[IndexedDB] Adding transaction and value support to observers

content/browser/indexed_db/indexed_db_connection.h

Index: content/browser/indexed_db/indexed_db_connection.h
diff --git a/content/browser/indexed_db/indexed_db_connection.h b/content/browser/indexed_db/indexed_db_connection.h
index 20acb7b7244c0ed10041c8a59b955c37bf2c2419..818a29240d31cf8eeca4846b0bfa7d3c5bd104a0 100644
--- a/content/browser/indexed_db/indexed_db_connection.h
+++ b/content/browser/indexed_db/indexed_db_connection.h
@@ -77,9 +77,18 @@ class CONTENT_EXPORT IndexedDBConnection {
   // 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_;