When upload-pack serves a client request, it turns to
pack-objects to do the heavy lifting of creating a
packfile. There's no easy way to intercept the call to
pack-objects, but there are a few good reasons to want to do
so:
1. If you're debugging a client or server issue with
fetching, you may want to store a copy of the generated
packfile.
2. If you're gathering data from real-world fetches for
performance analysis or debugging, storing a copy of
the arguments and stdin lets you replay the pack
generation at your leisure.
3. You may want to insert a caching layer around
pack-objects; it is the most CPU- and memory-intensive
part of serving a fetch, and its output is a pure
function[1] of its input, making it an ideal place to
consolidate identical requests.
This patch adds a simple "hook" interface to intercept calls
to pack-objects. The new test demonstrates how it can be
used for debugging (using it for caching is a
straightforward extension; the tricky part is writing the
actual caching layer).
This hook is unlike the normal hook scripts found in the
"hooks/" directory of a repository. Because we promise that
upload-pack is safe to run in an untrusted repository, we
cannot execute arbitrary code or commands found in the
repository (neither in hooks/, nor in the config). So
instead, this hook is triggered from a config variable that
is explicitly ignored in the per-repo config.
The config variable holds the actual shell command to run as
the hook. Another approach would be to simply treat it as a
boolean: "should I respect the upload-pack hooks in this
repo?", and then run the script from "hooks/" as we usually
do. However, that isn't as flexible; there's no way to run a
hook approved by the site administrator (e.g., in
"/etc/gitconfig") on a repository whose contents are not
trusted. The approach taken by this patch is more
fine-grained, if a little less conventional for git hooks
(it does behave similar to other configured commands like
diff.external, etc).
[1] Pack-objects isn't _actually_ a pure function. Its
output depends on the exact packing of the object
database, and if multi-threading is used for delta
compression, can even differ racily. But for the
purposes of caching, that's OK; of the many possible
outputs for a given input, it is sufficient only that we
output one of them.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Jeff King