Re: fatal: unable to write sha1 file git 1.6.2.1

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mar 24, 2009, 22:30 UTC
Message-ID: <alpine.LFD.2.00.0903241510010.3032@localhost.localdomain>
In-Reply-To: <49C94B02.5010604@mycircuit.org>

On Tue, 24 Mar 2009, Peter wrote:

Show 7 quoted lines

> > 
> > What OS? What filesystem? Are you perhaps running out of space?
>   
> Its Lenny Debian 5.0.0, Diskspace is ample . Filesystem is cifs ( this is a
> windows 2000 share mounted with samba in a VMware Workstation Debian client (
> yes, I know ... )). Memory usage, according to htop, is constant = 140/504 MB
> during the whole process until git fails.

Ok, it sounds very much like a possible CIFS problem.

Getting the exact error code for the "close()" will be interesting, because the only thing that can return an error under Linux in close() is if the filesystem "->flush()" function fails with an error.

In cifs, the cifs_flush() thing does a filemap_fdatawrite(), forcing the data out, and that in turn calls do_writepages() which in turn calls either generic_writepages() or cifs_writepages() depending on random cifs crap.

I'm not seeing any obvious errors there. But I would _not_ be surprised if the fchmod(fd, 0444) that we did before the close could be causing this: cifs gets confused and thinks that it must not write to the file because the file has been turned read-only.

Here's an idea: if this is reproducible for you, does the behavior change if you do

	[core]
		core.fsyncobjectfiles = true

in your .git/config file? That causes git to always fsync() the written data _before_ it does that fchmod(), which in turn means that by the time the close() rolls around, there should be no data to write, and thus no possibility that anybody gets confused when there is still dirty data on a file that has been marked read-only.

Anyway, I'm cc'ing Steve French and Jeff Layton, as the major CIFS go-to guys. It does seem like a CIFS bug is likely.

Steve, Jeff: git does basically

	open(".git/objects/xy/tmp_obj_xyzzy", O_RDWR|O_CREAT|O_EXCL, 0600) = 5
	write(5, ".."..., len)
	fchmod(5, 0444)
	close(5)
	link(".git/objects/xy/tmp_obj_xyzzy", ".git/objects/xy/xyzzy");
	unlink(".git/objects/xy/tmp_obj_xyzzy");

to write a new datafile. Under CIFS, that "close()" apparently sometimes returns with an error, and fails.

I guess we could change the "fchmod()" to happen after the close(), just to make it easier for filesystems to get this right. And yes, as outlined above, there's a config option to make git use fdatasync() before it does that fchmod() too. But it does seem like CIFS is just buggy.

If CIFS has problems with the above sequence (say, if some timeout refreshes the inode data or causes a re-connect with the server or whatever), then maybe cifs should always do an implicit fdatasync() when doing fchmod(), just to make sure that the fchmod won't screw up any cached dirty data?

		Linus