Converts std::unique_lock to std::lock_guard. Tested by compiling for CXX threads, OpenMP, no threads, and TBB. Change-Id: If1ba5cfce83e2ad4e1015354ce67f5b23e89101f

diff --git a/internal/ceres/concurrent_queue.h b/internal/ceres/concurrent_queue.h
index c4e076f..52e2903 100644
--- a/internal/ceres/concurrent_queue.h
+++ b/internal/ceres/concurrent_queue.h

@@ -83,7 +83,7 @@
   // Atomically push an element onto the queue.  If a thread was waiting for an
   // element, wake it up.
   void Push(const T& value) {
-    std::unique_lock<std::mutex> lock(mutex_);
+    std::lock_guard<std::mutex> lock(mutex_);
     queue_.push(value);
     work_pending_condition_.notify_one();
   }
@@ -93,7 +93,7 @@
   bool Pop(T* value) {
     CHECK(value != nullptr);
 
-    std::unique_lock<std::mutex> lock(mutex_);
+    std::lock_guard<std::mutex> lock(mutex_);
     return PopUnlocked(value);
   }
 
@@ -114,14 +114,14 @@
   // exit Wait() without getting a value. All future Wait requests will return
   // immediately if no element is present until EnableWaiters is called.
   void StopWaiters() {
-    std::unique_lock<std::mutex> lock(mutex_);
+    std::lock_guard<std::mutex> lock(mutex_);
     wait_ = false;
     work_pending_condition_.notify_all();
   }
 
   // Enable threads to block on Wait calls.
   void EnableWaiters() {
-    std::unique_lock<std::mutex> lock(mutex_);
+    std::lock_guard<std::mutex> lock(mutex_);
     wait_ = true;
   }
 

diff --git a/internal/ceres/concurrent_queue_test.cc b/internal/ceres/concurrent_queue_test.cc
index 3b15c4b..698966a 100644
--- a/internal/ceres/concurrent_queue_test.cc
+++ b/internal/ceres/concurrent_queue_test.cc

@@ -189,7 +189,7 @@
 
   std::thread thread([&]() {
     {
-      std::unique_lock<std::mutex> lock(mutex);
+      std::lock_guard<std::mutex> lock(mutex);
       waiting = true;
     }
 
@@ -197,7 +197,7 @@
     bool valid = queue.Wait(&element);
 
     {
-      std::unique_lock<std::mutex> lock(mutex);
+      std::lock_guard<std::mutex> lock(mutex);
       waiting = false;
       value = element;
       valid_value = valid;
@@ -209,7 +209,7 @@
 
   // Ensure nothing is has been popped off the queue
   {
-    std::unique_lock<std::mutex> lock(mutex);
+    std::lock_guard<std::mutex> lock(mutex);
     EXPECT_TRUE(waiting);
     ASSERT_FALSE(valid_value);
     ASSERT_EQ(0, value);
@@ -234,7 +234,7 @@
 
   auto task = [&]() {
     {
-      std::unique_lock<std::mutex> lock(mutex);
+      std::lock_guard<std::mutex> lock(mutex);
       waiting = true;
     }
 
@@ -242,7 +242,7 @@
     bool valid = queue.Wait(&element);
 
     {
-      std::unique_lock<std::mutex> lock(mutex);
+      std::lock_guard<std::mutex> lock(mutex);
       waiting = false;
       value = element;
       valid_value = valid;
@@ -256,7 +256,7 @@
 
   // Ensure the thread is waiting.
   {
-    std::unique_lock<std::mutex> lock(mutex);
+    std::lock_guard<std::mutex> lock(mutex);
     EXPECT_TRUE(waiting);
   }
 
@@ -286,7 +286,7 @@
 
   // Ensure nothing is popped off the queue.
   {
-    std::unique_lock<std::mutex> lock(mutex);
+    std::lock_guard<std::mutex> lock(mutex);
     EXPECT_TRUE(waiting);
     ASSERT_FALSE(valid_value);
     ASSERT_EQ(0, value);

diff --git a/internal/ceres/parallel_for_cxx.cc b/internal/ceres/parallel_for_cxx.cc
index 3da5a87..20a689d 100644
--- a/internal/ceres/parallel_for_cxx.cc
+++ b/internal/ceres/parallel_for_cxx.cc

@@ -60,7 +60,7 @@
   // Increment the number of jobs that have finished and signal the blocking
   // thread if all jobs have finished.
   void Finished() {
-    std::unique_lock<std::mutex> lock(mutex_);
+    std::lock_guard<std::mutex> lock(mutex_);
     ++num_finished_;
     CHECK_LE(num_finished_, num_total_);
     if (num_finished_ == num_total_) {
@@ -196,7 +196,7 @@
     {
       // Get the next available chunk of work to be performed. If there is no
       // work, return false.
-      std::unique_lock<std::mutex> lock(shared_state->mutex_i);
+      std::lock_guard<std::mutex> lock(shared_state->mutex_i);
       if (shared_state->i >= shared_state->num_work_items) {
         return false;
       }

diff --git a/internal/ceres/thread_pool.cc b/internal/ceres/thread_pool.cc
index 9c7bb89..8fc7f83 100644
--- a/internal/ceres/thread_pool.cc
+++ b/internal/ceres/thread_pool.cc

@@ -62,7 +62,7 @@
 }
 
 ThreadPool::~ThreadPool() {
-  std::unique_lock<std::mutex> lock(thread_pool_mutex_);
+  std::lock_guard<std::mutex> lock(thread_pool_mutex_);
   // Signal the thread workers to stop and wait for them to finish all scheduled
   // tasks.
   Stop();
@@ -72,7 +72,7 @@
 }
 
 void ThreadPool::Resize(int num_threads) {
-  std::unique_lock<std::mutex> lock(thread_pool_mutex_);
+  std::lock_guard<std::mutex> lock(thread_pool_mutex_);
 
   const int num_current_threads = thread_pool_.size();
   if (num_current_threads >= num_threads) {
@@ -92,7 +92,7 @@
 }
 
 int ThreadPool::Size() {
-  std::unique_lock<std::mutex> lock(thread_pool_mutex_);
+  std::lock_guard<std::mutex> lock(thread_pool_mutex_);
   return thread_pool_.size();
 }
 

diff --git a/internal/ceres/thread_pool_test.cc b/internal/ceres/thread_pool_test.cc
index 5485fe4..2b1bf87 100644
--- a/internal/ceres/thread_pool_test.cc
+++ b/internal/ceres/thread_pool_test.cc

@@ -59,7 +59,7 @@
 
     for (int i = 0; i < num_tasks; ++i) {
       thread_pool.AddTask([&]() {
-          std::unique_lock<std::mutex> lock(mutex);
+          std::lock_guard<std::mutex> lock(mutex);
           ++value;
           condition.notify_all();
         });
@@ -96,7 +96,7 @@
     auto task = [&]() {
       // This will block until the mutex is released inside the condition
       // variable.
-      std::unique_lock<std::mutex> lock(mutex);
+      std::lock_guard<std::mutex> lock(mutex);
       ++value;
       condition.notify_all();
     };
@@ -150,7 +150,7 @@
       thread_pool.AddTask([&]() {
         // This will block until the mutex is released inside the condition
         // variable.
-        std::unique_lock<std::mutex> lock(mutex);
+        std::lock_guard<std::mutex> lock(mutex);
         ++value;
         condition.notify_all();
       });