MB-47604: Make it possible to swap Flush::manifest

NexusKVStore needs to be able to swap the VB::Manifest of a
Collections::Flush object so that the copy created for the secondary
KVStore does not break the stats tracked in the primary VB::Manifest.

Change-Id: I3a14dbb18422cb6574214b711bc88fd1c9a99504
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/158672
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Paolo Cocchi <paolo.cocchi@couchbase.com>

engines/ep/src/collections/flush.cc

@@ -43,7 +43,7 @@ void Flush::saveCollectionStats(
         }
 
         // Get the current stats of the collection (for the seqno).
-        auto collsFlushStats = manifest.lock().getStatsForFlush(
+        auto collsFlushStats = manifest.get().lock().getStatsForFlush(
                 cid, flushStats.getPersistedHighSeqno());
 
         // Generate new stats, add the deltas from this flush batch for count
@@ -93,7 +93,8 @@ uint32_t Flush::countNonEmptyDroppedCollections() const {
         if (sItr == flushAccounting.getStats().end() ||
             !sItr->second.itemInBatch()) {
             const auto highSeqno =
-                    manifest.lock()
+                    manifest.get()
+                            .lock()
                             .getStatsForFlush(cid, dropped.endSeqno)
                             .highSeqno;
             if (highSeqno != 0 && highSeqno != dropped.startSeqno) {
@@ -130,7 +131,7 @@ void Flush::forEachDroppedCollection(
 // longer applicable and are not pushed to the VB::Manifest.
 void Flush::postCommitMakeStatsVisible() {
     for (const auto& [cid, flushStats] : flushAccounting.getStats()) {
-        auto lock = manifest.lock(cid);
+        auto lock = manifest.get().lock(cid);
         if (!lock.valid() ||
             lock.isLogicallyDeleted(flushStats.getPersistedHighSeqno())) {
             // Can be flushing for a dropped collection (no longer in the
@@ -161,7 +162,7 @@ void Flush::flushSuccess(Vbid vbid, EPBucket& bucket) {
 void Flush::notifyManifestOfAnyDroppedCollections() {
     for (const auto& [cid, droppedData] :
          flushAccounting.getDroppedCollections()) {
-        manifest.collectionDropPersisted(cid, droppedData.endSeqno);
+        manifest.get().collectionDropPersisted(cid, droppedData.endSeqno);
     }
 }
 
@@ -603,7 +604,7 @@ flatbuffers::DetachedBuffer Flush::encodeOpenScopes(
 }
 
 bool Flush::droppedCollectionsExists() const {
-    return manifest.isDropInProgress();
+    return manifest.get().isDropInProgress();
 }
 
 FlushAccounting::StatisticsUpdate Flush::getDroppedStats(CollectionID cid) {
@@ -670,4 +671,8 @@ void Flush::updateStats(const DocKey& key,
     }
 }
 
+void Flush::setManifest(Manifest& newManifest) {
+    manifest = newManifest;
+}
+
 } // namespace Collections::VB

engines/ep/src/collections/flush.h

@@ -128,6 +128,14 @@ class Flush {
      */
     void setManifestUid(ManifestUid uid);
 
+    /**
+     * Set the manifest to the given manifest. Required by the NexusKVStore to
+     * generate a VB::Commit/Collections::Flush to track collections stats
+     * for the secondary KVStore without altering the stats tracked in the
+     * primary VB::Manifest.
+     */
+    void setManifest(Manifest& newManifest);
+
     /**
      * Called after a flush was successful so that purging can be triggered and
      * statistic changes applied.
@@ -351,7 +359,7 @@ class Flush {
      * ref to the 'parent' manifest for this VB::Flusher, this will receive item
      * count updates
      */
-    Manifest& manifest;
+    std::reference_wrapper<Manifest> manifest;
 
     /**
      * Flushing counts how many non-empty collections were committed and uses