fix: add mutual exclusion locks for cancelable select

In a couple of occurences, tests with enabled race detector exposed
some concurrent accesses to the cancelation callback used in select and
channel operations send and recv for EvalWithContext. This change ensure
that all accesses to this object are protected by mutex.

Fixes #815.

interp/run.go

@@ -94,9 +94,13 @@ func (interp *Interpreter) run(n *node, cf *frame) {
 		f = newFrame(cf, len(n.types), interp.runid())
 	}
 	interp.mutex.RLock()
-	f.done = reflect.SelectCase{Dir: reflect.SelectRecv, Chan: reflect.ValueOf(interp.done)}
+	c := reflect.ValueOf(interp.done)
 	interp.mutex.RUnlock()
 
+	f.mutex.Lock()
+	f.done = reflect.SelectCase{Dir: reflect.SelectRecv, Chan: c}
+	f.mutex.Unlock()
+
 	for i, t := range n.types {
 		f.data[i] = reflect.New(t).Elem()
 	}
@@ -2216,7 +2220,11 @@ func rangeChan(n *node) {
 	tnext := getExec(n.tnext)
 
 	n.exec = func(f *frame) bltn {
-		chosen, v, ok := reflect.Select([]reflect.SelectCase{f.done, {Dir: reflect.SelectRecv, Chan: value(f)}})
+		f.mutex.RLock()
+		done := f.done
+		f.mutex.RUnlock()
+
+		chosen, v, ok := reflect.Select([]reflect.SelectCase{done, {Dir: reflect.SelectRecv, Chan: value(f)}})
 		if chosen == 0 {
 			return nil
 		}
@@ -2701,7 +2709,11 @@ func recv(n *node) {
 					return fnext
 				}
 				// Slow: channel read blocks, allow cancel
-				chosen, v, _ := reflect.Select([]reflect.SelectCase{f.done, {Dir: reflect.SelectRecv, Chan: ch}})
+				f.mutex.RLock()
+				done := f.done
+				f.mutex.RUnlock()
+
+				chosen, v, _ := reflect.Select([]reflect.SelectCase{done, {Dir: reflect.SelectRecv, Chan: ch}})
 				if chosen == 0 {
 					return nil
 				}
@@ -2719,8 +2731,12 @@ func recv(n *node) {
 					return tnext
 				}
 				// Slow: channel is blocked, allow cancel
+				f.mutex.RLock()
+				done := f.done
+				f.mutex.RUnlock()
+
 				var chosen int
-				chosen, getFrame(f, l).data[i], _ = reflect.Select([]reflect.SelectCase{f.done, {Dir: reflect.SelectRecv, Chan: ch}})
+				chosen, getFrame(f, l).data[i], _ = reflect.Select([]reflect.SelectCase{done, {Dir: reflect.SelectRecv, Chan: ch}})
 				if chosen == 0 {
 					return nil
 				}
@@ -2765,7 +2781,11 @@ func recv2(n *node) {
 				return tnext
 			}
 			// Slow: channel is blocked, allow cancel
-			chosen, v, ok := reflect.Select([]reflect.SelectCase{f.done, {Dir: reflect.SelectRecv, Chan: ch}})
+			f.mutex.RLock()
+			done := f.done
+			f.mutex.RUnlock()
+
+			chosen, v, ok := reflect.Select([]reflect.SelectCase{done, {Dir: reflect.SelectRecv, Chan: ch}})
 			if chosen == 0 {
 				return nil
 			}
@@ -2887,7 +2907,11 @@ func send(n *node) {
 				return next
 			}
 			// Slow: send on channel blocks, allow cancel
-			chosen, _, _ := reflect.Select([]reflect.SelectCase{f.done, {Dir: reflect.SelectSend, Chan: ch, Send: data}})
+			f.mutex.RLock()
+			done := f.done
+			f.mutex.RUnlock()
+
+			chosen, _, _ := reflect.Select([]reflect.SelectCase{done, {Dir: reflect.SelectSend, Chan: ch, Send: data}})
 			if chosen == 0 {
 				return nil
 			}
@@ -2979,7 +3003,10 @@ func _select(n *node) {
 	}
 
 	n.exec = func(f *frame) bltn {
+		f.mutex.RLock()
 		cases[nbClause] = f.done
+		f.mutex.RUnlock()
+
 		for i := range cases[:nbClause] {
 			switch cases[i].Dir {
 			case reflect.SelectRecv: