This is the mail archive of the
mailing list for the Cygwin project.
Re: fstat st_size on open files on Parallels filesystem is wrong
- From: lennox at cs dot columbia dot edu
- To: cygwin at cygwin dot com
- Date: Tue, 22 Apr 2014 12:09:45 -0400
- Subject: Re: fstat st_size on open files on Parallels filesystem is wrong
- Authentication-results: sourceware.org; auth=none
- References: <21333 dot 25325 dot 11106 dot 958642 at compute01 dot cs dot columbia dot edu> <227151856 dot 20140421223417 at yandex dot ru> <21333 dot 26515 dot 393838 dot 380071 at compute01 dot cs dot columbia dot edu> <20140422081628 dot GC2339 at calimero dot vinschen dot de>
On Tuesday, April 22 2014, "Corinna Vinschen" wrote to "cygwin at cygwin.com" saying:
> On Apr 21 14:46, lennox at cs.columbia.edu wrote:
> > On Monday, April 21 2014, "Andrey Repin" wrote to "lennox at
> > cs.columbia.edu, cygwin at cygwin.com" saying:
> > > Greetings, lennox at cs.columbia.edu!
> > >
> > > > Iâm running cygwin64 1.7.29 in a Windows 8.1 Pro virtual machine, running in
> > > > Parallels Desktop 9.0.24229 on Mac OS X 10.9.2.
> > >
> > > > Parallels Desktop automatically mounts my Mac OS X home directory as a Z:
> > > > drive in Windows. Cygwin mount reports this drive as being type "prlsf".
> > >
> > > > Unfortunately, I've discovered that if I have an open file on this
> > > > filesystem which has been written to, the size returned by Cygwin fstat() on
> > > > the open file is wrong. A stat() of the file after it's been closed is
> > > > correct.
> > >
> > > > This has the consequence that emacs always thinks saved files have been
> > > > modified externally, since emacs looks at files' sizes (as well as their
> > > > modification times) to detect external changes. This makes emacs
> > > > near-unusable.
> > >
> > > > This problem does not occur for files in my Cygwin home directory, or other
> > > > locations mounted on my Windows C: drive.
> > >
> > > > I've attached a simple unit test program that illustrates the problem.
> > > > I've also attached my cygcheck -s -v -r output.
> > >
> > > > Any ideas? Is this a Cygwin bug, a Parallels bug, or something else?
> > > > Glancing over the Cygwin code, I see that there are a few cases where fstat
> > > > has special cases for certain filesystem types.
> > >
> > > You never flushing the buffer in your test code, or I'm reading it wrong?
> > This is using Posix APIs -- open() / write() -- not C APIs, fopen() /
> > fwrite(), so there shouldn't be a buffer? Notice that the test behaves as I
> > expect for a file on NTFS.
> > Adding a call to fsync() prior to the fstat() call doesn't change anything.
> This is actually a bad sign. The problem you're describing occurs on
> NFS, too. If you write to the file, a subsequent call to fetch stat
> attributes does not return the actual size of the file, but the size at
> the time the handle has been opened.
> However, on NFS, a call to FlushFileBuffers helps to kick stat back into
> shape. That's the Win32 function called from fsync as well. What is
> Cygwin supposed to do if that doesn't work?
It's certainly possible that the file system driver in the Parallels Tools
has a bug, and if so I'm happy to report it to Parallels.
I'll see if I can reproduce the test case using just Win32 APIs. Are there
any particular gotchas I should watch out for, or should just looking at
what Win32 functions are called in fhandler_disk_file.cc be sufficient?
lennox at cs.columbia.edu
Problem reports: http://cygwin.com/problems.html
Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple