[vm/isolate] Provide more verbose information when second mutator thread attempts to enter an isolate.

This should help with troubleshooting vm crashes caused by multiple mutator threads entering an isolate.

TEST=flutter/engine runtime_unittests that exhibits this flaky problem with DartIsolateTest.ValidLoadingUnitSucceeds

Change-Id: Ib503df844cceafa58882a695a1fe7ceaa88eaed4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/174900
Commit-Queue: Alexander Aprelev <aam@google.com>
Reviewed-by: Siva Annamalai <asiva@google.com>

Author: aam

runtime/vm/dart_api_impl.cc

@@ -1700,10 +1700,16 @@ DART_EXPORT void Dart_EnterIsolate(Dart_Isolate isolate) {
   // TODO(http://dartbug.com/16615): Validate isolate parameter.
   Isolate* iso = reinterpret_cast<Isolate*>(isolate);
   if (!Thread::EnterIsolate(iso)) {
-    FATAL(
-        "Unable to Enter Isolate : "
-        "Multiple mutators entering an isolate / "
-        "Dart VM is shutting down");
+    if (iso->IsScheduled()) {
+      FATAL(
+          "Isolate %s is already scheduled on mutator thread %p, "
+          "failed to schedule from os thread 0x%" Px "\n",
+          iso->name(), iso->scheduled_mutator_thread(),
+          OSThread::ThreadIdToIntPtr(OSThread::GetCurrentThreadId()));
+    } else {
+      FATAL("Unable to enter isolate %s as Dart VM is shutting down",
+            iso->name());
+    }
   }
   // A Thread structure has been associated to the thread, we do the
   // safepoint transition explicitly here instead of using the

runtime/vm/isolate.h

@@ -812,7 +812,8 @@ class Isolate : public BaseIsolate, public IntrusiveDListEntry<Isolate> {
     return thread == nullptr ? nullptr : thread->isolate();
   }
 
-  bool IsScheduled() { return scheduled_mutator_thread_ != nullptr; }
+  bool IsScheduled() { return scheduled_mutator_thread() != nullptr; }
+  Thread* scheduled_mutator_thread() const { return scheduled_mutator_thread_; }
 
   // Register a newly introduced class.
   void RegisterClass(const Class& cls);