midx: allow marking a pack as preferred

When multiple packs in the multi-pack index contain the same object, the
MIDX machinery must make a choice about which pack it associates with
that object. Prior to this patch, the lowest-ordered[1] pack was always
selected.

Pack selection for duplicate objects is relatively unimportant today,
but it will become important for multi-pack bitmaps. This is because we
can only invoke the pack-reuse mechanism when all of the bits for reused
objects come from the reuse pack (in order to ensure that all reused
deltas can find their base objects in the same pack).

To encourage the pack selection process to prefer one pack over another
(the pack to be preferred is the one a caller would like to later use as
a reuse pack), introduce the concept of a "preferred pack". When
provided, the MIDX code will always prefer an object found in a
preferred pack over any other.

No format changes are required to store the preferred pack, since it
will be able to be inferred with a corresponding MIDX bitmap, by looking
up the pack associated with the object in the first bit position (this
ordering is described in detail in a subsequent commit).

[1]: the ordering is specified by MIDX internals; for our purposes we
can consider the "lowest ordered" pack to be "the one with the
most-recent mtime.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 42 insertions, 0 deletions

diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index b4afab1dfc..5641d158df 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -234,6 +234,48 @@ test_expect_success 'warn on improper hash version' '
 	)
 '
 
+test_expect_success 'midx picks objects from preferred pack' '
+	test_when_finished rm -rf preferred.git &&
+	git init --bare preferred.git &&
+	(
+		cd preferred.git &&
+
+		a=$(echo "a" | git hash-object -w --stdin) &&
+		b=$(echo "b" | git hash-object -w --stdin) &&
+		c=$(echo "c" | git hash-object -w --stdin) &&
+
+		# Set up two packs, duplicating the object "B" at different
+		# offsets.
+		#
+		# Note that the "BC" pack (the one we choose as preferred) sorts
+		# lexically after the "AB" pack, meaning that omitting the
+		# --preferred-pack argument would cause this test to fail (since
+		# the MIDX code would select the copy of "b" in the "AB" pack).
+		git pack-objects objects/pack/test-AB <<-EOF &&
+		$a
+		$b
+		EOF
+		bc=$(git pack-objects objects/pack/test-BC <<-EOF
+		$b
+		$c
+		EOF
+		) &&
+
+		git multi-pack-index --object-dir=objects \
+			write --preferred-pack=test-BC-$bc.idx 2>err &&
+		test_must_be_empty err &&
+
+		test-tool read-midx --show-objects objects >out &&
+
+		ofs=$(git show-index <objects/pack/test-BC-$bc.idx | grep $b |
+			cut -d" " -f1) &&
+		printf "%s %s\tobjects/pack/test-BC-%s.pack\n" \
+			"$b" "$ofs" "$bc" >expect &&
+		grep ^$b out >actual &&
+
+		test_cmp expect actual
+	)
+'
 
 test_expect_success 'verify multi-pack-index success' '
 	git multi-pack-index verify --object-dir=$objdir