[IndexedDB] Adding transaction and value support to observers

content/browser/indexed_db/indexed_db_connection.h

 // Copyright 2013 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.
 
 #ifndef CONTENT_BROWSER_INDEXED_DB_INDEXED_DB_CONNECTION_H_
 #define CONTENT_BROWSER_INDEXED_DB_INDEXED_DB_CONNECTION_H_
 
 #include <memory>
 #include <set>
 #include <unordered_map>
@@ skipping 59 matching lines @@
 
   IndexedDBTransaction* GetTransaction(int64_t id) const;
 
   base::WeakPtr<IndexedDBTransaction> AddTransactionForTesting(
       std::unique_ptr<IndexedDBTransaction> transaction);
 
   // We ignore calls where the id doesn't exist to facilitate the AbortAll call.
   // TODO(dmurph): Change that so this doesn't need to ignore unknown ids.
   void RemoveTransaction(int64_t id);
 
+  // Returns a new transaction id for an observer transaction.
+  // Unique per connection.
+  int64_t NewObserverTransactionId();
+
  private:
   const int32_t id_;
 
+  // The renderer-created transactions are in the right 32 bits, and the

[palmer, 2017/01/18 19:56:01]

Nit: Clarify: "...the right 32 bits of a transaction ID..."

But, it sounds like TransactionID should be of a type that documents itself
better?

struct TransactionID {
  int64_t renderer_created_id : 32;
  int64_t browser_created_id : 32;
};

or, if you ever need to refer to the whole 64-bit object at once:

struct TransationID {
  union {
    int64_t storage;
    int32_t renderer_created_id;
    int32_t browser_created_id;
  };
};

or similarly:

struct TransationID {
  union {
    int64_t storage;
    int64_t renderer_created_id : 32;
    int64_t browser_created_id : 32;
  };
};

...or any number of other ideas.

And then you can say:

int32_t next_browser_created_id = 1;

without needing any comments.

(Note also the int32_t vs. uint32_t mismatch, and the terminology mismatch:
"browser-created" vs. "observer" — pick 1 and use it consistently.)

[dmurph, 2017/01/18 21:00:57]

I tried to clarify. I'm going to add a bug for updating the Transaction id
scheme, we have it as an int64 everywhere and it would be nice to change it. 
https://bugs.chromium.org/p/chromium/issues/detail?id=682378

+  // browser-created transactions are in the left 32 bits.
+  // This keeps track of the left 32 bits.
+  uint32_t next_observer_transaction_id_ = 1;
+
   // The process id of the child process this connection is associated with.
   // Tracked for IndexedDBContextImpl::GetAllOriginsDetails and debugging.
   const int child_process_id_;
 
   // NULL in some unit tests, and after the connection is closed.
   scoped_refptr<IndexedDBDatabase> database_;
 
   // The connection owns transactions created on this connection.
   std::unordered_map<int64_t, std::unique_ptr<IndexedDBTransaction>>
       transactions_;
 
   // The callbacks_ member is cleared when the connection is closed.
   // May be NULL in unit tests.
   scoped_refptr<IndexedDBDatabaseCallbacks> callbacks_;
   std::vector<std::unique_ptr<IndexedDBObserver>> active_observers_;
   base::WeakPtrFactory<IndexedDBConnection> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(IndexedDBConnection);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_INDEXED_DB_INDEXED_DB_CONNECTION_H_