Avoid disk accesses on the wrong thread in URLRequestFileJob

Avoid disk accesses on the wrong thread in URLRequestFileJob
while opening and positioning in the file.

Relanding of https://chromiumcodereview.appspot.com/10695110/. First attempt to
commit was reverted because FileStream blocked deletion of temporary files in
tests on Windows. After https://chromiumcodereview.appspot.com/11440008/ FileStream
always opens files with PLATFORM_FILE_SHARE_DELETE allowing tests to successfully
delete temporary files. Thus this CL should be safe to commit now (without changes
compared to initial commit).

BUG=59849, 114783
TEST=net_unittests

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=173425

[wtc, 2012-12-11 22:34:03]

Patch set 1 LGTM.

Can you explain why opening files with the FILE_SHARE_DELETE
flag on Windows fixed the test failures? After reading your
comment in
https://chromiumcodereview.appspot.com/11440008/diff/1/net/base/file_stream_c...
I still don't understand it.

Does the FILE_SHARE_DELETE flag allow a DeleteFile() call
to succeed when there is still an open handle to the file?
I saw this documented in the DeleteFile MSDN page:

  The DeleteFile function fails if an application attempts
  to delete a file that has other handles open for normal
  I/O or as a memory-mapped file (FILE_SHARE_DELETE must
  have been specified when other handles were opened).

I think your comment should include such additional info.

[pivanof, 2012-12-11 22:45:26]

On 2012/12/11 22:34:03, wtc wrote:
> Does the FILE_SHARE_DELETE flag allow a DeleteFile() call
> to succeed when there is still an open handle to the file?

That's right, if all open handles were open with FILE_SHARE_DELETE then
DeleteFile() will succeed.

> I saw this documented in the DeleteFile MSDN page:
> 
>   The DeleteFile function fails if an application attempts
>   to delete a file that has other handles open for normal
>   I/O or as a memory-mapped file (FILE_SHARE_DELETE must
>   have been specified when other handles were opened).
> 
> I think your comment should include such additional info.

I've included additional info about this in the commit message. PTAL.

[wtc, 2012-12-12 20:07:04]

On 2012/12/11 22:45:26, pivanof wrote:
> 
> I've included additional info about this in the commit message. PTAL.

Thanks -- it's important to say "temporary files with open handles".

Note: I actually just wanted you to improve your comment in
net/base/file_stream_context.cc, which is more important than
the commit message for this CL. Thanks.

[pivanof, 2012-12-17 05:22:55]

OK. I commit this CL as is and make a comment improvement in the separate CL.

[commit-bot: I haz the power, 2012-12-17 05:23:15]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/paivanof@gmail.com/11490024/1

[commit-bot: I haz the power, 2012-12-17 07:11:05]

Change committed as 173425