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 propagate through the program.
Eventually this could cause a panic downstream, which is difficult to trace
back to the source of the error.

Author: knusbaum

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 TestInvalidDerivedFail(t testingT) {
+	panicVal := recoveredValue(func() { WithCancel(nil) })
+	if panicVal == nil {
+		t.Error("expected panic")
+	}
+	panicVal := recoveredValue(func() { WithDeadline(nil, time.Second) })
+	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()