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Re: New draft of FileSystem API posted

From: Eric Uhrhane <ericu@google.com>
Date: Fri, 13 Aug 2010 10:58:28 -0700
Message-ID: <AANLkTikg7f4kpBrziTbKHbnmezhUre06bKjjaPaEmdgZ@mail.gmail.com>
To: Kinuko Yasuda <kinuko@chromium.org>
Cc: Web Applications Working Group WG <public-webapps@w3.org>
On Thu, Aug 12, 2010 at 11:08 PM, Kinuko Yasuda <kinuko@chromium.org> wrote:
> Hi Eric,
> Thanks for your reply.
> Actually after sending that email I had started to think that caching isFile
> / isDirectory information in memory would be ok if user could get an
> informative error code when an entry becomes stale -- and seems like that's
> the case.  So I'm almost convinced :)
> One thing I'm not fully convinced yet about FileEntry / DirectoryEntry is
> if we really want to make FileEntry a subclass of File.
> To me it looks like they have slightly different semantics - File is an
> immutable snapshot of file content, while Entry is more like a handle for
> meta-level operations that actually modify the underlying disk image.
> File's size attribute and slice() method assume its snapshot doesn't change
> after it was captured, but FileEntry has createWriter method and it will
> likely change the file content.  It brings several questions: if FileEntry
> == File in what timing are UAs supposed to capture a snapshot?  Do we want
> to have a synchronous size attribute on a mutable Entry?  If we move an
> Entry to another name, does its File.type need to change according to its
> new name?

You're quite right.  A mutable FileEntry shouldn't have a synchronous
size member, and slice() and type are likewise problematic.

How about if we add a getFile() method that returns a File object,
snapshotted at the time of the call?  Hmm...that name's going to be a
bit confusing, though, since there's a getFile on DirectoryEntry that
does something completely different.  Perhaps just file()?  It should
definitely be a method, not a member, to make clear that it's making
calls to the underlying filesystem.

> On Wed, Aug 11, 2010 at 1:25 PM, Eric Uhrhane <ericu@google.com> wrote:
>>
>> My apologies for the slow response; I'm now back from my vacation.
>>
>> On Tue, Jul 20, 2010 at 6:27 PM, Kinuko Yasuda <kinuko@google.com> wrote:
>> > Hi Eric,
>> > Thanks for brushing up the draft.
>> > We had some internal discussion about the API details and came up with a
>> > question regarding is{File,Directory} attributes of Entry interface.
>> > It seems like user agent needs to be able to tell if a given entry is
>> > file
>> > or directory synchronously (or from its cache), but we wonder what
>> > should
>> > happen if the underlying file object is changed later.  For example, we
>> > may
>> > have a situation like this:
>> > 1. a user code gets (or creates) a file entry.
>> > 2. another flow of the code (or another code in the same origin) removes
>> > the
>> > same entry and creates a *directory* at the same path.
>> > 3. the original code refers isFile attribute of the entry. -- should it
>> > be
>> > true or false?
>>
>> My intention was that the UA would look up that information when first
>> creating the Entry and keep it cached.  It's not the kind of thing
>> that changes very often, so I didn't think we needed it to be a live
>> query every time.  If the user tries to write to a file that's become
>> a directory, or vice-versa, that's what INVALID_STATE_ERROR is for.
>>
>> This is an unusual error, thus it's appropriate that it be handled via
>> exception.
>>
>> > If an Entry is just a reference (i.e. a pathname) of a system file
>> > entity,
>> > it would look natural that it is resolved at run-time thus returns false
>> > in
>> > the above case.   But if so we'll have two problems: 1) we'll need to
>> > make
>> > synchronous stat calls to get the attribute values, and 2) as we have
>> > different interfaces for file and directory, we may end up with having
>> > invalid FileEntry objects for directories - or vice versa.
>> > Would it be possible to have a single unified interface for file and
>> > directory and let scripts figure out the info at runtime (e.g. in each
>> > asynchronous filesystem operation)?
>>
>> Do you mean, allow all file operations and directory operation, and
>> fail if you use the wrong one?  UAs already have to fail on operations
>> that don't make sense [or fail in the underlying implementation],
>> whether or not we keep a unified interface, so I think that would just
>> clutter up each subtype with the others' methods.
>
>
>>
>> If I'm misunderstanding you, please give an example of your proposed
>> interface and a situation which it would improve.
>
> You're not misunderstanding me.   I meant if UAs didn't cache the
> information that an entry is file or directory it would be natural to make
> wrong operations fail at run-time, but otherwise I fully agree that
> cluttering up subtypes with irrelevant methods is not a good idea.
>>
>> > On Wed, Jul 7, 2010 at 6:50 PM, Eric Uhrhane <ericu@google.com> wrote:
>> >>
>> >> I've posted a new draft of File API: Directories and System [1].  In
>> >> this draft I've rolled in quite a bit of feedback that I received
>> >> since first posting it on DAP--many apologies for the delay.  This is
>> >> the first draft produced since we agreed to move this spec from DAP to
>> >> WebApps; I hope those of you who have time will give it a look and let
>> >> me know what you think.
>> >>
>> >> In general I've tried to address any comment I was sent and had not
>> >> already addressed via email.  The few that didn't make it in, I've
>> >> responded to below.
>> >>
>> >> My thanks to Robin Berjon and Mike Clement for all their feedback.
>> >>
>> >> Robin:
>> >>  - "data stored there by the application should not be deleted by the
>> >> UA without user intervention", "UA should require permission from the
>> >> user", "The application may of course delete it at will" -> these
>> >> sound like real conformance statements, therefore SHOULD, SHOULD NOT,
>> >> and MAY.
>> >>
>> >> Those are in a non-normative section; is that language still
>> >> appropriate
>> >> there?
>> >>
>> >> Robin:
>> >> [discussion about speccing the URI format]
>> >>
>> >> Left as an open issue.
>> >>
>> >> Mike:
>> >> [discussion about multiple sandboxes per origin]
>> >>
>> >> I think that would be very easy and clean to add later if desired, and
>> >> in the mean time, one can use subdirectories.
>> >>
>> >> Mike:
>> >> getFile/getDirectory are a bit overloaded.  How about including
>> >> methods like exists(), createFile() and createDirectory()?  Though
>> >> these methods are easily implemented in terms of getFile/getDirectory,
>> >> I always prefer more direct API methods that help make the code easier
>> >> to understand.  I expect, though, that you are attempting to be a low
>> >> level as possible here.
>> >>
>> >> As Robin pointed out, adding extra round-trips will slow things down.
>> >> Also, it can encourage race conditions.  These are easy for libraries
>> >> to implement via wrappers.
>> >>
>> >> Mike:
>> >> [request for creation time in getMetadata]
>> >>
>> >> It may be hard to support reliably cross-platform [2].
>> >>
>> >> Robin:
>> >> [specifying a single locale everywhere]
>> >>
>> >> I don't think that'll make folks very happy if it's not their locale.
>> >> If I e.g. try to force my locale on Turkish Windows users, they're
>> >> going to see some interesting errors trying to share files with apps
>> >> outside the browser, or for that matter even saving certain groups of
>> >> files from inside the browser.
>> >>
>> >>    Eric
>> >>
>> >> [1] http://dev.w3.org/2009/dap/file-system/file-dir-sys.html
>> >> [2] http://en.wikipedia.org/wiki/MAC_times
>> >>
>> >
>> >
>
>
Received on Friday, 13 August 2010 17:59:14 GMT

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