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Re: [FileAPI] Deterministic release of Blob proposal

From: Arun Ranganathan <aranganathan@mozilla.com>
Date: Tue, 6 Mar 2012 13:27:03 -0800 (PST)
To: Feras Moussa <ferasm@microsoft.com>
Cc: Adrian Bateman <adrianba@microsoft.com>, public-webapps@w3.org, Ian Hickson <ian@hixie.ch>, Anne van Kesteren <annevk@opera.com>
Message-ID: <1421190408.6079926.1331069223949.JavaMail.root@zimbra1.shared.sjc1.mozilla.com>
Feras, 

In practice, I think this is important enough and manageable enough to include in the spec., and I'm willing to slow the train down if necessary, but I'd like to understand a few things first. Below: 

----- Original Message -----

> At TPAC we discussed the ability to deterministically close blobs
> with a few
> others.

> As we’ve discussed in the createObjectURL thread[1], a Blob may
> represent
> an expensive resource (eg. expensive in terms of memory, battery, or
> disk
> space). At present there is no way for an application to
> deterministically
> release the resource backing the Blob. Instead, an application must
> rely on
> the resource being cleaned up through a non-deterministic garbage
> collector
> once all references have been released. We have found that not having
> a way
> to deterministically release the resource causes a performance impact
> for a
> certain class of applications, and is especially important for mobile
> applications
> or devices with more limited resources.

> In particular, we’ve seen this become a problem for media intensive
> applications
> which interact with a large number of expensive blobs. For example, a
> gallery
> application may want to cycle through displaying many large images
> downloaded
> through websockets, and without a deterministic way to immediately
> release
> the reference to each image Blob, can easily begin to consume vast
> amounts of
> resources before the garbage collector is executed.

> To address this issue, we propose that a close method be added to the
> Blob
> interface.
> When called, the close method should release the underlying resource
> of the
> Blob, and future operations on the Blob will return a new erro r , a
> ClosedError.
> This allows an application to signal when it's finished using the
> Blob.

Do you agree that Transferable (http://dev.w3.org/html5/spec/Overview.html#transferable-objects) seems to be what we're looking for, and that Blob should implement Transferable? 

Transferable addresses the use case of copying across threads, and "neuters" the source object (though honestly, the word "neuter" makes me wince -- naming is a problem on the web). We can have a more generic method on Transferable that serves our purpose here, rather than *.close(), and Blob can avail of that. This is something we can work out with HTML, and might be the right thing to do for the platform (although this creates something to think about for MessagePort and for ArrayBuffer, which also implement Transferable). 

I agree with your changes, but am confused by some edge cases: 

> To support this change, the following changes in the File API spec
> are needed:

> * In section 6 (The Blob Interface)
> - Addition of a close method. When called, the close method releases
> the
> underlying resource of the Blob. Close renders the blob invalid, and
> further
> operations such as URL.createObjectURL or the FileReader read methods
> on
> the closed blob will fail and return a ClosedError. If there are any
> non-revoked
> URLs to the Blob, these URLs will continue to resolve until they have
> been
> revoked.
> - For the slice method, state that the returned Blob is a new Blob
> with its own
> lifetime semantics – calling close on the new Blob is independent of
> calling close
> on the original Blob.

> *In section 8 (The FIleReader Interface)
> - State the FileReader reads directly over the given Blob, and not a
> copy with
> an independent lifetime.

> * In section 10 (Errors and Exceptions)
> - Addition of a ClosedError. If the File or Blob has had the close
> method called,
> then for asynchronous read methods the error attribute MUST return a
> “ClosedError” DOMError and synchronous read methods MUST throw a
> ClosedError exception.

> * In section 11.8 (Creating and Revoking a Blob URI)
> - For createObjectURL – If this method is called with a closed Blob
> argument,
> then user agents must throw a ClosedError exception.

> Similarly to how slice() clones the initial Blob to return one with
> its own
> independent lifetime, the same notion will be needed in other APIs
> which
> conceptually clone the data – namely FormData, any place the
> Structured Clone
> Algorithm is used, and BlobBuilder.
> Similarly to how FileReader must act directly on the Blob’s data, the
> same notion
> will be needed in other APIs which must act on the data - namely
> XHR.send and
> WebSocket. These APIs will need to throw an error if called on a Blob
> that was
> closed and the resources are released.
So Blob.slice() already presumes a new Blob, but I can certainly make this clearer. And I agree with the changes above, including the addition of something liked ClosedError (though I suppose this is an important enough error + exception to hash out with HTML and DOM4, and once again, the name is TBD). 

In your implementation, what happens exactly to Eric's edge cases, namely: 

xhr.send(blob); 
blob.close(); // method name TBD 

// AND 

frameRef.src = URL.createObjectURL(blob); 
blob.close() // method name TBD 

In my opinion, the first (using xhr) should succeed. In the second, frameRef.src works, but subsequent attempts to mint a Blob URI for the same 'blob' resource fail. Does this hold true for you? 

-- A* 
Received on Tuesday, 6 March 2012 21:27:32 GMT

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