Re: fetching packs and storing them as packs

[Shawn Pearce <spearce@spearce.org>]

Alex Riesen <raa.lkml@gmail.com> wrote:
> >>>So the receive-pack process becomes:
> >>>
> >>> a. Create temporary pack file in $GIT_DIR/objects/pack_XXXXX.
> >>>b. Create temporary index file in $GIT_DIR/objects/index_XXXXX.
> >>
> >>Why not $GIT_DIR/objects/tmp/pack... and ignore it everywhere?
> >
> >Because there is a race condition.
> 
> Oh, right. Incidentally, is there a lockfile for packs?

No, we have never needed them before.  And I think we'd like to
avoid adding them now.

We do have the rule that a pack can only be accessed if its .idx
file exists; therefore we generate a pack and its index, then place
the pack into the object directory, then the index.  That way the
pack is there before the index but another process won't attempt
to read the pack until the index exists.  It also means we should
never put an index into the pack directory before its associated
pack.  :-)

If we are pruneing packs or loose objects after the repack we delete
only after the pack and index are in place.  Running processes
rescan the existing packs once if they look for an object and
cannot find it in the loose objects directory or in the packs it
already knows about; this lets most running processes automatically
recover should a parallel repack delete the loose objects it needs
(as they were just packed).

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