[heap] Support safepoint->AssertActive() for shared isolates

Lock mutex for shared isolate in global safepoints, such that e.g. the
StringTable can use isolate->heap()->safepoint()->AssertActive() even
for shared isolates.

Bug: v8:11708, v8:12749
Change-Id: I8d99203581dfa2d7225846e19fa981300f88589e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3563138Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79715}

src/heap/safepoint.cc

@@ -358,6 +358,12 @@ void GlobalSafepoint::EnterGlobalSafepointScope(Isolate* initiator) {
         initiator, &clients.back());
   });
 
+  // Make it possible to use AssertActive() on shared isolates.
+  CHECK(shared_isolate_->heap()->safepoint()->local_heaps_mutex_.TryLock());
+
+  // Shared isolates should never have multiple threads.
+  shared_isolate_->heap()->safepoint()->AssertMainThreadIsOnlyThread();
+
   // Iterate all clients again to initiate the safepoint for all of them - even
   // if that means blocking.
   for (PerClientSafepointData& client : clients) {
@@ -381,6 +387,8 @@ void GlobalSafepoint::EnterGlobalSafepointScope(Isolate* initiator) {
 }
 
 void GlobalSafepoint::LeaveGlobalSafepointScope(Isolate* initiator) {
+  shared_isolate_->heap()->safepoint()->local_heaps_mutex_.Unlock();
+
   IterateClientIsolates([initiator](Isolate* client) {
     Heap* client_heap = client->heap();
     client_heap->safepoint()->LeaveGlobalSafepointScope(initiator);

src/snapshot/snapshot.cc

@@ -313,10 +313,6 @@ void Snapshot::SerializeDeserializeAndVerifyForTesting(
   // Test serialization.
   {
     GlobalSafepointScope global_safepoint(isolate);
-    base::Optional<SafepointScope> shared_isolate_safepoint_scope;
-    if (Isolate* shared_isolate = isolate->shared_isolate()) {
-      shared_isolate_safepoint_scope.emplace(shared_isolate->heap());
-    }
     DisallowGarbageCollection no_gc;
 
     Snapshot::SerializerFlags flags(

test/mjsunit/mjsunit.status

@@ -1456,8 +1456,6 @@
   # Script referenced only through context-dependent SourceTextModule
   # https://bugs.chromium.org/p/v8/issues/detail?id=11073
   'tools/processor': [SKIP],
-  # https://crbug.com/v8/12749
-  'shared-memory/shared-struct-atomics-workers': [SKIP],
 }],  # variant == stress_snapshot and arch == x64
 
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