[PATCH 2/2] fetch-pack: avoid repeatedly re-scanning pack directory

When we look up a sha1 object for reading, we first check packfiles, and then loose objects. If we still haven't found it, we re-scan the list of packfiles in `objects/pack`. This final step ensures that we can co-exist with a simultaneous repack process which creates a new pack and then prunes the old object.

This extra re-scan usually does not have a performance impact for two reasons:

  1. If an object is missing, then typically the re-scan
     will find a new pack, then no more misses will occur.
     Or if it truly is missing, then our next step is
     usually to die().

  2. Re-scanning is cheap enough that we do not even notice.

However, these do not always hold. The assumption in (1) is that the caller is expecting to find the object. This is usually the case, but the call to `parse_object` in `everything_local` does not follow this pattern. It is looking to see whether we have objects that the remote side is advertising, not something we expect to have. Therefore if we are fetching from a remote which has many refs pointing to objects we do not have, we may end up re-scanning the pack directory many times.

Even with this extra re-scanning, the impact is often not noticeable due to (2); we just readdir() the packs directory and skip any packs that are already loaded. However, if there are a large number of packs, then even enumerating the directory directory can be expensive (especially if we do it repeatedly). Having this many packs is a good sign the user should run `git gc`, but it would still be nice to avoid having to scan the directory at all.

Signed-off-by: Jeff King <peff@peff.net>
---
 fetch-pack.c | 3 +++
 1 file changed, 3 insertions(+)