upload-pack: provide a hook for running pack-objects

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>

1 files changed, 62 insertions, 0 deletions

diff --git a/t/t5544-pack-objects-hook.sh b/t/t5544-pack-objects-hook.sh
new file mode 100755
index 0000000000..4357af1525
--- /dev/null
+++ b/t/t5544-pack-objects-hook.sh
@@ -0,0 +1,62 @@
+#!/bin/sh
+
+test_description='test custom script in place of pack-objects'
+. ./test-lib.sh
+
+test_expect_success 'create some history to fetch' '
+	test_commit one &&
+	test_commit two
+'
+
+test_expect_success 'create debugging hook script' '
+	write_script .git/hook <<-\EOF
+		echo >&2 "hook running"
+		echo "$*" >hook.args
+		cat >hook.stdin
+		"$@" <hook.stdin >hook.stdout
+		cat hook.stdout
+	EOF
+'
+
+clear_hook_results () {
+	rm -rf .git/hook.* dst.git
+}
+
+test_expect_success 'hook runs via global config' '
+	clear_hook_results &&
+	test_config_global uploadpack.packObjectsHook ./hook &&
+	git clone --no-local . dst.git 2>stderr &&
+	grep "hook running" stderr
+'
+
+test_expect_success 'hook outputs are sane' '
+	# check that we recorded a usable pack
+	git index-pack --stdin <.git/hook.stdout &&
+
+	# check that we recorded args and stdin. We do not check
+	# the full argument list or the exact pack contents, as it would make
+	# the test brittle. So just sanity check that we could replay
+	# the packing procedure.
+	grep "^git" .git/hook.args &&
+	$(cat .git/hook.args) <.git/hook.stdin >replay
+'
+
+test_expect_success 'hook runs from -c config' '
+	clear_hook_results &&
+	git clone --no-local \
+	  -u "git -c uploadpack.packObjectsHook=./hook upload-pack" \
+	  . dst.git 2>stderr &&
+	grep "hook running" stderr
+'
+
+test_expect_success 'hook does not run from repo config' '
+	clear_hook_results &&
+	test_config uploadpack.packObjectsHook "./hook" &&
+	git clone --no-local . dst.git 2>stderr &&
+	! grep "hook running" stderr &&
+	test_path_is_missing .git/hook.args &&
+	test_path_is_missing .git/hook.stdin &&
+	test_path_is_missing .git/hook.stdout
+'
+
+test_done