fix assertion

arangod/Wal/Slots.cpp

@@ -146,7 +146,13 @@ SlotInfo Slots::nextUnused(TRI_voc_tick_t databaseId, TRI_voc_cid_t collectionId
       Slot* slot = &_slots[_handoutIndex];
       TRI_ASSERT(slot != nullptr);
 
-      if (slot->isUnused()) {
+      // check if next slot is free for writing
+      // and also is the slot following it is also free for writing
+      // this is required because in some cases we need two free slots
+      // to write a WAL entry: the first slot for the prologue marker
+      // and the second slot for the actual marker
+      if (slot->isUnused() && 
+          _slots[nextHandoutIndex()].isUnused()) {
         if (hasWaited) {
           CONDITION_LOCKER(guard, _condition);
           TRI_ASSERT(_waiting > 0);
@@ -275,13 +281,14 @@ SlotInfo Slots::nextUnused(TRI_voc_tick_t databaseId, TRI_voc_cid_t collectionId
     CONDITION_LOCKER(guard, _condition);
     if (!hasWaited) {
       ++_waiting;
+      _logfileManager->signalSync(true);
       hasWaited = true;
     }
 
     bool mustWait;
     {
       MUTEX_LOCKER(mutexLocker, _lock);
-      mustWait = (_freeSlots == 0);
+      mustWait = (_freeSlots < 2);
     }
 
     if (mustWait) {
@@ -581,7 +588,7 @@ int Slots::closeLogfile(Slot::TickType& lastCommittedTick, bool& worked) {
     bool mustWait;
     {
       MUTEX_LOCKER(mutexLocker, _lock);
-      mustWait = (_freeSlots == 0);
+      mustWait = (_freeSlots < 2);
     }
 
     if (mustWait) {
@@ -667,6 +674,16 @@ Slot::TickType Slots::handout() {
   return _lastAssignedTick;
 }
 
+/// @brief return the next slots that would be handed out, without 
+/// actually handing it out
+size_t Slots::nextHandoutIndex() const {
+  size_t handoutIndex = _handoutIndex;
+  if (++handoutIndex == _numberOfSlots) {
+    handoutIndex = 0;
+  }
+  return handoutIndex;
+}
+
 /// @brief wait until all data has been synced up to a certain marker
 bool Slots::waitForTick(Slot::TickType tick) {
   static uint64_t const SleepTime = 10000;

arangod/Wal/Slots.h

@@ -137,6 +137,10 @@ class Slots {
   /// @brief handout a region and advance the handout index
   Slot::TickType handout();
 
+  /// @brief return the next slots that would be handed out, without 
+  /// actually handing it out
+  size_t nextHandoutIndex() const;
+
   /// @brief wait until all data has been synced up to a certain marker
   bool waitForTick(Slot::TickType);