V8: avoid deadlock when profiling is active

A deadlock happens when sampler initiated by SIGPROF tries to lock
the thread and the thread is already locked by the same thread. As
a result, other thread involved in sampling process hangs. The
patch adds a check for thread lock before continuing sampler
operation.

The fix has been tested on a sample app under load with and without
profiling turned on.

Fixes issue #14576 and specifically the duplicate issue #25295

deps/v8/src/isolate.h

@@ -384,6 +384,11 @@ class Isolate {
  public:
   ~Isolate();
 
+  // ISSUE-14576: allow to access process_wide_mutex_ from sampler.cc
+  static base::LazyMutex GetProcessWideMutex() {
+    return process_wide_mutex_;
+  }
+
   // A thread has a PerIsolateThreadData instance for each isolate that it has
   // entered. That instance is allocated when the isolate is initially entered
   // and reused on subsequent entries.

deps/v8/src/sampler.cc

@@ -342,6 +342,15 @@ void SignalHandler::HandleProfilerSignal(int signal, siginfo_t* info,
     // We require a fully initialized and entered isolate.
     return;
   }
+
+  // ISSUE-14576: To avoid deadlock, return if there is a thread lock
+  if (Isolate::GetProcessWideMutex().Pointer()->TryLock()) {
+    Isolate::GetProcessWideMutex().Pointer()->Unlock();
+  }
+  else {
+    return;
+  }
+
   if (v8::Locker::IsActive() &&
       !isolate->thread_manager()->IsLockedByCurrentThread()) {
     return;