are required")
 	}
 
-	closedPortDelay, expectClosedPortDelay := dialClosedPort()
+	closedPortDelay, expectClosedPortDelay := dialClosedPort(t)
 	if closedPortDelay > expectClosedPortDelay {
 		t.Errorf("got %v; want <= %v", closedPortDelay, expectClosedPortDelay)
 	}