runtime: fix heap corruption during GC

The 'n' variable is used during rescan initiation in GC_END case,
but it's overwritten with chan capacity in GC_CHAN case.
As the result rescan is done with the wrong object size.
Fixes #5554.

R=golang-dev, khr
CC=golang-dev
https://golang.org/cl/9831043

src/pkg/runtime/gc_test.go

@@ -121,3 +121,31 @@ func TestGcArraySlice(t *testing.T) {
 		}
 	}
 }
+
+func TestGcRescan(t *testing.T) {
+	type X struct {
+		c     chan error
+		nextx *X
+	}
+	type Y struct {
+		X
+		nexty *Y
+		p     *int
+	}
+	var head *Y
+	for i := 0; i < 10; i++ {
+		p := &Y{}
+		p.c = make(chan error)
+		p.nextx = &head.X
+		p.nexty = head
+		p.p = new(int)
+		*p.p = 42
+		head = p
+		runtime.GC()
+	}
+	for p := head; p != nil; p = p.nexty {
+		if *p.p != 42 {
+			t.Fatal("corrupted heap")
+		}
+	}
+}

src/pkg/runtime/mgc0.c

@@ -623,7 +623,7 @@ scanblock(Workbuf *wbuf, Obj *wp, uintptr nobj, bool keepworking)
 	byte *b, *arena_start, *arena_used;
 	uintptr n, i, end_b, elemsize, size, ti, objti, count, type;
 	uintptr *pc, precise_type, nominal_size;
-	uintptr *map_ret, mapkey_size, mapval_size, mapkey_ti, mapval_ti, *chan_ret;
+	uintptr *map_ret, mapkey_size, mapval_size, mapkey_ti, mapval_ti, *chan_ret, chancap;
 	void *obj;
 	Type *t;
 	Slice *sliceptr;
@@ -1062,13 +1062,13 @@ scanblock(Workbuf *wbuf, Obj *wp, uintptr nobj, bool keepworking)
 			if(!(chantype->elem->kind & KindNoPointers)) {
 				// Channel's buffer follows Hchan immediately in memory.
 				// Size of buffer (cap(c)) is second int in the chan struct.
-				n = ((uintgo*)chan)[1];
-				if(n > 0) {
+				chancap = ((uintgo*)chan)[1];
+				if(chancap > 0) {
 					// TODO(atom): split into two chunks so that only the
 					// in-use part of the circular buffer is scanned.
 					// (Channel routines zero the unused part, so the current
 					// code does not lead to leaks, it's just a little inefficient.)
-					*objbufpos++ = (Obj){(byte*)chan+runtime·Hchansize, n*chantype->elem->size,
+					*objbufpos++ = (Obj){(byte*)chan+runtime·Hchansize, chancap*chantype->elem->size,
 						(uintptr)chantype->elem->gc | PRECISE | LOOP};
 					if(objbufpos == objbuf_end)
 						flushobjbuf(objbuf, &objbufpos, &wp, &wbuf, &nobj);