pack-objects: don't reuse deltas with path walk

builtin/push.c

@@ -624,6 +624,10 @@ int cmd_push(int argc,
 	else if (recurse_submodules == RECURSE_SUBMODULES_ONLY)
 		flags |= TRANSPORT_RECURSE_SUBMODULES_ONLY;
 
+	prepare_repo_settings(the_repository);
+	if (the_repository->settings.pack_use_path_walk)
+		flags |= TRANSPORT_PUSH_NO_REUSE_DELTA;
+
 	if (tags)
 		refspec_append(&rs, "refs/tags/*");
 

send-pack.c

@@ -93,6 +93,8 @@ static int pack_objects(int fd, struct ref *refs, struct oid_array *advertised,
 		strvec_push(&po.args, "--shallow");
 	if (args->disable_bitmaps)
 		strvec_push(&po.args, "--no-use-bitmap-index");
+	if (args->no_reuse_delta)
+		strvec_push(&po.args, "--no-reuse-delta");
 	po.in = -1;
 	po.out = args->stateless_rpc ? -1 : fd;
 	po.git_cmd = 1;

send-pack.h

@@ -22,6 +22,7 @@ struct send_pack_args {
 		force_update:1,
 		use_thin_pack:1,
 		use_ofs_delta:1,
+		no_reuse_delta:1,
 		dry_run:1,
 		/* One of the SEND_PACK_PUSH_CERT_* constants. */
 		push_cert:2,

t/t5590-push-path-walk.sh

@@ -0,0 +1,109 @@
+#!/bin/sh
+
+test_description='verify that push respects `pack.usePathWalk`'
+
+TEST_PASSES_SANITIZE_LEAK=true
+. ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-pack.sh
+
+test_expect_success 'setup bare repository and clone' '
+	git init --bare -b main bare.git &&
+	git --git-dir=bare.git config receive.unpackLimit 0 &&
+	git --git-dir bare.git commit-tree -m initial $EMPTY_TREE >head_oid &&
+	git --git-dir bare.git update-ref refs/heads/main $(cat head_oid) &&
+	git clone --bare bare.git clone.git
+'
+test_expect_success 'avoid reusing deltified objects' '
+	# construct two commits, one containing a file with the hex digits
+	# repeated 16 times, the next reducing that to 8 times. The crucial
+	# part is that the blob of the second commit is deltified _really_
+	# badly and it is therefore easy to detect if a `git push` reused that
+	# delta.
+	x="0123456789abcdef" &&
+	printf "$x$x$x$x$x$x$x$x" >x128 &&
+	printf "$x$x$x$x$x$x$x$x$x$x$x$x$x$x$x$x" >x256 &&
+
+	pack=clone.git/objects/pack/pack-tmp.pack &&
+	pack_header 2 >$pack &&
+
+	# add x256 as a non-deltified object, using an uncompressed zlib stream
+	# for simplicity
+	# 060 = OBJ_BLOB << 4, 0200 = size larger than 15,
+	# 0 = lower 4 bits of size, 020 = bits 5-9 of size (size = 256)
+	printf "\260\020" >>$pack &&
+	# Uncompressed zlib stream always starts with 0170 1 1, followed
+	# by two bytes encoding the size, little endian, then two bytes with
+	# the bitwise-complement of that size, then the payload, and then the
+	# Adler32 checksum. For some reason, the checksum is in big-endian
+	# format.
+	printf "\170\001\001\0\001\377\376" >>$pack &&
+	cat x256 >>$pack &&
+	# Manually-computed Adler32 checksum: 0xd7ae4621
+	printf "\327\256\106\041" >>$pack &&
+
+	# add x128 as a very badly deltified object
+	# 0120 = OBJ_OFS_DELTA << 4, 0200 = total size larger than 15,
+	# 4 = lower 4 bits of size, 030 = bits 5-9 of size
+	# (size = 128 * 3 + 2 + 2)
+	printf "\344\030" >>$pack &&
+	# 0415 = size (i.e. the relative negative offset) of the previous
+	# object (x256, used as base object)
+	# encoded as 0200 | ((0415 >> 7) - 1), 0415 & 0177
+	printf "\201\015" >>$pack &&
+	# Uncompressed zlib stream, as before, size = 2 + 2 + 128 * 3 (i.e.
+	# 0604)
+	printf "\170\001\001\204\001\173\376" >>$pack &&
+	# base object size = 0400 (encoded as 0200 | (0400 & 0177),
+	# 0400 >> 7)
+	printf "\200\002" >>$pack &&
+	# object size = 0200 (encoded as 0200 | (0200 & 0177), 0200 >> 7
+	printf "\200\001" >>$pack &&
+	# massively badly-deltified object: copy every single byte individually
+	# 0200 = copy, 1 = use 1 byte to encode the offset (counter),
+	# 020 = use 1 byte to encode the size (1)
+	printf "$(printf "\\\\221\\\\%03o\\\\001" $(test_seq 0 127))" >>$pack &&
+	# Manually-computed Adler32 checksum: 0x99c369c4
+	printf "\231\303\151\304" >>$pack &&
+
+	pack_trailer $pack &&
+	git index-pack -v $pack &&
+
+	oid256=$(git hash-object x256) &&
+	printf "100755 blob $oid256\thex\n" >tree &&
+	tree_oid="$(git --git-dir=clone.git mktree <tree)" &&
+	commit_oid=$(git --git-dir=clone.git commit-tree \
+		-p $(git --git-dir=clone.git rev-parse main) \
+		-m 256 $tree_oid) &&
+
+	oid128=$(git hash-object x128) &&
+	printf "100755 blob $oid128\thex\n" >tree &&
+	tree_oid="$(git --git-dir=clone.git mktree <tree)" &&
+	commit_oid=$(git --git-dir=clone.git commit-tree \
+		-p $commit_oid \
+		-m 128 $tree_oid) &&
+
+	# Verify that the on-disk size of the delta object is suboptimal in the
+	# clone (see below why 18 bytes or smaller is the optimal size):
+	git index-pack --verify-stat clone.git/objects/pack/pack-*.pack >verify &&
+	size="$(sed -n "s/^$oid128 blob *\([^ ]*\).*/\1/p" <verify)" &&
+	test $size -gt 18 &&
+
+	git --git-dir=clone.git update-ref refs/heads/main $commit_oid &&
+	git --git-dir=clone.git -c pack.usePathWalk=true push origin main &&
+	git index-pack --verify-stat bare.git/objects/pack/pack-*.pack >verify &&
+	size="$(sed -n "s/^$oid128 blob *\([^ ]*\).*/\1/p" <verify)" &&
+	# The on-disk size of the delta object should be smaller than, or equal
+	# to, 18 bytes, as that would be the size if storing the payload
+	# uncompressed:
+	#   3 bytes: 0170 01 01
+	# + 2 bytes: zlib stream size
+	# + 2 bytes: but-wise complement of the zlib stream size
+	# + 7 bytes: payload
+	#   (= 2 bytes for the size of tbe base object
+	#    + 2 bytes for the size of the delta command
+	#    + 3 bytes for the copy command)
+	# + 2 + 2 bytes: Adler32 checksum
+	test $size -le 18
+'
+
+test_done

transport.c

@@ -893,6 +893,7 @@ static int git_transport_push(struct transport *transport, struct ref *remote_re
 	args.dry_run = !!(flags & TRANSPORT_PUSH_DRY_RUN);
 	args.porcelain = !!(flags & TRANSPORT_PUSH_PORCELAIN);
 	args.atomic = !!(flags & TRANSPORT_PUSH_ATOMIC);
+	args.no_reuse_delta = !!(flags & TRANSPORT_PUSH_NO_REUSE_DELTA);
 	args.push_options = transport->push_options;
 	args.url = transport->url;
 

transport.h

@@ -158,6 +158,7 @@ struct transport {
 #define TRANSPORT_RECURSE_SUBMODULES_ONLY	(1<<15)
 #define TRANSPORT_PUSH_FORCE_IF_INCLUDES	(1<<16)
 #define TRANSPORT_PUSH_AUTO_UPSTREAM		(1<<17)
+#define TRANSPORT_PUSH_NO_REUSE_DELTA		(1<<18)
 
 int transport_summary_width(const struct ref *refs);