Add SafeThread to fix thread JNI shutdown races.

During JVM shutdown, some JNI calls may not return, so it's not possible to
reliably perform a join() during static variable destruction (which occurs
as the JVM unloads the JNI module).

Also, due to static variable destruction, it's not safe to use any members
of a static class instance from a separate thread of execution.

SafeThread is a templated thread class and a related owner class that's
designed for safe operation and shutdown of threads in the presence of
callbacks that may not return.  It also passes ownership of variables from
the static instance to the thread, so the thread can safely operate until
it exits (the last operation of the thread being to destroy its instance).

Notifiers, RpcServer, and Logger now use SafeThread to ensure race-free
destruction in both C++ and Java.

All Java callback threads are now marked as Java daemon threads so they
don't keep the JVM running after main() terminates.

All Java callback threads are now named so their purpose is more easily
identified in a debugger.

Add SetRpcServerOnStart and SetRpcServerOnExit (similar to Listener).

src/SafeThread.h

@@ -0,0 +1,93 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) FIRST 2015. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#ifndef NT_SAFETHREAD_H_
+#define NT_SAFETHREAD_H_
+
+#include <atomic>
+#include <condition_variable>
+#include <mutex>
+#include <thread>
+
+namespace nt {
+
+// Base class for SafeThreadOwner threads.
+class SafeThread {
+ public:
+  virtual ~SafeThread() = default;
+  virtual void Main() = 0;
+
+  std::mutex m_mutex;
+  bool m_active = true;
+  std::condition_variable m_cond;
+};
+
+namespace detail {
+
+// Non-template proxy base class for common proxy code.
+class SafeThreadProxyBase {
+ public:
+  SafeThreadProxyBase(SafeThread* thr) : m_thread(thr) {
+    if (!m_thread) return;
+    std::unique_lock<std::mutex>(m_thread->m_mutex).swap(m_lock);
+    if (!m_thread->m_active) {
+      m_lock.unlock();
+      m_thread = nullptr;
+      return;
+    }
+  }
+  explicit operator bool() const { return m_thread != nullptr; }
+  std::unique_lock<std::mutex>& GetLock() { return m_lock; }
+
+ protected:
+  SafeThread* m_thread;
+  std::unique_lock<std::mutex> m_lock;
+};
+
+// A proxy for SafeThread.
+// Also serves as a scoped lock on SafeThread::m_mutex.
+template <typename T>
+class SafeThreadProxy : public SafeThreadProxyBase {
+ public:
+  SafeThreadProxy(SafeThread* thr) : SafeThreadProxyBase(thr) {}
+  T& operator*() const { return *static_cast<T*>(m_thread); }
+  T* operator->() const { return static_cast<T*>(m_thread); }
+};
+
+// Non-template owner base class for common owner code.
+class SafeThreadOwnerBase {
+ public:
+  void Stop();
+
+ protected:
+  SafeThreadOwnerBase() { m_thread = nullptr; }
+  SafeThreadOwnerBase(const SafeThreadOwnerBase&) = delete;
+  SafeThreadOwnerBase& operator=(const SafeThreadOwnerBase&) = delete;
+  ~SafeThreadOwnerBase() { Stop(); }
+
+  void Start(SafeThread* thr);
+  SafeThread* GetThread() { return m_thread.load(); }
+
+ private:
+  std::atomic<SafeThread*> m_thread;
+};
+
+}  // namespace detail
+
+template <typename T>
+class SafeThreadOwner : public detail::SafeThreadOwnerBase {
+ public:
+  void Start() { Start(new T); }
+  void Start(T* thr) { detail::SafeThreadOwnerBase::Start(thr); }
+
+  using Proxy = typename detail::SafeThreadProxy<T>;
+  Proxy GetThread() { return Proxy(detail::SafeThreadOwnerBase::GetThread()); }
+};
+
+}  // namespace nt
+
+#endif  // NT_SAFETHREAD_H_