runtime (chan): fix blocking select on a nil channel

Previously, a blocking select on a nil channel would result in a nil panic inside the channel runtime code.
This change fixes the nil checks so that the select works as intended.

src/runtime/chan.go

@@ -85,10 +85,12 @@ func (b *channelBlockedList) detach() {
 	}
 	for i, v := range b.allSelectOps {
 		// cancel all other channel operations that are part of this select statement
-		if &b.allSelectOps[i] == b {
+		switch {
+		case &b.allSelectOps[i] == b:
+			// This entry is the one that was already detatched.
 			continue
-		}
+		case v.t == nil:
-		if v.s.ch == nil {
+			// This entry is not used (nil channel).
 			continue
 		}
 		v.s.ch.blocked = v.s.ch.blocked.remove(&b.allSelectOps[i])
@@ -512,6 +514,13 @@ func chanSelect(recvbuf unsafe.Pointer, states []chanSelectState, ops []channelB
 
 	// construct blocked operations
 	for i, v := range states {
+		if v.ch == nil {
+			// A nil channel receive will never complete.
+			// A nil channel send would have panicked during tryChanSelect.
+			ops[i] = channelBlockedList{}
+			continue
+		}
+
 		ops[i] = channelBlockedList{
 			next:         v.ch.blocked,
 			t:            task.Current(),

testdata/channel.go

@@ -134,6 +134,8 @@ func main() {
 	select {
 	case make(chan int) <- 3:
 		println("unreachable")
+	case <-(chan int)(nil):
+		println("unreachable")
 	case n := <-ch:
 		println("select n from chan:", n)
 	case n := <-make(chan int):