singleflight: example for using Group

Fixes golang/go#60208

Change-Id: I422a45c0f139ba47045f47cec1c96d300a2a424f
Reviewed-on: https://go-review.googlesource.com/c/sync/+/496535
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

singleflight/singleflight_test.go

@@ -327,3 +327,33 @@ func TestPanicDoSharedByDoChan(t *testing.T) {
 		t.Errorf("Test subprocess failed, but the crash isn't caused by panicking in Do")
 	}
 }
+
+func ExampleGroup() {
+	g := new(Group)
+
+	block := make(chan struct{})
+	res1c := g.DoChan("key", func() (interface{}, error) {
+		<-block
+		return "func 1", nil
+	})
+	res2c := g.DoChan("key", func() (interface{}, error) {
+		<-block
+		return "func 2", nil
+	})
+	close(block)
+
+	res1 := <-res1c
+	res2 := <-res2c
+
+	// Results are shared by functions executed with duplicate keys.
+	fmt.Println("Shared:", res2.Shared)
+	// Only the first function is executed: it is registered and started with "key",
+	// and doesn't complete before the second funtion is registered with a duplicate key.
+	fmt.Println("Equal results:", res1.Val.(string) == res2.Val.(string))
+	fmt.Println("Result:", res1.Val)
+
+	// Output:
+	// Shared: true
+	// Equal results: true
+	// Result: func 1
+}