Move common utility classes to wpiutil library. (#79)

This is a breaking change to dependencies that use the static ntcore
library.  Unless the wpiutil library is also linked, linker errors will
result.  This does not affect the shared ntcore library.

wpiutil/include/support/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 WPIUTIL_SUPPORT_SAFETHREAD_H_
+#define WPIUTIL_SUPPORT_SAFETHREAD_H_
+
+#include <atomic>
+#include <condition_variable>
+#include <mutex>
+#include <thread>
+
+namespace wpi {
+
+// 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 wpi
+
+#endif  // WPIUTIL_SUPPORT_SAFETHREAD_H_