context: add lock in Cause to avoid race

Change-Id: I8d970e8db859bdd17390cfbc22cc2ba0d326ed0c
Reviewed-on: https://go-review.googlesource.com/c/go/+/453735
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Sameer Ajmani <sameer@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

src/context/context.go

@@ -285,6 +285,8 @@ func withCancel(parent Context) *cancelCtx {
 // Cause returns nil if c has not been canceled yet.
 func Cause(c Context) error {
 	if cc, ok := c.Value(&cancelCtxKey).(*cancelCtx); ok {
+		cc.mu.Lock()
+		defer cc.mu.Unlock()
 		return cc.cause
 	}
 	return nil

src/context/context_test.go

@@ -5,6 +5,7 @@
 package context
 
 import (
+	"errors"
 	"fmt"
 	"math/rand"
 	"runtime"
@@ -934,3 +935,22 @@ func XTestCause(t testingT) {
 		}
 	}
 }
+
+func XTestCauseRace(t testingT) {
+	cause := errors.New("TestCauseRace")
+	ctx, cancel := WithCancelCause(Background())
+	go func() {
+		cancel(cause)
+	}()
+	for {
+		// Poll Cause, rather than waiting for Done, to test that
+		// access to the underlying cause is synchronized properly.
+		if err := Cause(ctx); err != nil {
+			if err != cause {
+				t.Errorf("Cause returned %v, want %v", err, cause)
+			}
+			break
+		}
+		runtime.Gosched()
+	}
+}

src/context/x_test.go

@@ -30,3 +30,4 @@ func TestInvalidDerivedFail(t *testing.T)              { XTestInvalidDerivedFail
 func TestDeadlineExceededSupportsTimeout(t *testing.T) { XTestDeadlineExceededSupportsTimeout(t) }
 func TestCustomContextGoroutines(t *testing.T)         { XTestCustomContextGoroutines(t) }
 func TestCause(t *testing.T)                           { XTestCause(t) }
+func TestCauseRace(t *testing.T)                       { XTestCauseRace(t) }