Some fleshing out the mojo based localstorage implementation.

content/common/leveldb_wrapper.mojom

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module content.mojom;
 
 import "components/leveldb/public/interfaces/leveldb.mojom";
 
 // Gives information about changes to a LevelDB database.
 // The reason this is a parameter to DeleteAll and GetAll below, instead of
 // being specified when opening a LevelDBWrapper, is to avoid the client getting
 // callbacks for changes that have already been applied to its database that
 // it's fetching via GetAll or it's clearing via DeleteAll.
 // In the methods below, |source| is a user-defined string which was passed to
 // the various LevelDBWrapper methods.
 // Note that observer methods are called before the callbacks for the
 // LevelDBWrapper methods are run.
 interface LevelDBObserver {
+  KeyAdded(array<uint8> key, array<uint8> value, string source);
   KeyChanged(array<uint8> key, array<uint8> new_value, array<uint8> old_value,
              string source);
   KeyDeleted(array<uint8> key, array<uint8> old_value, string source);
   AllDeleted(string source);
+  GetAllComplete(uint64 request_id);

[jam, 2016/03/30 17:15:16]

for consistency with the other methods, GetAll should take a "string source" and
this method should spit it back out

[michaeln, 2016/03/30 21:57:01]

Done

The magic cookie value for the localstorage usecase  is different for this
method compared to the others, but thats ok.

 };
 
 struct KeyValue {
   array<uint8> key;
   array<uint8> value;
 };
 
 // A wrapper around leveldb that supports giving notifications when values
 // change.
 interface LevelDBWrapper {
+  AddObserver(LevelDBObserver observer);

[jam, 2016/03/30 17:15:16]

how about combining this with StoragePartitionService::OpenLocalStorage? i.e. we
make it in lockstep that when one opens a database they specify an optional
observer.

the benefit would be to ensure that notifications aren't dropped.

[michaeln, 2016/03/30 21:57:01]

Done.

+
   // Sets the database entry for |key| to |value|. Returns OK on success.
-  Put(array<uint8> key, array<uint8> value, string source)
-    => (leveldb.DatabaseError status);
+  Put(array<uint8> key, array<uint8> value, string source) => (bool success);
 
   // Remove the database entry (if any) for |key|.  Returns OK on success, and a
   // non-OK status on error.  It is not an error if |key| did not exist in the
   // database.
-  Delete(array<uint8> key, string source) => (leveldb.DatabaseError status);
+  Delete(array<uint8> key, string source) => (bool success);
 
   // Removes all the entries.
-  DeleteAll(LevelDBObserver observer, string source)
-    => (leveldb.DatabaseError status);
-
-  // Returns the value of the |key|.
-  Get(array<uint8> key) => (leveldb.DatabaseError status, array<uint8> value);
+  DeleteAll(string source) => (bool success);
 
   // Only used with small databases. Returns all key/value pairs.
   [Sync]
-  GetAll(LevelDBObserver observer)
+  GetAll(uint64 reqeust_id)
     => (leveldb.DatabaseError status, array<KeyValue> data);
 };