runtime: prevent garbage collection during hashmap insertion

Inserting a key-value pair into a hashmap storing keys or values
indirectly can cause the garbage collector to find the hashmap in
an inconsistent	state.

Fixes #5074.

R=golang-dev, minux.ma, rsc
CC=golang-dev
https://golang.org/cl/7913043

src/pkg/runtime/gc_test.go

@@ -7,6 +7,7 @@ package runtime_test
 import (
 	"os"
 	"runtime"
+	"runtime/debug"
 	"testing"
 )
 
@@ -82,3 +83,17 @@ func TestGcDeepNesting(t *testing.T) {
 		t.Fail()
 	}
 }
+
+func TestGcHashmapIndirection(t *testing.T) {
+	defer debug.SetGCPercent(debug.SetGCPercent(1))
+	runtime.GC()
+	type T struct {
+		a [256]int
+	}
+	m := make(map[T]T)
+	for i := 0; i < 2000; i++ {
+		var a T
+		a.a[0] = i
+		m[a] = T{}
+	}
+}

src/pkg/runtime/hashmap.c

@@ -1016,10 +1016,12 @@ runtime·mapassign(MapType *t, Hmap *h, byte *ak, byte *av)
 	res = nil;
 	hit = hash_insert(t, h, ak, (void**)&res);
 	if(!hit) {
+		// Need to pass dogc=0 to runtime·mallocgc because the garbage collector
+		// is assuming that all hashmaps are in a consistent state.
 		if(h->flag & IndirectKey)
-			*(void**)res = runtime·mal(t->key->size);
+			*(void**)res = runtime·mallocgc(t->key->size, 0, 0, 1);
 		if(h->flag & IndirectVal)
-			*(void**)(res+h->valoff) = runtime·mal(t->elem->size);
+			*(void**)(res+h->valoff) = runtime·mallocgc(t->elem->size, 0, 0, 1);
 	}
 	t->key->alg->copy(t->key->size, hash_keyptr(h, res), ak);
 	t->elem->alg->copy(t->elem->size, hash_valptr(h, res), av);

src/pkg/runtime/mgc0.c

@@ -932,17 +932,29 @@ scanblock(Workbuf *wbuf, Obj *wp, uintptr nobj, bool keepworking)
 					if(!(mapkey_kind & KindNoPointers) || d.indirectkey) {
 						if(!d.indirectkey)
 							*objbufpos++ = (Obj){d.key_data, mapkey_size, mapkey_ti};
-						else
+						else {
+							if(Debug) {
+								obj = *(void**)d.key_data;
+								if(!(arena_start <= obj && obj < arena_used))
+									runtime·throw("scanblock: inconsistent hashmap");
+							}
 							*ptrbufpos++ = (PtrTarget){*(void**)d.key_data, mapkey_ti};
 						}
+					}
 					if(!(mapval_kind & KindNoPointers) || d.indirectval) {
 						if(!d.indirectval)
 							*objbufpos++ = (Obj){d.val_data, mapval_size, mapval_ti};
-						else
+						else {
+							if(Debug) {
+								obj = *(void**)d.val_data;
+								if(!(arena_start <= obj && obj < arena_used))
+									runtime·throw("scanblock: inconsistent hashmap");
+							}
 							*ptrbufpos++ = (PtrTarget){*(void**)d.val_data, mapval_ti};
 						}
 					}
 				}
+			}
 			if(map_ret == nil)
 				goto next_block;
 			pc = map_ret;