refs_from_alternate: helper to use refs from alternates

The receiving end of "git push" advertises the objects that the repository
itself does not use, but are at the tips of refs in other repositories
whose object databases are used as alternates for it. This helps it avoid
having to receive (and the pusher having to send) objects that are already
available to the receiving repository via the alternates mechanism.

Tweak the helper function that implements this feature, and move it to
transport.[ch] for future reuse by other programs.

The additional test demonstrates how this optimization is helping "git push",
and "git fetch" is ignorant about it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Acked-by: Shawn O. Pearce <spearce@spearce.org>

1 files changed, 66 insertions, 0 deletions

diff --git a/t/t5501-fetch-push-alternates.sh b/t/t5501-fetch-push-alternates.sh
new file mode 100755
index 0000000..16a4676
--- /dev/null
+++ b/t/t5501-fetch-push-alternates.sh
@@ -0,0 +1,66 @@
+#!/bin/sh
+
+test_description='fetch/push involving alternates'
+. ./test-lib.sh
+
+count_objects () {
+	loose=0 inpack=0
+	eval "$(
+		git count-objects -v |
+		sed -n -e 's/^count: \(.*\)/loose=\1/p' \
+		    -e 's/^in-pack: \(.*\)/inpack=\1/p'
+	)" &&
+	echo $(( $loose + $inpack ))
+}
+
+
+test_expect_success setup '
+	(
+		git init original &&
+		cd original &&
+		i=0 &&
+		while test $i -le 100
+		do
+			echo "$i" >count &&
+			git add count &&
+			git commit -m "$i" || exit
+			i=$(($i + 1))
+		done
+	) &&
+	(
+		git clone --reference=original "file:///$(pwd)/original" one &&
+		cd one &&
+		echo Z >count &&
+		git add count &&
+		git commit -m Z &&
+		count_objects >../one.count
+	) &&
+	A=$(pwd)/original/.git/objects &&
+	git init receiver &&
+	echo "$A" >receiver/.git/objects/info/alternates &&
+	git init fetcher &&
+	echo "$A" >fetcher/.git/objects/info/alternates
+'
+
+test_expect_success 'pushing into a repository with the same alternate' '
+	(
+		cd one &&
+		git push ../receiver master:refs/heads/it
+	) &&
+	(
+		cd receiver &&
+		count_objects >../receiver.count
+	) &&
+	test_cmp one.count receiver.count
+'
+
+test_expect_failure 'fetching from a repository with the same alternate' '
+	(
+		cd fetcher &&
+		git fetch ../one master:refs/heads/it &&
+		count_objects >../fetcher.count
+	) &&
+	test_cmp one.count fetcher.count
+'
+
+test_done