cmd/gc: fix race detector instrumentation of type switches.

A type switch on a value with map index expressions,
could get a spurious instrumentation from a OTYPESW node.
These nodes do not need instrumentation because after
walk the type switch has been turned into a sequence
of ifs.

Fixes #5890.

R=golang-dev, dvyukov
CC=golang-dev
https://golang.org/cl/11308043

src/cmd/gc/racewalk.c

@@ -323,10 +323,6 @@ racewalknode(Node **np, NodeList **init, int wr, int skip)
 		racewalknode(&n->left, init, 0, 0);
 		goto ret;
 
-	case OTYPESW:
-		racewalknode(&n->right, init, 0, 0);
-		goto ret;
-
 	// should not appear in AST by now
 	case OSEND:
 	case ORECV:
@@ -402,6 +398,7 @@ racewalknode(Node **np, NodeList **init, int wr, int skip)
 	case ONONAME:
 	case OLITERAL:
 	case OSLICESTR:  // always preceded by bounds checking, avoid double instrumentation.
+	case OTYPESW:    // ignored by code generation, do not instrument.
 		goto ret;
 	}
 

src/pkg/runtime/race/testdata/mop_test.go

@@ -262,6 +262,25 @@ func TestRaceCaseTypeBody(t *testing.T) {
 	<-c
 }
 
+func TestRaceCaseTypeIssue5890(t *testing.T) {
+	// spurious extra instrumentation of the initial interface
+	// value.
+	var x, y int
+	m := make(map[int]map[int]interface{})
+	m[0] = make(map[int]interface{})
+	c := make(chan int, 1)
+	go func() {
+		switch i := m[0][1].(type) {
+		case nil:
+		case *int:
+			*i = x
+		}
+		c <- 1
+	}()
+	m[0][1] = y
+	<-c
+}
+
 func TestNoRaceRange(t *testing.T) {
 	ch := make(chan int, 3)
 	a := [...]int{1, 2, 3}