Subject: Re: Git gc removes all packs

From: Jeff King <[email-masked]>
Date: Thu, 5 Feb 2015 15:03:32 -0500

On Thu, Feb 05, 2015 at 04:13:03PM +0100, Dmitry Neverov wrote:

> I'm using git p4 for synchronization with perforce. Sometimes after 'git
> p4 rebase' git starts a garbage collection. When gc finishes a local
> repository contains no pack files only loose objects, so I have to
> re-import repository from perforce. It also doesn't contain a temporary
> pack git gc was creating.

It sounds like git didn't find any refs; it will pack only objects which
are reachable. Unreachable objects are either:

  1. Exploded into loose objects if the mtime on the pack they contain
     is less than 2 weeks old (and will eventually expire when they
     become 2 weeks old).

  2. Dropped completely if older than 2 weeks.

> One more thing about my setup: since git p4 promotes a use of a linear
> history I use a separate repository for another branch in perforce. In
> order to be able to cherry-pick between repositories I added this
> another repo objects dir as an alternate and also added a ref which is a
> symbolic link to a branch in another repo (so I don't have to do any
> fetches).

You can't symlink refs like this. The loose refs in the filesystem may
be migrated into the "packed-refs" file, at which point your symlink
will be broken. That is a likely reason why git would not find any refs.

So your setup will not ever work reliably.  But IMHO, it is a bug that
git does not notice the broken symlink and abort an operation which is
computing reachability in order to drop objects. As you noticed, it
means a misconfiguration or filesystem error results in data loss.

-Peff