MB-56094: Update CM::numItems at ItemExpel

Quantity correctly updated in the CheckpointRemoval path, but we missed
to update it when items are released in the Expel path.

Change-Id: Ic05cbb3b9c98c6c9073b9b6f74fdfd6604cdcb74
Reviewed-on: https://review.couchbase.org/c/kv_engine/+/188832
Tested-by: Paolo Cocchi <paolo.cocchi@couchbase.com>
Reviewed-by: Vesko Karaganev <vesko.karaganev@couchbase.com>

engines/ep/src/checkpoint_manager.cc

@@ -687,6 +687,7 @@ CheckpointManager::expelUnreferencedCheckpointItems() {
     }
 
     stats.itemsExpelledFromCheckpoints.fetch_add(numItemsExpelled);
+    numItems.fetch_sub(numItemsExpelled);
 
     // queueLock already released here, O(N) deallocation is lock-free
     const auto queuedItemsMemReleased = extractRes.deleteItems();

engines/ep/tests/module_tests/checkpoint_test.cc

@@ -2216,7 +2216,9 @@ void CheckpointTest::testExpelCheckpointItems() {
      * 1002 - 2nd item (key1) <<<<<<< persistenceCursor
      * 1003 - 3rd item (key2)
      */
-    EXPECT_EQ(1 + itemCount, manager->getNumOpenChkItems());
+    const size_t expectedNumItemsPre = 1 + itemCount;
+    EXPECT_EQ(expectedNumItemsPre, manager->getNumOpenChkItems());
+    EXPECT_EQ(expectedNumItemsPre, manager->getNumItems());
 
     const auto expelResult = manager->expelUnreferencedCheckpointItems();
     EXPECT_EQ(2, expelResult.count);
@@ -2240,8 +2242,9 @@ void CheckpointTest::testExpelCheckpointItems() {
     }
 
     // 1 mutation removed from checkpoint
-    EXPECT_EQ(1 + (itemCount - expelResult.count),
-              manager->getNumOpenChkItems());
+    const size_t expectedNumItemsPost = 1 + (itemCount - expelResult.count);
+    EXPECT_EQ(expectedNumItemsPost, manager->getNumOpenChkItems());
+    EXPECT_EQ(expectedNumItemsPost, manager->getNumItems());
 
     // Try to register a DCP replication cursor from 1001 - an expelled item.
     std::string dcp_cursor1(DCP_CURSOR_PREFIX + std::to_string(1));