Re: RFC on packfile URIs and .gitmodules check

Jonathan Tan [off-list ref] writes:

Someone at $DAYJOB noticed that if a .gitmodules-containing tree and the
.gitmodules blob itself are sent in 2 separate packfiles during a fetch
(which can happen when packfile URIs are used), transfer.fsckobjects
causes the fetch to fail. You can reproduce it as follows (as of the
time of writing):

  $ git -c fetch.uriprotocols=https -c transfer.fsckobjects=true clone https://chromium.googlesource.com/chromiumos/codesearch
  Cloning into 'codesearch'...
  remote: Total 2242 (delta 0), reused 2242 (delta 0)
  Receiving objects: 100% (2242/2242), 1.77 MiB | 4.62 MiB/s, done.
  error: object 1f155c20935ee1154a813a814f03ef2b3976680f: gitmodulesMissing: unable to read .gitmodules blob
  fatal: fsck error in pack objects
  fatal: index-pack failed

This happens because the fsck part is currently being done in
index-pack, which operates on one pack at a time. When index-pack sees
the tree, it runs fsck on it (like any other object), and the fsck
subsystem remembers the .gitmodules target (specifically, in
gitmodules_found in fsck.c). Later, index-pack runs fsck_finish() which
checks if the target exists, but it doesn't, so it reports the failure.

Is this because the gitmodules blob is contained in the base image
served via the pack URI mechansim, and the "dynamic" packfile for
the latest part of the history refers to the gitmodules file that is
unchanged, hence the latter one lacks it?

Another option is to do it within the connectivity check instead - so,
update rev-list and the object walking mechanism to be able to detect
.gitmodules in trees and fsck the target blob whenever such an entry
occurs. This has the advantage that there is no extra re-inflation,
although it might be strange to have object walking be able to fsck.

The simplest solution would be to just relax this - check the blob if it
exists, but if it doesn't, it's OK. Some things in favor of this
solution:

 - This is something we already do in the partial clone case (although
   it could be argued that in this case, we're already trusting the
   server for far more than .gitmodules, so just because it's OK in the
   partial clone case doesn't mean that it's OK in the regular case).

 - Also, the commit message for this feature (from ed8b10f631 ("fsck: check
   .gitmodules content", 2018-05-21)) gives a rationale of a newer
   server being able to protect older clients.
    - Servers using receive-pack (instead of fetch-pack) to obtain
      objects would still be protected, since receive-pack still only
      accepts one packfile at a time (and there are currently no plans
      to expand this).
    - Also, malicious .gitobjects files could still be crafted that pass
      fsck checking - for example, by containing a URL (of another
      server) that refers to a repo with a .gitobjects that would fail
      fsck.

So I would rather go with just relaxing the check, but if consensus is
that we should still do it, I'll investigate doing it in the
connectivity check.

You've listed two possible solutions, i.e.

 (1) punt and declare that we assume an missing and uncheckable blob
     is OK,

 (2) defer the check after transfer completes.

Between the two, my gut feeling is that the latter is preferrable.
If we assume an missing and uncheckable one is OK, then even if a
blob is available to be checked, there is not much point in
checking, no?

As long as the quarantine of incoming pack works correctly,
streaming the incoming packdata (and packfile downloaded out of line
via a separate mechanism like pack URI) to index-pack that does not
check to complete the transfer, with a separate step to check the
sanity of these packs as a whole, should not harm the repository
even if it is interrupted in the middle, after transfer is done but
before checking says it is OK.

As a potential third option, I wonder if it is easier for everybody
involved (including third-party implementation of their
index-pack/fsck equivalent) if we made it a rule that a pack that
has a tree that refers to .git<something> must include the blob for
it?

Thanks.