context: avoid defer in the cancelCtx.Err method

name                  old time/op  new time/op  delta
CheckCanceled/Err-4   53.5ns ± 2%  20.8ns ± 0%  -61.05%  (p=0.008 n=5+5)
CheckCanceled/Done-4  44.4ns ± 1%  44.5ns ± 0%     ~     (p=0.889 n=5+5)

Change-Id: I2c68700a2b33f8feb3d307ce7c966590a3e960af
Reviewed-on: https://go-review.googlesource.com/107137
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/context/benchmark_test.go

@@ -96,3 +96,21 @@ func buildContextTree(root Context, depth int) {
 		root, _ = WithCancel(root)
 	}
 }
+
+func BenchmarkCheckCanceled(b *testing.B) {
+	ctx, cancel := WithCancel(Background())
+	cancel()
+	b.Run("Err", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			ctx.Err()
+		}
+	})
+	b.Run("Done", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			select {
+			case <-ctx.Done():
+			default:
+			}
+		}
+	})
+}

src/context/context.go

@@ -334,8 +334,9 @@ func (c *cancelCtx) Done() <-chan struct{} {
 
 func (c *cancelCtx) Err() error {
 	c.mu.Lock()
-	defer c.mu.Unlock()
-	return c.err
+	err := c.err
+	c.mu.Unlock()
+	return err
 }
 
 func (c *cancelCtx) String() string {