Re: [PATCH 0/8] Speed up connectivity checks via quarantine dir

On Wed, May 19, 2021 at 09:13:18PM +0200, Patrick Steinhardt wrote:

If I am reading the patches correctly, your definition of "satisfied" is: the referenced object exists already on the receiving side.

But that's subtly different from the current rule, which is: the object must be reachable from the current ref tips. The invariant that Git has traditionally tried to maintain (for a repo not to be corrupt) is only that we have the complete graph of objects reachable from the tips.

If we have an unreachable tree in the object database which references blobs we don't have, that doesn't make the repository corrupt. And with the current code, we would not accept a push that references that tree (unless it also pushes the necessary blobs). But after your patch, we would, and that would _make_ the repository corrupt.

I will say that:

  1. Modern versions of git-repack and git-prune try to keep even
     unreachable parts of the graph complete (if we are keeping object X
     that refers to Y, then we try to keep Y, too). But I don't know how
     foolproof it is (certainly the traversal we do there is "best
     effort"; if there's a missing reference that exists, we don't
     bail).

  2. This is not the only place that just checks object existence in the
     name of speed. When updating a ref, for example, we only check that
     the tip object exists.

So I suspect it might work OK in practice. But it is a pretty big loosening of the current rules for pushes, and that makes me nervous.

There's another related change here that is actually a tightening of the rules. The current code checks that the ref tips proposed by the sender are valid. If there are objects in the pack not needed for the ref update, their connectivity isn't checked (though normal senders would obviously avoid sending extra objects for no reason). Your "iterate over all quarantined objects" makes that stricter.

I'm of two minds there:

  1. We could easily keep the original rule by just traversing the
     object graph starting from the ref tips, as we do now, but ending
     the traversal any time we hit an object that we already have
     outside the quarantine.

  2. This tightening is actually important if we want to avoid letting
     people _intentionally_ introduce the unreachable-but-incomplete
     scenario. Without it, an easy denial-of-service corruption against
     a repository you can push to is:

       - push an update to change a ref from X to Y. Include all objects
	 necessary for X..Y, but _also_ include a tree T which points to
	 a missing blob B. This will be accepted by the current rules
	 (but not by your patch).

       - push an update to change the ref from Y to C, where C is a
	 commit whose root tree is T. Your patch allows this (because we
	 already have T in the repository). But the resulting repository
	 is corrupt (the ref now points to an incomplete object graph).

If we wanted to keep the existing rule (requiring that any objects that sender didn't provide are actually reachable from the current refs), then we'd want to be able to check reachability quickly. And there I'd probably turn to reachability bitmaps.

I suspect that "rev-list --use-bitmap-index" in the connectivity check might help in some cases. Especially when you are looking at the union of objects reachable from all refs, we can avoid a lot of fill-in traversal (because if the bitmap for a recent ref includes the object in an older ref, then we know the older ref is covered, even if it doesn't have an on-disk bitmap at its tip). But I would not be at all surprised if there are other slowdowns in the traversal code when you have a lot of refs (e.g., I think we're pretty eager to parse all of the traversal tips as part of the setup).

-Peff