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Re: Colliding FileWriters

From: Jonas Sicking <jonas@sicking.cc>
Date: Wed, 11 Jan 2012 12:25:57 -0800
Message-ID: <CA+c2ei8_E1nZNr+ZkTfLEtDUsvpErAePGfcoYpR9u_163yEjiQ@mail.gmail.com>
To: Eric U <ericu@google.com>
Cc: Webapps WG <public-webapps@w3.org>
On Tue, Jan 10, 2012 at 1:32 PM, Eric U <ericu@google.com> wrote:
> On Tue, Jan 10, 2012 at 1:08 PM, Jonas Sicking <jonas@sicking.cc> wrote:
>> Hi All,
>> We've been looking at implementing FileWriter and had a couple of questions.
>> First of all, what happens if multiple pages create a FileWriter for
>> the same FileEntry at the same time? Will both be able to write to the
>> file at the same time and whoever writes lasts to a given byte wins?
> This isn't currently specified, and that's a hole we should fill.  By
> not having it in the spec, my assumption would be that last-wins would
> hold, but it would be good to clarify it if that's the behavior we
> want.  It's especially important given that there's nothing like
> fflush(), which would help users know what "last" meant.  Speaking of
> which, should we add a flushing mechanism?
>> This is different from how file systems normally work since as long as
>> file is open for writing that tends to prevent other processes from
>> opening the same file.
> You're perhaps thinking of windows, where by default files are opened
> in exclusive mode?  On other operating systems, and on windows when
> you specify FILE_SHARE_WRITE in dwShareMode in CreateFile, multiple
> writers can exist simultaneously.

Ah. I didn't realize this was different on other OSs. It still seems
risky to not provide any means to get exclusive access. The only way I
can see websites dealing with this is to create their own locking
mechanism backed by using IndexedDB transactions as low-level atomic
primitive (local-storage doesn't work since you can implement
compare-and-swap in an atomic manner).

Having a 'exclusive' flag for createFileWriter seems much easier and
removes the IndexedDB dependency. I'd probably even say that it should
default to true since on the web defaulting to safe rather than fast
generally results in fewer bugs.

>> A second question is why is FileEntry.createWriter asynchronous? It
>> doesn't actually do any IO and so it seems like it could return an
>> answer synchronously.
> FileWriter has a synchronous length property, just as Blob does, so it
> needs to do IO at creation time to look it up.

So how does this work if you have two tabs running in different
processes create FileWriters for the same FileEntry. Each tab could
end up changing the file's size in which case the the other tabs
FileWriter will either have to synchronously update its .length, or it
will have an outdated length.

So the IO you do when creating the FileWriter is basically unreliable
as soon as it's done.

So it seems like you could get the size when creating the FileEntry
and then use that cached size when creating FileWriter instance.

Though I wonder if it wouldn't be better to remove the .length
property. If anything we could add a asynchronous length getter or a
write method which appends to the end of the file (since writing is
already asynchronous).

Though if we add the 'exclusive' flag described above, then we'll need
to keep createFileWriter async anyway.

>> Would this also explain why FileEntry.getFile is asynchronous? I.e. it
>> won't call it's callback until all current FileWriters have been
>> closed?
> Nope.  It's asynchronous because a File is a Blob, and has a
> synchronous length accessor, so we look up the length when we mint the
> File.  Note that the length can go stale if you have multiple writers,
> as we want to keep it fast.

This reminds me of something else that I intended to ask. I seem to
recall that you guys invalidate existing File instances pointing to a
FileEntry if the file is modified after the File object is
instantiated? How is this implemented? Especially given that the
FileWriter which modified the file might live in a different process
than the File reference. Do you guys grab a time-stamp when the File
instance is created and then check that against the last-modified time
of the os-file? What happens if the user modifies the OS time?

/ Jonas
Received on Wednesday, 11 January 2012 20:27:07 UTC

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