SERVER-38597 Add drop-pending ident counter to serverStatus

jstests/noPassthrough/rollback_wt_drop.js

@@ -74,6 +74,9 @@
                           mydb.getName() + ' after dropping collection ' + coll.getFullName() +
                           '. Before: ' + tojson(collectionsBeforeDrop) + '. After: ' +
                           tojson(collectionsAfterDrop));
+            assert.gt(mydb.serverStatus().storageEngine.dropPendingIdents,
+                      0,
+                      'There is no drop pending ident in the storage engine.');
         }
 
         const renameTargetColl = node.getCollection(renameTargetCollName);

src/mongo/db/catalog/catalog_control_test.cpp

@@ -91,6 +91,9 @@ public:
     std::string getFilesystemPathForDb(const std::string& dbName) const final {
         return "";
     }
+    std::set<std::string> getDropPendingIdents() const final {
+        return {};
+    }
 };
 
 /**

src/mongo/db/storage/kv/kv_storage_engine.h

@@ -355,6 +355,10 @@ public:
         return _timestampMonitor.get();
     }
 
+    std::set<std::string> getDropPendingIdents() const {
+        return _dropPendingIdentReaper.getAllIdents();
+    }
+
 private:
     using CollIter = std::list<std::string>::iterator;
 

src/mongo/db/storage/storage_engine.h

@@ -354,6 +354,11 @@ public:
      */
     virtual bool supportsPendingDrops() const = 0;
 
+    /**
+     * Returns a set of drop pending idents inside the storage engine.
+     */
+    virtual std::set<std::string> getDropPendingIdents() const = 0;
+
     /**
      * Clears list of drop-pending idents in the storage engine.
      * Used primarily by rollback after recovering to a stable timestamp.

src/mongo/db/storage/storage_init.cpp

@@ -60,6 +60,8 @@ public:
                            << engine->supportsReadConcernMajority()
                            << "supportsPendingDrops"
                            << engine->supportsPendingDrops()
+                           << "dropPendingIdents"
+                           << static_cast<long long>(engine->getDropPendingIdents().size())
                            << "supportsSnapshotReadConcern"
                            << engine->supportsReadConcernSnapshot()
                            << "readOnly"