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Re: Request for feedback: Filesystem API

From: Janusz Majnert <j.majnert@samsung.com>
Date: Mon, 19 Aug 2013 10:01:01 +0200
Message-id: <5211D0BD.9040508@samsung.com>
To: public-script-coord@w3.org

On 2013-08-17 13:33, Jonas Sicking wrote:
> On Fri, Aug 16, 2013 at 11:50 AM, Jonas Sicking <jonas@sicking.cc> wrote:
>>> Variation: what happens if I do:
>>>
>>> var fileHandle;
>>> navigator.getFilesystem().then(function(root) {
>>>      return root.openWrite("highscores"); //highscores is 100-bytes long
>>>   }).then(function(handle) {
>>>      fileHandle = handle;
>>>      return fileHandle.read(1000); //offset is set to 1000?
>>
>> This read would result in an error.
>>
>>>   }).then(function(buffer) {
>>>      result=calculateSomething(buffer);
>>>      fileHandle.write(result); //append result at offset 100 or 1000?
>>
>> Which means that this code isn't called at all. The first argument to
>> .then() is the success callback which is only called if the promise
>> successfully returned a value.
>
> Sorry, I misread the code here.
>
> I think reading past the end of the file should not be an error,
> though that's something that I'd love to get feedback on.
IMHO reading past the EOF should not result in an error. OTOH the offset 
behaviour makes things more complicated.
Is there any reason why offset needs to be updated synchronously? From 
what you wrote, consecutive reads/writes can only be scheduled from 
inside Promise resolvers, so offset has no significant meaning outside 
of fulfill/reject callbacks. More so if, as you proposed, offset's 
setter will also be blocked by the "active" flag.

>
> If that's the case then the initial read will be successful, but would
> only return an ArrayBuffer which is 100 bytes long.
>
> The call to .read() will synchronously set .offset to 1000.
>
> That means that when .write() is called, .offset is still 1000 and so
> we will write to offset 1000 by filling the area 100-1000 with
> 0-bytes, and then write 'result' after that.
>
> If you want to make sure to always append, do something like
>
> var fileHandle;
> navigator.getFilesystem().then
> (function(root) {
>      return root.openWrite("highscores"); //highscores is 100-bytes long
>   }).then(function(handle) {
>      fileHandle = handle;
>      return fileHandle.read(1000); //offset is set to 1000
>   }).then(function(buffer) {
>      result=calculateSomething(buffer);
>      fileHandle.offset = null; // append
>      fileHandle.write(result);
>   });
>
> If you want to make sure to always write the result after the read
> buffer, do something like:
>
> var fileHandle;
> navigator.getFilesystem().then
> (function(root) {
>      return root.openWrite("highscores"); //highscores is 100-bytes long
>   }).then(function(handle) {
>      fileHandle = handle;
>      return fileHandle.read(1000); //offset is set to 1000
>   }).then(function(buffer) {
>      result=calculateSomething(buffer);
>      fileHandle.offset = buffer.byteLength;
>      fileHandle.write(result);
>   });
>
> The .offset property is simply syntax to aid with consecutive
> reads/writes. It's not actually affected by any IO operations. This
> enables doing things like
>
> navigator.getFilesystem().then
> (function(root) {
>      return root.openWrite("highscores");
>   }).then(function(handle) {
>      return Promise.all(handle.read(1000), handle.read(5), handle.read(100));
>   }).then(function(results) {
>      // results is an array with 3 ArrayBuffers
>   });
>
> I.e. you don't need to wait for the previous IO operation to finish
> before scheduling the next one. In fact, there are performance
> benefits to not waiting.
>
> / Jonas
>
>

-- 
Janusz Majnert
Samsung R&D Institute Poland
Samsung Electronics
Received on Monday, 19 August 2013 08:02:11 UTC

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