MB-19641: Fix data race on ItemPager::phase

ItemPager::phase is accessed by multiple threads (when different
vbuckets are being processed by different executors), so it needs to
be atomic.

As reported by ThreadSanitizer:

   WARNING: ThreadSanitizer: data race (pid=180366)
     Read of size 4 at 0x7d1c0000d650 by thread T18 (mutexes: write M18437):
       #0 PagingVisitor::visit(StoredValue*) ep-engine/src/item_pager.cc:86 (ep.so+0x000000107734)
       #1 HashTable::visit(HashTableVisitor&) ep-engine/src/stored-value.cc:475 (ep.so+0x00000011d1b8)
       couchbase#2 VBCBAdaptor::run() ep-engine/src/ep.cc:3929 (ep.so+0x0000000a3c2f)
       couchbase#3 ExecutorThread::run() ep-engine/src/executorthread.cc:115 (ep.so+0x0000000fb086)

     Previous write of size 4 at 0x7d1c0000d650 by thread T17:
       #0 PagingVisitor::complete() ep-engine/src/item_pager.cc:179 (ep.so+0x000000108004)
       #1 VBCBAdaptor::run() ep-engine/src/ep.cc:3937 (ep.so+0x0000000a3d7e)
       couchbase#2 ExecutorThread::run() ep-engine/src/executorthread.cc:115 (ep.so+0x0000000fb086)

Change-Id: I9652a9f7f36f18967b6ec5903db53f3c7fb917fe
Reviewed-on: http://review.couchbase.org/64161
Tested-by: buildbot <build@couchbase.com>
Well-Formed: buildbot <build@couchbase.com>
Reviewed-by: Will Gardner <will.gardner@couchbase.com>

Author: daverigby

src/item_pager.cc

@@ -58,8 +58,8 @@ class PagingVisitor : public VBucketVisitor {
      */
     PagingVisitor(EventuallyPersistentStore &s, EPStats &st, double pcnt,
                   std::shared_ptr<AtomicValue<bool>> &sfin, pager_type_t caller,
-                  bool pause = false, double bias = 1,
-                  item_pager_phase *phase = NULL) :
+                  bool pause, double bias,
+                  std::atomic<item_pager_phase>* phase) :
         store(s), stats(st), percent(pcnt),
         activeBias(bias), ejected(0),
         startTime(ep_real_time()), stateFinalizer(sfin), owner(caller),
@@ -239,7 +239,7 @@ class PagingVisitor : public VBucketVisitor {
     bool completePhase;
     bool wasHighMemoryUsage;
     hrtime_t taskStart;
-    item_pager_phase *pager_phase;
+    std::atomic<item_pager_phase>* pager_phase;
 };
 
 ItemPager::ItemPager(EventuallyPersistentEngine *e, EPStats &st) :

src/item_pager.h

@@ -75,7 +75,10 @@ class ItemPager : public GlobalTask {
     EventuallyPersistentEngine     *engine;
     EPStats                        &stats;
     std::shared_ptr<AtomicValue<bool>>   available;
-    item_pager_phase                phase;
+
+    // Current pager phase. Atomic as may be accessed by multiple PagingVisitor
+    // objects running on different threads.
+    std::atomic<item_pager_phase> phase;
     bool                            doEvict;
 };