[vm/tags] Reset default/current tags as thread is released.

Fixes https://github.com/dart-lang/sdk/issues/61473
TEST=ci

Change-Id: I36606a38bf282ff1cae58d6c3184cc3212df29f9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/449070
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>

runtime/vm/dart.cc

@@ -431,6 +431,8 @@ char* Dart::DartInit(const Dart_InitializeParams* params) {
     StackZone zone(T);
     HandleScope handle_scope(T);
     Object::InitNullAndBool(vm_isolate_->group());
+    // Now that null is initialized properly.
+    group->tag_table_ = GrowableObjectArray::null();
     vm_isolate_->isolate_group_->set_object_store(new ObjectStore());
     vm_isolate_->isolate_object_store()->Init();
     vm_isolate_->finalizers_ = GrowableObjectArray::null();
@@ -1037,8 +1039,13 @@ ErrorPtr Dart::InitializeIsolate(Thread* T,
   I->debugger()->NotifyIsolateCreated();
 #endif
 
-  const UserTag& default_tag = UserTag::Handle(UserTag::DefaultTag(T));
-  T->set_current_tag(default_tag);
+  if (is_first_isolate_in_group) {
+    // IsolateGroup tag_table was not available when isolate was first
+    // created, but now it is.
+    ASSERT(T->current_tag() == UserTag::null());
+    const UserTag& default_tag = UserTag::Handle(UserTag::DefaultTag(T));
+    T->set_current_tag(default_tag);
+  }
 
   I->init_loaded_prefixes_set_storage();
 

runtime/vm/thread.cc

@@ -366,6 +366,9 @@ void Thread::AssertEmptyThreadInvariants() {
   ASSERT(stack_limit_.load() == 0);
   ASSERT(safepoint_state_ == 0);
 
+  ASSERT(default_tag_ == UserTag::null());
+  ASSERT(current_tag_ == UserTag::null());
+
   // Avoid running these asserts for `vm-isolate`.
   if (active_stacktrace_.untag() != 0) {
     ASSERT(field_table_values_ == nullptr);
@@ -434,6 +437,13 @@ void Thread::EnterIsolate(Isolate* isolate) {
     thread->ExitSafepoint();
   }
 
+  StackZone zone(thread);
+  HANDLESCOPE(thread);
+  if (group->tag_table() != GrowableObjectArray::null()) {
+    const UserTag& default_tag = UserTag::Handle(UserTag::DefaultTag(thread));
+    thread->set_current_tag(default_tag);
+  }
+
   ASSERT(!thread->IsAtSafepoint());
 }
 
@@ -764,6 +774,8 @@ void Thread::FreeActiveThread(Thread* thread,
   thread->stack_limit_.store(0);
   thread->safepoint_state_ = 0;
   thread->ResetStateLocked();
+  thread->current_tag_ = UserTag::null();
+  thread->default_tag_ = UserTag::null();
 
   thread->AssertEmptyThreadInvariants();
   thread_registry->ReturnThreadLocked(thread);