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diff --git a/third_party/re2/util/mutex.h b/third_party/re2/util/mutex.h
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+// Copyright 2007 The RE2 Authors.  All Rights Reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+ * A simple mutex wrapper, supporting locks and read-write locks.
+ * You should assume the locks are *not* re-entrant.
+ */
+
+#ifndef RE2_UTIL_MUTEX_H_
+#define RE2_UTIL_MUTEX_H_
+
+namespace re2 {
+
+#ifndef WIN32
+#define HAVE_PTHREAD 1
+#define HAVE_RWLOCK 1
+#endif
+
+#if defined(NO_THREADS)
+  typedef int MutexType;      // to keep a lock-count
+#elif defined(HAVE_PTHREAD) && defined(HAVE_RWLOCK)
+  // Needed for pthread_rwlock_*.  If it causes problems, you could take it
+  // out, but then you'd have to unset HAVE_RWLOCK (at least on linux -- it
+  // *does* cause problems for FreeBSD, or MacOSX, but isn't needed
+  // for locking there.)
+# ifdef __linux__
+#   undef _XOPEN_SOURCE
+#   define _XOPEN_SOURCE 500  // may be needed to get the rwlock calls
+# endif
+# include <pthread.h>
+  typedef pthread_rwlock_t MutexType;
+#elif defined(HAVE_PTHREAD)
+# include <pthread.h>
+  typedef pthread_mutex_t MutexType;
+#elif defined(WIN32)
+# ifndef WIN32_LEAN_AND_MEAN
+#  define WIN32_LEAN_AND_MEAN  // We only need minimal includes
+# endif
+# ifdef GMUTEX_TRYLOCK
+  // We need Windows NT or later for TryEnterCriticalSection().  If you
+  // don't need that functionality, you can remove these _WIN32_WINNT
+  // lines, and change TryLock() to assert(0) or something.
+#   ifndef _WIN32_WINNT
+#     define _WIN32_WINNT 0x0400
+#   endif
+# endif
+# include <windows.h>
+  typedef CRITICAL_SECTION MutexType;
+#else
+# error Need to implement mutex.h for your architecture, or #define NO_THREADS
+#endif
+
+class Mutex {
+ public:
+  // Create a Mutex that is not held by anybody.
+  inline Mutex();
+
+  // Destructor
+  inline ~Mutex();
+
+  inline void Lock();    // Block if needed until free then acquire exclusively
+  inline void Unlock();  // Release a lock acquired via Lock()
+  inline bool TryLock(); // If free, Lock() and return true, else return false
+  // Note that on systems that don't support read-write locks, these may
+  // be implemented as synonyms to Lock() and Unlock().  So you can use
+  // these for efficiency, but don't use them anyplace where being able
+  // to do shared reads is necessary to avoid deadlock.
+  inline void ReaderLock();   // Block until free or shared then acquire a share
+  inline void ReaderUnlock(); // Release a read share of this Mutex
+  inline void WriterLock() { Lock(); }     // Acquire an exclusive lock
+  inline void WriterUnlock() { Unlock(); } // Release a lock from WriterLock()
+  inline void AssertHeld() { }
+
+ private:
+  MutexType mutex_;
+
+  // Catch the error of writing Mutex when intending MutexLock.
+  Mutex(Mutex *ignored) {}
+  // Disallow "evil" constructors
+  Mutex(const Mutex&);
+  void operator=(const Mutex&);
+};
+
+// Now the implementation of Mutex for various systems
+#if defined(NO_THREADS)
+
+// When we don't have threads, we can be either reading or writing,
+// but not both.  We can have lots of readers at once (in no-threads
+// mode, that's most likely to happen in recursive function calls),
+// but only one writer.  We represent this by having mutex_ be -1 when
+// writing and a number > 0 when reading (and 0 when no lock is held).
+//
+// In debug mode, we assert these invariants, while in non-debug mode
+// we do nothing, for efficiency.  That's why everything is in an
+// assert.
+#include <assert.h>
+
+Mutex::Mutex() : mutex_(0) { }
+Mutex::~Mutex()            { assert(mutex_ == 0); }
+void Mutex::Lock()         { assert(--mutex_ == -1); }
+void Mutex::Unlock()       { assert(mutex_++ == -1); }
+bool Mutex::TryLock()      { if (mutex_) return false; Lock(); return true; }
+void Mutex::ReaderLock()   { assert(++mutex_ > 0); }
+void Mutex::ReaderUnlock() { assert(mutex_-- > 0); }
+
+#elif defined(HAVE_PTHREAD) && defined(HAVE_RWLOCK)
+
+#include <stdlib.h>      // for abort()
+#define SAFE_PTHREAD(fncall)  do { if ((fncall) != 0) abort(); } while (0)
+
+Mutex::Mutex()             { SAFE_PTHREAD(pthread_rwlock_init(&mutex_, NULL)); }
+Mutex::~Mutex()            { SAFE_PTHREAD(pthread_rwlock_destroy(&mutex_)); }
+void Mutex::Lock()         { SAFE_PTHREAD(pthread_rwlock_wrlock(&mutex_)); }
+void Mutex::Unlock()       { SAFE_PTHREAD(pthread_rwlock_unlock(&mutex_)); }
+bool Mutex::TryLock()      { return pthread_rwlock_trywrlock(&mutex_) == 0; }
+void Mutex::ReaderLock()   { SAFE_PTHREAD(pthread_rwlock_rdlock(&mutex_)); }
+void Mutex::ReaderUnlock() { SAFE_PTHREAD(pthread_rwlock_unlock(&mutex_)); }
+
+#undef SAFE_PTHREAD
+
+#elif defined(HAVE_PTHREAD)
+
+#include <stdlib.h>      // for abort()
+#define SAFE_PTHREAD(fncall)  do { if ((fncall) != 0) abort(); } while (0)
+
+Mutex::Mutex()             { SAFE_PTHREAD(pthread_mutex_init(&mutex_, NULL)); }
+Mutex::~Mutex()            { SAFE_PTHREAD(pthread_mutex_destroy(&mutex_)); }
+void Mutex::Lock()         { SAFE_PTHREAD(pthread_mutex_lock(&mutex_)); }
+void Mutex::Unlock()       { SAFE_PTHREAD(pthread_mutex_unlock(&mutex_)); }
+bool Mutex::TryLock()      { return pthread_mutex_trylock(&mutex_) == 0; }
+void Mutex::ReaderLock()   { Lock(); }      // we don't have read-write locks
+void Mutex::ReaderUnlock() { Unlock(); }
+#undef SAFE_PTHREAD
+
+#elif defined(WIN32)
+
+Mutex::Mutex()             { InitializeCriticalSection(&mutex_); }
+Mutex::~Mutex()            { DeleteCriticalSection(&mutex_); }
+void Mutex::Lock()         { EnterCriticalSection(&mutex_); }
+void Mutex::Unlock()       { LeaveCriticalSection(&mutex_); }
+bool Mutex::TryLock()      { return TryEnterCriticalSection(&mutex_) != 0; }
+void Mutex::ReaderLock()   { Lock(); }      // we don't have read-write locks
+void Mutex::ReaderUnlock() { Unlock(); }
+
+#endif
+
+
+// --------------------------------------------------------------------------
+// Some helper classes
+
+// MutexLock(mu) acquires mu when constructed and releases it when destroyed.
+class MutexLock {
+ public:
+  explicit MutexLock(Mutex *mu) : mu_(mu) { mu_->Lock(); }
+  ~MutexLock() { mu_->Unlock(); }
+ private:
+  Mutex * const mu_;
+  // Disallow "evil" constructors
+  MutexLock(const MutexLock&);
+  void operator=(const MutexLock&);
+};
+
+// ReaderMutexLock and WriterMutexLock do the same, for rwlocks
+class ReaderMutexLock {
+ public:
+  explicit ReaderMutexLock(Mutex *mu) : mu_(mu) { mu_->ReaderLock(); }
+  ~ReaderMutexLock() { mu_->ReaderUnlock(); }
+ private:
+  Mutex * const mu_;
+  // Disallow "evil" constructors
+  ReaderMutexLock(const ReaderMutexLock&);
+  void operator=(const ReaderMutexLock&);
+};
+
+class WriterMutexLock {
+ public:
+  explicit WriterMutexLock(Mutex *mu) : mu_(mu) { mu_->WriterLock(); }
+  ~WriterMutexLock() { mu_->WriterUnlock(); }
+ private:
+  Mutex * const mu_;
+  // Disallow "evil" constructors
+  WriterMutexLock(const WriterMutexLock&);
+  void operator=(const WriterMutexLock&);
+};
+
+// Catch bug where variable name is omitted, e.g. MutexLock (&mu);
+#define MutexLock(x) COMPILE_ASSERT(0, mutex_lock_decl_missing_var_name)
+#define ReaderMutexLock(x) COMPILE_ASSERT(0, rmutex_lock_decl_missing_var_name)
+#define WriterMutexLock(x) COMPILE_ASSERT(0, wmutex_lock_decl_missing_var_name)
+
+}  // namespace re2
+
+#endif  /* #define RE2_UTIL_MUTEX_H_ */