impl(spanner): lock mutex in total_sessions accessor (#15017)

Author: scotthart

google/cloud/spanner/internal/session_pool.cc

@@ -370,9 +370,13 @@ std::shared_ptr<SpannerStub> SessionPool::GetStub(Session const& session) {
   return GetStub(std::unique_lock<std::mutex>(mu_));
 }
 
-void SessionPool::DecrementSessionCount(
-    std::unique_lock<std::mutex> const&,
-    google::cloud::spanner_internal::Session& session) {
+int SessionPool::total_sessions() const {
+  std::lock_guard<std::mutex> lk(mu_);
+  return total_sessions_;
+}
+
+void SessionPool::DecrementSessionCount(std::unique_lock<std::mutex> const&,
+                                        Session const& session) {
   --total_sessions_;
   auto const& channel = session.channel();
   if (channel) {

google/cloud/spanner/internal/session_pool.h

@@ -118,8 +118,11 @@ class SessionPool : public std::enable_shared_from_this<SessionPool> {
   /**
    * Returns the number of sessions in the session pool plus the number of
    * sessions allocated to running transactions.
+   *
+   * @note This function should only be used for testing as other threads
+   *   could be modifying the underlying value immediately after it returns.
    */
-  int total_sessions() const { return total_sessions_; }
+  int total_sessions() const;
 
  private:
   friend std::shared_ptr<SessionPool> MakeSessionPool(
@@ -215,7 +218,7 @@ class SessionPool : public std::enable_shared_from_this<SessionPool> {
 
   // Performs the necessary bookkeeping when a session is removed from use.
   void DecrementSessionCount(std::unique_lock<std::mutex> const& lk,
-                             Session& session);
+                             Session const& session);
 
   spanner::Database const db_;
   google::cloud::CompletionQueue cq_;
@@ -226,7 +229,7 @@ class SessionPool : public std::enable_shared_from_this<SessionPool> {
   int const max_pool_size_;
   std::mt19937 random_generator_;
 
-  std::mutex mu_;
+  mutable std::mutex mu_;
   std::condition_variable cond_;
   SessionHolder multiplexed_session_;               // GUARDED_BY(mu_)
   std::vector<std::unique_ptr<Session>> sessions_;  // GUARDED_BY(mu_)