Re: [Bug] data loss with cyclic alternates

From: Jeff King <peff@peff.net>
Date: Jul 12, 2014, 05:57 UTC
Message-ID: <20140712055703.GA9655@sigill.intra.peff.net>
In-Reply-To: <1405101705.22963.27.camel@jekeller-desk1.amr.corp.intel.com>

On Fri, Jul 11, 2014 at 06:01:46PM +0000, Keller, Jacob E wrote:

Show 9 quoted lines

> > Yeah, don't do that.  A thinks "eh, the other guy must have it" and
> > B thinks the same.  In general, do not prune or gc a repository
> > other repositories borrow from, even if there is no cycle, because
> > the borrowee does not know anythning about objects that it itself no
> > longer needs but are still needed by its borrowers.
> > 
> 
> Doesn't gc get run automatically at some points? Is the automatic gc run
> setup to avoid this problem?

No, the automatic gc doesn't avoid this. It can't in the general case, as the parent repository does not know how many or which children are pointed to it as an alternate. And the borrowing repository does not even need to have write permission to the parent, so it cannot write a backpointer.

If people are using alternates, they should probably turn off gc.auto in the borrowee (it doesn't seem unreasonable to me to do so automatically via "clone -s" in cases where we can write to the alternates repo, and to issue a warning otherwise).

-Peff