Add priority_mutex and priority_condition_variable. (#50)

Also provide wpi::mutex and wpi::condition_variable as wrappers for these
on Linux (where they're available), and for std::mutex and
std::condition_variable on other platforms.

src/main/native/include/support/priority_mutex.h

@@ -0,0 +1,84 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2016-2017 FIRST. 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+// Allows usage with std::lock_guard without including <mutex> separately
+#include <mutex>
+
+#ifdef __linux__
+#include <pthread.h>
+#endif
+
+namespace wpi {
+
+#ifdef __linux__
+
+#define WPI_HAVE_PRIORITY_MUTEX 1
+
+class priority_recursive_mutex {
+ public:
+  typedef pthread_mutex_t* native_handle_type;
+
+  constexpr priority_recursive_mutex() noexcept = default;
+  priority_recursive_mutex(const priority_recursive_mutex&) = delete;
+  priority_recursive_mutex& operator=(const priority_recursive_mutex&) = delete;
+
+  // Lock the mutex, blocking until it's available.
+  void lock() { pthread_mutex_lock(&m_mutex); }
+
+  // Unlock the mutex.
+  void unlock() { pthread_mutex_unlock(&m_mutex); }
+
+  // Tries to lock the mutex.
+  bool try_lock() noexcept { return !pthread_mutex_trylock(&m_mutex); }
+
+  pthread_mutex_t* native_handle() { return &m_mutex; }
+
+ private:
+// Do the equivalent of setting PTHREAD_PRIO_INHERIT and
+// PTHREAD_MUTEX_RECURSIVE_NP.
+#ifdef __PTHREAD_MUTEX_HAVE_PREV
+  pthread_mutex_t m_mutex = {
+      {0, 0, 0, 0, 0x20 | PTHREAD_MUTEX_RECURSIVE_NP, __PTHREAD_SPINS, {0, 0}}};
+#else
+  pthread_mutex_t m_mutex = {
+      {0, 0, 0, 0x20 | PTHREAD_MUTEX_RECURSIVE_NP, 0, {__PTHREAD_SPINS}}};
+#endif
+};
+
+class priority_mutex {
+ public:
+  typedef pthread_mutex_t* native_handle_type;
+
+  constexpr priority_mutex() noexcept = default;
+  priority_mutex(const priority_mutex&) = delete;
+  priority_mutex& operator=(const priority_mutex&) = delete;
+
+  // Lock the mutex, blocking until it's available.
+  void lock() { pthread_mutex_lock(&m_mutex); }
+
+  // Unlock the mutex.
+  void unlock() { pthread_mutex_unlock(&m_mutex); }
+
+  // Tries to lock the mutex.
+  bool try_lock() noexcept { return !pthread_mutex_trylock(&m_mutex); }
+
+  pthread_mutex_t* native_handle() { return &m_mutex; }
+
+ private:
+// Do the equivalent of setting PTHREAD_PRIO_INHERIT.
+#ifdef __PTHREAD_MUTEX_HAVE_PREV
+  pthread_mutex_t m_mutex = {{0, 0, 0, 0, 0x20, __PTHREAD_SPINS, {0, 0}}};
+#else
+  pthread_mutex_t m_mutex = {{0, 0, 0, 0x20, 0, {__PTHREAD_SPINS}}};
+#endif
+};
+
+#endif  // __linux__
+
+}  // namespace wpi