From: Jeff King <peff@peff.net>
Date: Thu, 17 Nov 2016 01:43:07 GMT
Subject: Re: Re* Protecting old temporary objects being reused from concurrent "git gc"?
Message-ID: <20161117014306.2ptqd56gur7dlb4c@sigill.intra.peff.net>
In-Reply-To: <xmqqvavmopl8.fsf_-_@gitster.mtv.corp.google.com>

On Wed, Nov 16, 2016 at 05:35:47PM -0800, Junio C Hamano wrote:

> OK, here is what I have queued.
> 
> -- >8 --
> Subject: cache-tree: make sure to "touch" tree objects the cache-tree records
> 
> The cache_tree_fully_valid() function is called by callers that want
> to know if they need to call cache_tree_update(), i.e. as an attempt
> to optimize. They all want to have a fully valid cache-tree in the
> end so that they can write a tree object out.

That makes sense. I was focusing on cache_tree_update() call, but we do
not even get there in the fully-valid case.

So I think this approach is nice as long as there is not a caller who
asks "are we fully valid? I do not need to write, but was just
wondering". That should be a read-only operation, but the freshen calls
may fail with EPERM, for example.

I do not see any such callers, nor do I really expect any. Just trying
to think through the possible consequences.

> Strictly speaking, freshing these tree objects at each and every
> level is probably unnecessary, given that anything reachable from a
> young object inherits the youth from the referring object to be
> protected from pruning.  It should be sufficient to freshen only the
> very top-level tree instead.  Benchmarking and optimization is left
> as an exercise for later days.

Good observation, and nicely explained all around.

-Peff