create independent executeLockedRead and executeLockedWrite to speed performance (#4178)

arangod/Agency/Agent.cpp

@@ -1529,7 +1529,14 @@ arangodb::consensus::index_t Agent::readDB(Node& node) const {
   return _commitIndex;
 }
 
-void Agent::executeLocked(std::function<void()> const& cb) {
+void Agent::executeLockedRead(std::function<void()> const& cb) {
+  _tiLock.assertNotLockedByCurrentThread();
+  MUTEX_LOCKER(ioLocker, _ioLock);
+  READ_LOCKER(oLocker, _outputLock);
+  cb();
+}
+
+void Agent::executeLockedWrite(std::function<void()> const& cb) {
   _tiLock.assertNotLockedByCurrentThread();
   MUTEX_LOCKER(ioLocker, _ioLock);
   WRITE_LOCKER(oLocker, _outputLock);

arangod/Agency/Agent.h

@@ -199,18 +199,32 @@ class Agent : public arangodb::Thread,
   State const& state() const;
 
   /// @brief execute a callback while holding _ioLock
-  void executeLocked(std::function<void()> const& cb);
+  ///  and read lock for _readDB
+  void executeLockedRead(std::function<void()> const& cb);
+
+  /// @brief execute a callback while holding _ioLock
+  ///  and write lock for _readDB
+  void executeLockedWrite(std::function<void()> const& cb);
 
   /// @brief Get read store and compaction index
   index_t readDB(Node&) const;
 
   /// @brief Get read store
+  ///  WARNING: this assumes caller holds appropriate
+  ///  locks or will use executeLockedRead() or
+  ///  executeLockedWrite() with a lambda function
   Store const& readDB() const;
 
   /// @brief Get spearhead store
+  ///  WARNING: this assumes caller holds appropriate
+  ///  locks or will use executeLockedRead() or
+  ///  executeLockedWrite() with a lambda function
   Store const& spearhead() const;
 
   /// @brief Get transient store
+  ///  WARNING: this assumes caller holds appropriate
+  ///  locks or will use executeLockedRead() or
+  ///  executeLockedWrite() with a lambda function
   Store const& transient() const;
 
   /// @brief Serve active agent interface

arangod/Agency/RestAgencyHandler.cpp

@@ -158,7 +158,7 @@ RestStatus RestAgencyHandler::handleStores() {
     {
       VPackObjectBuilder b(&body);
       {
-        _agent->executeLocked([&]() {
+        _agent->executeLockedRead([&]() {
           body.add(VPackValue("spearhead"));
           {
             VPackArrayBuilder bb(&body);
@@ -167,9 +167,7 @@ RestStatus RestAgencyHandler::handleStores() {
           body.add(VPackValue("read_db"));
           {
             VPackArrayBuilder bb(&body);
-            _agent->executeLocked([&]() {
             _agent->readDB().dumpToBuilder(body);
-              });
           }
           body.add(VPackValue("transient"));
           {

arangod/Agency/Supervision.cpp

@@ -542,7 +542,7 @@ bool Supervision::updateSnapshot() {
     return false;
   }
 
-  _agent->executeLocked([&]() {
+  _agent->executeLockedRead([&]() {
       if (_agent->readDB().has(_agencyPrefix)) {
         _snapshot = _agent->readDB().get(_agencyPrefix);
       }
@@ -582,7 +582,7 @@ void Supervision::run() {
 
     bool done = false;
     MUTEX_LOCKER(locker, _lock);
-    _agent->executeLocked([&]() {
+    _agent->executeLockedRead([&]() {
       if (_agent->readDB().has(supervisionNode)) {
         try {
           _snapshot = _agent->readDB().get(supervisionNode);