Avoid retain cycles in parent pointers (#2621)

All the various table drivers that need to navigate back must use weak
pointers to get there because the parent owns the children.

Author: wilhuff

Firestore/core/src/firebase/firestore/local/leveldb_remote_document_cache.h

@@ -57,7 +57,8 @@ class LevelDbRemoteDocumentCache : public RemoteDocumentCache {
   FSTMaybeDocument* DecodeMaybeDocument(absl::string_view encoded,
                                         const model::DocumentKey& key);
 
-  FSTLevelDB* db_;
+  // This instance is owned by FSTLevelDB; avoid a retain cycle.
+  __weak FSTLevelDB* db_;
   FSTLocalSerializer* serializer_;
 };
 

Firestore/core/src/firebase/firestore/local/memory_mutation_queue.h

@@ -111,7 +111,8 @@ class MemoryMutationQueue : public MutationQueue {
    */
   int IndexOfBatchId(model::BatchId batch_id);
 
-  FSTMemoryPersistence* persistence_;
+  // This instance is owned by FSTMemoryPersistence; avoid a retain cycle.
+  __weak FSTMemoryPersistence* persistence_;
   /**
    * A FIFO queue of all mutations to apply to the backend. Mutations are added
    * to the end of the queue as they're written, and removed from the front of

Firestore/core/src/firebase/firestore/local/memory_query_cache.h

@@ -95,7 +95,9 @@ class MemoryQueryCache : public QueryCache {
   void SetLastRemoteSnapshotVersion(model::SnapshotVersion version) override;
 
  private:
-  FSTMemoryPersistence* persistence_;
+  // This instance is owned by FSTMemoryPersistence; avoid a retain cycle.
+  __weak FSTMemoryPersistence* persistence_;
+
   /** The highest sequence number encountered */
   model::ListenSequenceNumber highest_listen_sequence_number_;
   /** The highest numbered target ID encountered. */

Firestore/core/src/firebase/firestore/local/memory_remote_document_cache.h

@@ -62,7 +62,8 @@ class MemoryRemoteDocumentCache : public RemoteDocumentCache {
   /** Underlying cache of documents. */
   model::MaybeDocumentMap docs_;
 
-  FSTMemoryPersistence *persistence_;
+  // This instance is owned by FSTMemoryPersistence; avoid a retain cycle.
+  __weak FSTMemoryPersistence *persistence_;
 };
 
 }  // namespace local