Don't allow breakpoints in Java debugger worker thread.

Debugging the debugger may cause deadlock.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=111432504

Author: vladlf

src/agent/jvmti_agent.cc

@@ -247,6 +247,12 @@ void JvmtiAgent::JvmtiOnBreakpoint(
     jthread thread,
     jmethodID method,
     jlocation location) {
+  // Ignore breakpoint events from debugger worker threads. Debugging
+  // the debugger may cause deadlock.
+  if (JvmtiAgentThread::IsInAgentThread()) {
+    return;
+  }
+
   std::shared_ptr<Debugger> debugger = debugger_;
 
   if (debugger != nullptr) {

src/agent/jvmti_agent_thread.cc

@@ -21,6 +21,11 @@
 namespace devtools {
 namespace cdbg {
 
+// Thread local variable indicating whether the current thread is JVMTI
+// agent thread created by "JvmtiAgentThread".
+static __thread bool g_is_agent_thread = false;
+
+
 JvmtiAgentThread::JvmtiAgentThread() {
 }
 
@@ -85,6 +90,9 @@ bool JvmtiAgentThread::StartAgentThread(
       [] (jvmtiEnv* jvmti, JNIEnv* jni, void* arg) {
         set_thread_jni(jni);
 
+        DCHECK(!g_is_agent_thread);
+        g_is_agent_thread = true;
+
         auto* agent_arg = reinterpret_cast<
             std::pair<string, std::function<bool()>>*>(arg);
 
@@ -94,6 +102,8 @@ bool JvmtiAgentThread::StartAgentThread(
 
         LOG(INFO) << "Agent thread exited: " << agent_arg->first;
 
+        g_is_agent_thread = false;
+
         delete agent_arg;
       },
       agent_arg,
@@ -111,6 +121,10 @@ bool JvmtiAgentThread::StartAgentThread(
 }
 
 
+bool JvmtiAgentThread::IsInAgentThread() {
+  return g_is_agent_thread;
+}
+
 }  // namespace cdbg
 }  // namespace devtools
 

src/agent/jvmti_agent_thread.h

@@ -40,6 +40,10 @@ class JvmtiAgentThread : public AgentThread {
 
   void Sleep(int ms) override;
 
+  // Returns true if the current thread is JVMTI agent thread created by
+  // this class.
+  static bool IsInAgentThread();
+
  private:
   // Creates the thread objects and starts the agent thread.
   bool StartAgentThread(