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Re: git archive generates tar with malformed pax extended attribute

On Wed, May 29, 2019 at 07:54:44PM +0200, René Scharfe wrote:

> Am 29.05.19 um 03:17 schrieb Jeff King:
> > On Wed, May 29, 2019 at 01:34:32AM +0200, René Scharfe wrote:
> >> Parsing trees with symlinks twice is not ideal, but keeps the set
> >> structure simple -- a standard oidset suffices.
> >
> > If blobs comes after trees (and they usually do in a pack), you can do
> > it in a single pass by marking the blob as a symlink target, and then
> > when we actually see that blob's contents, marking it as either OK or
> > problematic. And then the finish() step just correlates those with the
> > tree.
> Good idea.  Is that ordering guaranteed?  (Stumbling about the "usually"
> in your first sentence.)

It's not guaranteed. Our implementation of pack-objects does order blobs
after trees, but I suspect this could be violated in rare cases with
some of the delta-island pack layering stuff.

I think it makes sense to be sure that the receiver is correct no matter
what, but optimize for this expected case (that's what I tried to do
with the .gitmodules checks).

> An ordering where dependent objects (like trees) follow the objects they
> reference would be better suited for these kinds of checks..

But worse for others (e.g., like .gitmodules where it's cheap to
identify a candidate blob, but the blob check is involved; there it's
much more optimal to see the tree first).

> > But here the problem is in the tree, not the blob. So we're not finding
> > suspect blobs, but rather re-checking each tree. And no matter what we
> > do (whether it's visiting the object again, or creating a set or mapping
> > with the object names) is going to be linear there. And a repository
> > with a symlink in the root tree is going to revisit or put in our
> > mapping every single root tree.
> That's true, potentially it needs remember and/or reprocess all trees,
> meaning this check may double the run time of fsck in the worst case.
> Example from the wild: The kernel repo currently has 36 symlinks and
> 6+ million objects are checked in total, and the symlink check processes
> 18943 trees_with_symlinks entries there.

That sounds about right. It's basically every version of every tree that
has a symlink. Did it make a noticeable difference in timing? Indexing
the whole kernel history is already a horribly slow process. :)

> > TBH, I'm not sure this fsck check was worth it even without the
> > implementation complexity.
> Hmm.  git status reports such truncated symlinks as changed, so the
> issue *is* already detectable.

Hmm, yeah. That makes sense, since the filesystem (well, really the
syscall API layer) cannot represent the data we are feeding it.