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>

1 files changed, 4 insertions, 2 deletions

diff --git a/src/context/example_test.go b/src/context/example_test.go
index b91a8acef3..72ac5d2e49 100644
--- a/src/context/example_test.go
+++ b/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 {