interp: fix data race for composite literal creation

This change fixes two data races related to composite literal creation.

The first one isn't controversial as it is just about initializing the
variable that contains the values in the right place, i.e. within the
n.exec, so that this variable is local to each potential goroutine,
instead of being racily shared by all goroutines.

The second one is more worrying, i.e. having to protect the node typ
with a mutex, because a call to func (t *itype) refType actually
modifies the itype itself, which means it is not concurrent safe.
The change seems to work, and does not seem to introduce regression, but
it is still a concern as it probably is a sign that more similar
guarding has to be done in several other places.

interp/run.go

@@ -7,6 +7,7 @@ import (
 	"go/constant"
 	"log"
 	"reflect"
+	"sync"
 	"unsafe"
 )
 
@@ -2069,6 +2070,8 @@ func doCompositeLit(n *node, hasType bool) {
 	if typ.cat == ptrT || typ.cat == aliasT {
 		typ = typ.val
 	}
+	var mu sync.Mutex
+	typ.mu = &mu
 	child := n.child
 	if hasType {
 		child = n.child[1:]
@@ -2095,7 +2098,12 @@ func doCompositeLit(n *node, hasType bool) {
 	i := n.findex
 	l := n.level
 	n.exec = func(f *frame) bltn {
+		// TODO: it seems fishy that the typ might be modified post-compilation, and
+		// hence that several goroutines might be using the same typ that they all modify.
+		// We probably need to revisit that.
+		typ.mu.Lock()
 		a := reflect.New(typ.TypeOf()).Elem()
+		typ.mu.Unlock()
 		for i, v := range values {
 			a.Field(i).Set(v(f))
 		}
@@ -2122,6 +2130,8 @@ func doCompositeSparse(n *node, hasType bool) {
 	if typ.cat == ptrT || typ.cat == aliasT {
 		typ = typ.val
 	}
+	var mu sync.Mutex
+	typ.mu = &mu
 	child := n.child
 	if hasType {
 		child = n.child[1:]
@@ -2129,7 +2139,6 @@ func doCompositeSparse(n *node, hasType bool) {
 	destInterface := destType(n).cat == interfaceT
 
 	values := make(map[int]func(*frame) reflect.Value)
-	a, _ := typ.zero()
 	for _, c := range child {
 		c1 := c.child[1]
 		field := typ.fieldIndex(c.child[0].ident)
@@ -2149,6 +2158,9 @@ func doCompositeSparse(n *node, hasType bool) {
 	}
 
 	n.exec = func(f *frame) bltn {
+		typ.mu.Lock()
+		a, _ := typ.zero()
+		typ.mu.Unlock()
 		for i, v := range values {
 			a.Field(i).Set(v(f))
 		}