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Re: svn commit: r1040832 - Port a fix for a FSFS packing race

From: Julian Foad <julian.foad_at_wandisco.com>
Date: Thu, 02 Dec 2010 12:15:19 +0000

On Wed, 2010-12-01, Daniel Shahaf wrote:
> Julian Foad wrote on Wed, Dec 01, 2010 at 12:32:45 +0000:
> > On Wed, 2010-12-01, stefan2_at_apache.org wrote:
> > > Port (not merge) a fix for a FSFS packing race condition from the
> > > performance branch to /trunk: There is a slight time window
> > > between finding the name of a rev file and actually open it. If
> > > the revision in question gets packed just within this window,
> > > we will find the new (and final) file name with just one retry.
> > >
> > > * subversion/libsvn_fs_fs/fs_fs.c
> > > (open_pack_or_rev_file): retry once upon "missing file" error
> >
> > Hi Stefan.
> >
> > (1) I think the doc string of svn_fs_fs__path_rev_absolute() should note
> > that the returned path may no longer be valid by the time you come to
> > use it, and the reason for that. Does my patch below sound right?
> >
>
> Well, yes, and Stefan already added such a comment at the definition at
> my suggestion. But +1 to move that to the declaration.
>
> > (2) Doesn't the exact same race exist in *all* uses of
> > svn_fs_fs__path_rev_absolute()? There are five other calls to it,
>
> As far as I can tell, apart from open_pack_or_rev_file(), all callers
> always[1] run under the write lock. Since pack operation take the write
> lock too, it's not possible for the result of svn_fs_fs__path_rev_absolute()
> to become outdated for those callers.

OK, that's good. No change needed then.

> [1] with the exception of set_revision_proplist(), which can be called
> either under the write lock or under write_revision_zero() --- but in
> the latter case it's called before the format file has been created.
>
> > all of them using the result as a permissions reference to set the
> > file permissions on some new file. It seems to me that those uses can
> > fail in the same way.
> >
> > The root problem is the existence of a "get the path of this file" API
> > in the presence of semantics whereby the file might be removed from that
> > path at any time.
> >
> > Perhaps your fix is the best fix for this caller, but for the other
> > callers I think creating and using a "copy file permissions from
> > rev-file of rev R" API would be a better solution than adding re-tries
> > there. That API can do the re-try internally.
> >
> > What do you think?
> >
> >
> > Patch for (1) above:
> > [[[
> > Index: subversion/libsvn_fs_fs/fs_fs.h
> > ===================================================================
> > --- subversion/libsvn_fs_fs/fs_fs.h (revision 1040662)
> > +++ subversion/libsvn_fs_fs/fs_fs.h (working copy)
> > @@ -404,13 +404,16 @@ svn_error_t *svn_fs_fs__txn_changes_fetc
> > PERMS_REFERENCE. Temporary allocations are from POOL. */
> > svn_error_t *svn_fs_fs__move_into_place(const char *old_filename,
> > const char *new_filename,
> > const char *perms_reference,
> > apr_pool_t *pool);
> >
> > -/* Sets *PATH to the path of REV in FS, whether in a pack file or not.
> > +/* Set *PATH to the path of REV in FS, whether in a pack file or not.
> > + The path is not guaranteed to remain correct after the function returns,
> > + because it is possible that the revision will become packed just after
> > + this call, so the caller should re-try once if the path is not found.
> > Allocate in POOL. */
>
> Sounds right.
>
> Bordering on bikeshed, I would suggest some changes:
>
> * Separate describing what the function does ("Sets *PATH to FOO and
> allocates in POOL") from describing the conditions the caller should
> beware of / check for ("sometimes the return value will be out-of-date
> by the time you receive it").
>
> * Mention that the non-guarantee is not in effect if the caller has the
> write lock.

OK, committed in r1041056 with these improvements.

> > svn_error_t *
> > svn_fs_fs__path_rev_absolute(const char **path,
> > svn_fs_t *fs,
> > svn_revnum_t rev,
> > apr_pool_t *pool);
>
> Also, svn_fs_fs__path_rev_absolute() also does, internally, a single
> repeat loop. Theoretically this isn't needed any more now (since all
> callers either run under the write lock or retry)...

Can you check the attached patch for this, please?

- Julian

Received on 2010-12-02 13:15:59 CET

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