context: add APIs for setting a cancelation cause when deadline or timer expires

Fixes #56661

Change-Id: I1c23ebc52e6b7ae6ee956614e1a0a45d6ecbd5b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/449318
Run-TryBot: Sameer Ajmani <sameer@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

api/next/56661.txt

@@ -0,0 +1,2 @@
+pkg context, func WithDeadlineCause(Context, time.Time, error) (Context, CancelFunc) #56661
+pkg context, func WithTimeoutCause(Context, time.Duration, error) (Context, CancelFunc) #56661

src/context/context.go

@@ -492,6 +492,13 @@ func (c *cancelCtx) cancel(removeFromParent bool, err, cause error) {
 // Canceling this context releases resources associated with it, so code should
 // call cancel as soon as the operations running in this Context complete.
 func WithDeadline(parent Context, d time.Time) (Context, CancelFunc) {
+	return WithDeadlineCause(parent, d, nil)
+}
+
+// WithDeadlineCause behaves like WithDeadline but also sets the cause of the
+// returned Context when the deadline is exceeded. The returned CancelFunc does
+// not set the cause.
+func WithDeadlineCause(parent Context, d time.Time, cause error) (Context, CancelFunc) {
 	if parent == nil {
 		panic("cannot create context from nil parent")
 	}
@@ -506,14 +513,14 @@ func WithDeadline(parent Context, d time.Time) (Context, CancelFunc) {
 	propagateCancel(parent, c)
 	dur := time.Until(d)
 	if dur <= 0 {
-		c.cancel(true, DeadlineExceeded, nil) // deadline has already passed
+		c.cancel(true, DeadlineExceeded, cause) // deadline has already passed
 		return c, func() { c.cancel(false, Canceled, nil) }
 	}
 	c.mu.Lock()
 	defer c.mu.Unlock()
 	if c.err == nil {
 		c.timer = time.AfterFunc(dur, func() {
-			c.cancel(true, DeadlineExceeded, nil)
+			c.cancel(true, DeadlineExceeded, cause)
 		})
 	}
 	return c, func() { c.cancel(true, Canceled, nil) }
@@ -567,6 +574,13 @@ func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {
 	return WithDeadline(parent, time.Now().Add(timeout))
 }
 
+// WithTimeoutCause behaves like WithTimeout but also sets the cause of the
+// returned Context when the timout expires. The returned CancelFunc does
+// not set the cause.
+func WithTimeoutCause(parent Context, timeout time.Duration, cause error) (Context, CancelFunc) {
+	return WithDeadlineCause(parent, time.Now().Add(timeout), cause)
+}
+
 // WithValue returns a copy of parent in which the value associated with key is
 // val.
 //

src/context/context_test.go

@@ -793,8 +793,11 @@ func XTestCustomContextGoroutines(t testingT) {
 
 func XTestCause(t testingT) {
 	var (
-		parentCause = fmt.Errorf("parentCause")
+		forever       = 1e6 * time.Second
-		childCause  = fmt.Errorf("childCause")
+		parentCause   = fmt.Errorf("parentCause")
+		childCause    = fmt.Errorf("childCause")
+		tooSlow       = fmt.Errorf("tooSlow")
+		finishedEarly = fmt.Errorf("finishedEarly")
 	)
 	for _, test := range []struct {
 		name  string
@@ -926,6 +929,58 @@ func XTestCause(t testingT) {
 			err:   DeadlineExceeded,
 			cause: DeadlineExceeded,
 		},
+		{
+			name: "WithTimeout canceled",
+			ctx: func() Context {
+				ctx, cancel := WithTimeout(Background(), forever)
+				cancel()
+				return ctx
+			}(),
+			err:   Canceled,
+			cause: Canceled,
+		},
+		{
+			name: "WithTimeoutCause",
+			ctx: func() Context {
+				ctx, cancel := WithTimeoutCause(Background(), 0, tooSlow)
+				cancel()
+				return ctx
+			}(),
+			err:   DeadlineExceeded,
+			cause: tooSlow,
+		},
+		{
+			name: "WithTimeoutCause canceled",
+			ctx: func() Context {
+				ctx, cancel := WithTimeoutCause(Background(), forever, tooSlow)
+				cancel()
+				return ctx
+			}(),
+			err:   Canceled,
+			cause: Canceled,
+		},
+		{
+			name: "WithTimeoutCause stacked",
+			ctx: func() Context {
+				ctx, cancel := WithCancelCause(Background())
+				ctx, _ = WithTimeoutCause(ctx, 0, tooSlow)
+				cancel(finishedEarly)
+				return ctx
+			}(),
+			err:   DeadlineExceeded,
+			cause: tooSlow,
+		},
+		{
+			name: "WithTimeoutCause stacked canceled",
+			ctx: func() Context {
+				ctx, cancel := WithCancelCause(Background())
+				ctx, _ = WithTimeoutCause(ctx, forever, tooSlow)
+				cancel(finishedEarly)
+				return ctx
+			}(),
+			err:   Canceled,
+			cause: finishedEarly,
+		},
 	} {
 		if got, want := test.ctx.Err(), test.err; want != got {
 			t.Errorf("%s: ctx.Err() = %v want %v", test.name, got, want)