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Re: [IDB] Lifetime of IDB objects

From: Jonas Sicking <jonas@sicking.cc>
Date: Mon, 22 Oct 2012 02:00:27 -0700
Message-ID: <CA+c2ei8s_zhZhUu6jvCavXBkcnSewDbVUdKFy67V=80-TNiJMQ@mail.gmail.com>
To: Joćo Eiras <joaoe@opera.com>
Cc: "public-webapps@w3.org" <public-webapps@w3.org>
On Sun, Oct 21, 2012 at 5:01 PM, Joćo Eiras <joaoe@opera.com> wrote:
>
> Hi !
>
> The specification does not specify in detail what happens to several of the
> object types once they have reached their purpose.
>
> For instance, IDBTransaction's abort and objectStore methods dispatch
> InvalidStateError.
>
> However, IDBRequest and IDBCursor have properties which return other objects
> like IDBRequest.source, IDBRequest.result, IDBRequest.transaction,
> IDBCursor.source,

The intent was for these properties to return the same value as they
always did. Suggestions for how to make this more clear would be
welcome.

> IDBCursor.key, IDBCursor.primaryKey which behavior, after
> the request has completed, is undefined or defined as returning the same
> value (for source only it seems).

These too don't change their value when a transaction is committed.
The spec is hopefully pretty clear that these values are set to
'undefined' once the cursor has been iterated to the end though?

> Having these objects keeping references to other objects after they have
> completed, can represent extra memory overhead, while not very useful,
> specially if the application is data heavy, like an offline main client with
> lots of requests, or long blobs are used, and it prevents the garbage
> collector from cleaning up more than it could, specially while a transaction
> is active.
>
> I suggest that after an IDBRequest, IDBTransaction or IDBCursor complete,
> all their properties are cleared (at least the non-trivial ones) so the
> garbage collector can do it work. However, since that would cause the
> properties to return later undefined/null, it is better if they just all
> throw InvalidStateError when accessed after the object has reached it's
> purpose.

I definitely don't think we should be throwing more exceptions here. I
don't see that someone is doing something inherently wrong when trying
to access these properties after a transaction has been committed or
aborted, so throwing an exception seems like it can just introduce
breakage for authors.

Likewise, returning null/undefined for these objects can cause code
like "myrequest.source.name" to throw if accessed "too late".

I don't think that the "retaining memory" problem is a particularly
big one. Note that we'd only be retaining a small number of extra
objects at the most. Only if a page holds on to a request do we end up
keeping the store and transaction objects alive. Holding a transaction
alive never ends up holding all the requests alive.

> Btw, an error in http://www.w3.org/TR/IndexedDB/#widl-IDBCursor-source "This
> function never returns null or throws an exception". Should be "This
> property".

Would be great if you could file a bug on this since I'm likely to
forget otherwise.

/ Jonas
Received on Monday, 22 October 2012 09:01:24 GMT

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