context: fix a flaky timeout in TestLayersTimeout

In CL 223019, I reduced the short timeout in the testLayers helper to
be even shorter than it was. That exposed a racy (time-dependent)
select later in the function, which failed in one of the slower
builders (android-386-emu).

Also streamline the test to make it easier to test with a very high -count flag:
- Run tests that sleep for shortDuration in parallel to reduce latency.
- Use shorter durations in examples to reduce test running time.
- Avoid mutating global state (in package math/rand) in testLayers.

After this change (but not before it),
'go test -run=TestLayersTimeout -count=100000 context' passes on my workstation.

Fixes #38161

Change-Id: Iaf4abe7ac308b2100d8828267cda9f4f8ae4be82
Reviewed-on: https://go-review.googlesource.com/c/go/+/226457
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/context/context_test.go

@@ -27,6 +27,7 @@ type testingT interface {
 	Log(args ...interface{})
 	Logf(format string, args ...interface{})
 	Name() string
+	Parallel()
 	Skip(args ...interface{})
 	SkipNow()
 	Skipf(format string, args ...interface{})
@@ -284,6 +285,8 @@ func testDeadline(c Context, name string, t testingT) {
 }
 
 func XTestDeadline(t testingT) {
+	t.Parallel()
+
 	c, _ := WithDeadline(Background(), time.Now().Add(shortDuration))
 	if got, prefix := fmt.Sprint(c), "context.Background.WithDeadline("; !strings.HasPrefix(got, prefix) {
 		t.Errorf("c.String() = %q want prefix %q", got, prefix)
@@ -307,6 +310,8 @@ func XTestDeadline(t testingT) {
 }
 
 func XTestTimeout(t testingT) {
+	t.Parallel()
+
 	c, _ := WithTimeout(Background(), shortDuration)
 	if got, prefix := fmt.Sprint(c), "context.Background.WithDeadline("; !strings.HasPrefix(got, prefix) {
 		t.Errorf("c.String() = %q want prefix %q", got, prefix)
@@ -417,9 +422,9 @@ func XTestAllocs(t testingT, testingShort func() bool, testingAllocsPerRun func(
 			gccgoLimit: 3,
 		},
 		{
-			desc: "WithTimeout(bg, 15*time.Millisecond)",
+			desc: "WithTimeout(bg, 1*time.Nanosecond)",
 			f: func() {
-				c, _ := WithTimeout(bg, 15*time.Millisecond)
+				c, _ := WithTimeout(bg, 1*time.Nanosecond)
 				<-c.Done()
 			},
 			limit:      12,
@@ -545,7 +550,9 @@ func XTestLayersTimeout(t testingT) {
 }
 
 func testLayers(t testingT, seed int64, testTimeout bool) {
-	rand.Seed(seed)
+	t.Parallel()
+
+	r := rand.New(rand.NewSource(seed))
 	errorf := func(format string, a ...interface{}) {
 		t.Errorf(fmt.Sprintf("seed=%d: %s", seed, format), a...)
 	}
@@ -560,7 +567,7 @@ func testLayers(t testingT, seed int64, testTimeout bool) {
 		ctx       = Background()
 	)
 	for i := 0; i < minLayers || numTimers == 0 || len(cancels) == 0 || len(vals) == 0; i++ {
-		switch rand.Intn(3) {
+		switch r.Intn(3) {
 		case 0:
 			v := new(value)
 			ctx = WithValue(ctx, v, v)
@@ -587,11 +594,13 @@ func testLayers(t testingT, seed int64, testTimeout bool) {
 			}
 		}
 	}
+	if !testTimeout {
 		select {
 		case <-ctx.Done():
 			errorf("ctx should not be canceled yet")
 		default:
 		}
+	}
 	if s, prefix := fmt.Sprint(ctx), "context.Background."; !strings.HasPrefix(s, prefix) {
 		t.Errorf("ctx.String() = %q want prefix %q", s, prefix)
 	}
@@ -608,7 +617,7 @@ func testLayers(t testingT, seed int64, testTimeout bool) {
 		}
 		checkValues("after timeout")
 	} else {
-		cancel := cancels[rand.Intn(len(cancels))]
+		cancel := cancels[r.Intn(len(cancels))]
 		cancel()
 		select {
 		case <-ctx.Done():

src/context/example_test.go

@@ -10,6 +10,8 @@ import (
 	"time"
 )
 
+const shortDuration = 1 * time.Millisecond // a reasonable duration to block in an example
+
 // This example demonstrates the use of a cancelable context to prevent a
 // goroutine leak. By the end of the example function, the goroutine started
 // by gen will return without leaking.
@@ -55,7 +57,7 @@ func ExampleWithCancel() {
 // This example passes a context with an arbitrary deadline to tell a blocking
 // function that it should abandon its work as soon as it gets to it.
 func ExampleWithDeadline() {
-	d := time.Now().Add(50 * time.Millisecond)
+	d := time.Now().Add(shortDuration)
 	ctx, cancel := context.WithDeadline(context.Background(), d)
 
 	// Even though ctx will be expired, it is good practice to call its
@@ -79,7 +81,7 @@ func ExampleWithDeadline() {
 func ExampleWithTimeout() {
 	// Pass a context with a timeout to tell a blocking function that it
 	// should abandon its work after the timeout elapses.
-	ctx, cancel := context.WithTimeout(context.Background(), 50*time.Millisecond)
+	ctx, cancel := context.WithTimeout(context.Background(), shortDuration)
 	defer cancel()
 
 	select {