From 582d3e5fd18d3056eadb2ab8b76f331b36f037c2 Mon Sep 17 00:00:00 2001
From: Jan <jsteemann@users.noreply.github.com>
Date: Wed, 26 Jul 2017 21:49:41 +0200
Subject: [PATCH] only sync RocksDB WAL in MMFiles engine when there is a
 reason to do so (#2863)

---
 arangod/MMFiles/MMFilesTransactionState.cpp | 18 ++++++++++++++++--
 arangod/StorageEngine/TransactionState.cpp  | 10 ++++++++++
 arangod/StorageEngine/TransactionState.h    |  3 +++
 3 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/arangod/MMFiles/MMFilesTransactionState.cpp b/arangod/MMFiles/MMFilesTransactionState.cpp
index befd8b6b6c..ebc4c761a1 100644
--- a/arangod/MMFiles/MMFilesTransactionState.cpp
+++ b/arangod/MMFiles/MMFilesTransactionState.cpp
@@ -478,8 +478,22 @@ int MMFilesTransactionState::writeCommitMarker() {
     TRI_IF_FAILURE("TransactionWriteCommitMarkerNoRocksSync") { return TRI_ERROR_NO_ERROR; }
 
     if (_options.waitForSync) {
-      // also sync RocksDB WAL
-      MMFilesPersistentIndexFeature::syncWal();
+      // also sync RocksDB WAL if required
+      bool hasPersistentIndex = false;
+      allCollections([&hasPersistentIndex](TransactionCollection* collection) {
+        auto c = static_cast<MMFilesTransactionCollection*>(collection);
+        if (c->canAccess(AccessMode::Type::WRITE) && 
+            c->collection()->getPhysical()->hasIndexOfType(arangodb::Index::TRI_IDX_TYPE_PERSISTENT_INDEX)) {
+          hasPersistentIndex = true;
+          // abort iteration
+          return false;
+        }
+
+        return true;
+      });
+      if (hasPersistentIndex) {
+        MMFilesPersistentIndexFeature::syncWal();
+      }
     }
     
     TRI_IF_FAILURE("TransactionWriteCommitMarkerThrow") { 
diff --git a/arangod/StorageEngine/TransactionState.cpp b/arangod/StorageEngine/TransactionState.cpp
index 932a792c5d..194abf29df 100644
--- a/arangod/StorageEngine/TransactionState.cpp
+++ b/arangod/StorageEngine/TransactionState.cpp
@@ -198,6 +198,16 @@ int TransactionState::addCollection(TRI_voc_cid_t cid,
 Result TransactionState::ensureCollections(int nestingLevel) {
   return useCollections(nestingLevel);
 }
+  
+/// @brief run a callback on all collections
+void TransactionState::allCollections(std::function<bool(TransactionCollection*)> const& cb) {
+  for (auto& trxCollection : _collections) {
+    if (!cb(trxCollection)) {
+      // abort early
+      return;
+    }
+  }
+}
 
 /// @brief use all participating collections of a transaction
 Result TransactionState::useCollections(int nestingLevel) {
diff --git a/arangod/StorageEngine/TransactionState.h b/arangod/StorageEngine/TransactionState.h
index a1bd898493..cbca9c39a8 100644
--- a/arangod/StorageEngine/TransactionState.h
+++ b/arangod/StorageEngine/TransactionState.h
@@ -123,6 +123,9 @@ class TransactionState {
   /// @brief use all participating collections of a transaction
   Result useCollections(int nestingLevel);
 
+  /// @brief run a callback on all collections
+  void allCollections(std::function<bool(TransactionCollection*)> const& cb);
+
   /// @brief release collection locks for a transaction
   int unuseCollections(int nestingLevel);