cmd/cgo: retain Go pointer passed to C call for duration of call

Fixes #6397.

R=golang-dev, bradfitz, iant
CC=golang-dev
https://golang.org/cl/13858043

src/cmd/cc/pgen.c

@@ -651,9 +651,10 @@ bcomplex(Node *n, Node *c)
 // Updates the bitvector with a set bit for each pointer containing
 // value in the type description starting at offset.
 static void
-walktype1(Type *t, int32 offset, Bvec *bv)
+walktype1(Type *t, int32 offset, Bvec *bv, int param)
 {
 	Type *t1;
+	int32 o;
 
 	switch(t->etype) {
 	case TCHAR:
@@ -672,21 +673,29 @@ walktype1(Type *t, int32 offset, Bvec *bv)
 		break;
 
 	case TIND:
-	case TARRAY: // unlike Go, C passes arrays by reference
+	pointer:
 		// pointer types
 		if((offset + t->offset) % ewidth[TIND] != 0)
 			yyerror("unaligned pointer");
 		bvset(bv, ((offset + t->offset) / ewidth[TIND])*BitsPerPointer);
 		break;
 
+	case TARRAY:
+		if(param)	// unlike Go, C passes arrays by reference
+			goto pointer;
+		// array in struct or union is an actual array
+		for(o = 0; o < t->width; o += t->link->width)
+			walktype1(t->link, offset+o, bv, 0);
+		break;
+
 	case TSTRUCT:
 		// build map recursively
 		for(t1 = t->link; t1 != T; t1 = t1->down)
-			walktype1(t1, offset, bv);
+			walktype1(t1, offset, bv, 0);
 		break;
 
 	case TUNION:
-		walktype1(t->link, offset, bv);
+		walktype1(t->link, offset, bv, 0);
 		break;
 
 	default:
@@ -728,7 +737,7 @@ dumpgcargs(Type *fn, Sym *sym)
 			if(t->etype == TVOID)
 				continue;
 			argoffset = align(argoffset, t, Aarg1, nil);
-			walktype1(t, argoffset, bv);
+			walktype1(t, argoffset, bv, 1);
 			argoffset = align(argoffset, t, Aarg2, nil);
 		}
 		gextern(sym, nodconst(bv->n), 0, 4);

src/cmd/cgo/out.go

@@ -413,7 +413,17 @@ func (p *Package) writeDefsFunc(fc, fgo2 *os.File, n *Name) {
 	if argSize == 0 {
 		argSize++
 	}
-	fmt.Fprintf(fc, "·%s(struct{uint8 x[%d];}p)\n", n.Mangle, argSize)
+	// TODO(rsc): The struct here should declare pointers only where
+	// there are pointers in the actual argument frame.
+	// This is a workaround for golang.org/issue/6397.
+	fmt.Fprintf(fc, "·%s(struct{", n.Mangle)
+	if n := argSize / p.PtrSize; n > 0 {
+		fmt.Fprintf(fc, "void *y[%d];", n)
+	}
+	if n := argSize % p.PtrSize; n > 0 {
+		fmt.Fprintf(fc, "uint8 x[%d];", n)
+	}
+	fmt.Fprintf(fc, "}p)\n")
 	fmt.Fprintf(fc, "{\n")
 	fmt.Fprintf(fc, "\truntime·cgocall(_cgo%s%s, &p);\n", cPrefix, n.Mangle)
 	if n.AddError {