Re: [PATCH 1/7] cat-file: disable object/refname ambiguity check for batch mode

On Fri, Jul 12, 2013 at 12:30:07PM +0200, Michael Haggerty wrote:

But with particular respect to "git cat-file", I see problems:

1. get_ref_snapshot() would have to read all loose and packed refs
within the specified subtree, because loose refs have to be read before
packed refs.  So the call would be expensive if there are a lot of loose
refs.  And DWIM wouldn't know in advance where the references might be,
so it would have to set prefix="".  If many refs are looked up, then it
would presumably be worth it.  But if only a couple of lookups are done
and there are a lot of loose refs, then using a cache would probably
slow things down.

The slowdown could be ameliorated by adding some more intelligence, for
example only populating the loose refs cache after a certain number of
lookups have already been done.

I had assumed we would load the loose ref cache progressively by
directory. Of course that only helps avoiding "refs/foo/*" when you are
interested in "refs/heads/foo". If your "refs/heads/*" is very large,
you have to load all of it, and at some point it is cheaper to do a few
stat() calls than to actually readdir() the whole directory. On the
other hand, that is basically how packed-refs work now (if we hit any
ref, we have to load the whole file), and repos with many refs would
typically pack them all anyway.

2. A "git cat-file --batch" process can be long-lived.  What guarantees
would users expect regarding its lookup results?

I hadn't really intended this for general lookups, but just to speed up
the refname warning, where being out-of-date is more acceptable (since
the warning is a purely informational hint).

-Peff