context: prevent creation of invalid contexts

This commit makes it impossible to create derived contexts with nil parents.
Previously it was possible to create derived contexts with nil parents, and
invalid contexts could propogate through the program. Eventually this can
cause a panic downstream, which is difficult to trace back to the source
of the error.

Although `WithCancel` and `WithDeadline` already panic if `parent` is `nil`, this adds explicit checks to give a useful message in the panic.

Fixes #37908

Change-Id: I70fd01f6539c1b0da0e775fc5457e32e7075e52c
GitHub-Last-Rev: 1b7dadd7db
GitHub-Pull-Request: golang/go#37898
Reviewed-on: https://go-review.googlesource.com/c/go/+/223777
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/context/context.go

@@ -230,6 +230,9 @@ type CancelFunc func()
 // Canceling this context releases resources associated with it, so code should
 // call cancel as soon as the operations running in this Context complete.
 func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
+	if parent == nil {
+		panic("cannot create context from nil parent")
+	}
 	c := newCancelCtx(parent)
 	propagateCancel(parent, &c)
 	return &c, func() { c.cancel(true, Canceled) }
@@ -425,6 +428,9 @@ func (c *cancelCtx) cancel(removeFromParent bool, err 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) {
+	if parent == nil {
+		panic("cannot create context from nil parent")
+	}
 	if cur, ok := parent.Deadline(); ok && cur.Before(d) {
 		// The current deadline is already sooner than the new one.
 		return WithCancel(parent)
@@ -511,6 +517,9 @@ func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {
 // struct{}. Alternatively, exported context key variables' static
 // type should be a pointer or interface.
 func WithValue(parent Context, key, val interface{}) Context {
+	if parent == nil {
+		panic("cannot create context from nil parent")
+	}
 	if key == nil {
 		panic("nil key")
 	}

src/context/context_test.go

@@ -667,6 +667,21 @@ func XTestWithValueChecksKey(t testingT) {
 	}
 }
 
+func XTestInvalidDerivedFail(t testingT) {
+	panicVal := recoveredValue(func() { WithCancel(nil) })
+	if panicVal == nil {
+		t.Error("expected panic")
+	}
+	panicVal = recoveredValue(func() { WithDeadline(nil, time.Now().Add(shortDuration)) })
+	if panicVal == nil {
+		t.Error("expected panic")
+	}
+	panicVal = recoveredValue(func() { WithValue(nil, "foo", "bar") })
+	if panicVal == nil {
+		t.Error("expected panic")
+	}
+}
+
 func recoveredValue(fn func()) (v interface{}) {
 	defer func() { v = recover() }()
 	fn()

src/context/x_test.go

@@ -26,5 +26,6 @@ func TestLayersTimeout(t *testing.T)                   { XTestLayersTimeout(t) }
 func TestCancelRemoves(t *testing.T)                   { XTestCancelRemoves(t) }
 func TestWithCancelCanceledParent(t *testing.T)        { XTestWithCancelCanceledParent(t) }
 func TestWithValueChecksKey(t *testing.T)              { XTestWithValueChecksKey(t) }
+func TestInvalidDerivedFail(t *testing.T)              { XTestInvalidDerivedFail(t) }
 func TestDeadlineExceededSupportsTimeout(t *testing.T) { XTestDeadlineExceededSupportsTimeout(t) }
 func TestCustomContextGoroutines(t *testing.T)         { XTestCustomContextGoroutines(t) }