context: eliminate arbitrary timeouts in examples

ExampleWithDeadline and ExampleWithTimeout used an arbitrary 1-second
timeout for a “blocked” select case, which could fail if the test
goroutine happens to be descheduled for over a second, or perhaps if
an NTP synchronization happens to jump by a second at just the right
time.

Either case is plausible, especially on a heavily-loaded or slow
machine (as is often the case for builders for unusual ports).

Instead of an arbitrary timeout, use a “ready” channel that is never
actually ready.

Fixes #57594.

Change-Id: I9ff68f50b041a3382e7b267c28c5259e886a9d23
Reviewed-on: https://go-review.googlesource.com/c/go/+/460999
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Sameer Ajmani <sameer@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>

Author: Bryan C. Mills

src/context/example_test.go

@@ -12,6 +12,8 @@ import (
 
 const shortDuration = 1 * time.Millisecond // a reasonable duration to block in an example
 
+var neverReady = make(chan struct{}) // never closed
+
 // 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.
@@ -66,8 +68,8 @@ func ExampleWithDeadline() {
 	defer cancel()
 
 	select {
-	case <-time.After(1 * time.Second):
-		fmt.Println("overslept")
+	case <-neverReady:
+		fmt.Println("ready")
 	case <-ctx.Done():
 		fmt.Println(ctx.Err())
 	}
@@ -85,8 +87,8 @@ func ExampleWithTimeout() {
 	defer cancel()
 
 	select {
-	case <-time.After(1 * time.Second):
-		fmt.Println("overslept")
+	case <-neverReady:
+		fmt.Println("ready")
 	case <-ctx.Done():
 		fmt.Println(ctx.Err()) // prints "context deadline exceeded"
 	}