Re: [PATCH v3] unpack-trees: avoid duplicate ODB lookups during checkout
- Date: Thu, 13 Apr 2017 20:59:25 -0400
- From: Jeff King <peff@xxxxxxxx>
- Subject: Re: [PATCH v3] unpack-trees: avoid duplicate ODB lookups during checkout
On Thu, Apr 13, 2017 at 04:11:31PM -0700, Junio C Hamano wrote:
> git@xxxxxxxxxxxxxxxxx writes:
> > + /*
> > + * Fetch the tree from the ODB for each peer directory in the
> > + * n commits.
> > + *
> > + * For 2- and 3-way traversals, we try to avoid hitting the
> > + * ODB twice for the same OID. This should yield a nice speed
> > + * up in checkouts and merges when the commits are similar.
> > + *
> > + * We don't bother doing the full O(n^2) search for larger n,
> > + * because wider traversals don't happen that often and we
> > + * avoid the search setup.
> > + *
> > + * When 2 peer OIDs are the same, we just copy the tree
> > + * descriptor data. This implicitly borrows the buffer
> > + * data from the earlier cell.
> This "borrowing" made me worried, but it turns out that this is
> perfectly OK.
> fill_tree_descriptor() uses read_sha1_file() to give a tree_desc its
> own copy of the tree object data, so the code that calls into the
> tree traversal API is responsible for freeing the buffer returned
> from fill_tree_descriptor(). The updated code avoids double freeing
> by setting buf[i] to NULL for borrowing [i].
Yeah, I think that logic is fine. We could also keep a separate counter
for the size of buf, like:
buf[nr_buf++] = fill_tree_descriptor(t+i, sha1);
and then just count from zero to nr_buf in the freeing loop. I don't
think it matters much either way (the free(NULL) calls are presumably