gc: disallow close on receive-only channels

Fixes #2353.
Fixes #2246.

R=golang-dev, r, gri
CC=golang-dev
https://golang.org/cl/5282042

doc/go_spec.html

@@ -4531,12 +4531,13 @@ BuiltinArgs = Type [ "," ExpressionList ] | ExpressionList .
 
 <p>
 For a channel <code>c</code>, the built-in function <code>close(c)</code>
-marks the channel as unable to accept more values through a send operation;
-sending to or closing a closed channel causes a <a href="#Run_time_panics">run-time panic</a>.
+records that no more values will be sent on the channel.
+It is an error if <code>c</code> is a receive-only channel.
+Sending to or closing a closed channel causes a <a href="#Run_time_panics">run-time panic</a>.
+Closing the nil channel also causes a <a href="#Run_time_panics">run-time panic</a>.
 After calling <code>close</code>, and after any previously
 sent values have been received, receive operations will return
 the zero value for the channel's type without blocking.
-
 The multi-valued <a href="#Receive_operator">receive operation</a>
 returns a received value along with an indication of whether the channel is closed.
 </p>

src/cmd/gc/typecheck.c

@@ -984,6 +984,10 @@ reswitch:
 			yyerror("invalid operation: %#N (non-chan type %T)", n, t);
 			goto error;
 		}
+		if(!(t->chan & Csend)) {
+			yyerror("invalid operation: %#N (cannot close receive-only channel)", n);
+			goto error;
+		}
 		ok |= Etop;
 		goto ret;
 

src/pkg/runtime/chan.c

@@ -1052,6 +1052,9 @@ runtime·closechan(Hchan *c)
 	SudoG *sg;
 	G* gp;
 
+	if(c == nil)
+		runtime·panicstring("close of nil channel");
+
 	if(runtime·gcwaiting)
 		runtime·gosched();
 

test/chan/perm.go

@@ -48,4 +48,8 @@ func main() {
 	case x := <-cs: // ERROR "receive"
 		_ = x
 	}
+	
+	close(c)
+	close(cs)
+	close(cr)  // ERROR "receive"
 }

test/closedchan.go

@@ -327,4 +327,15 @@ func main() {
 			testclosed(mk(closedasync()))
 		}
 	}
+	
+	var ch chan int	
+	shouldPanic(func() {
+		close(ch)
+	})
+	
+	ch = make(chan int)
+	close(ch)
+	shouldPanic(func() {
+		close(ch)
+	})
 }