index-pack: prefetch missing REF_DELTA bases

When fetching, the client sends "have" commit IDs indicating that the
server does not need to send any object referenced by those commits,
reducing network I/O. When the client is a partial clone, the client
still sends "have"s in this way, even if it does not have every object
referenced by a commit it sent as "have".

If a server omits such an object, it is fine: the client could lazily
fetch that object before this fetch, and it can still do so after.

The issue is when the server sends a thin pack containing an object that
is a REF_DELTA against such a missing object: index-pack fails to fix
the thin pack. When support for lazily fetching missing objects was
added in 8b4c0103a9 ("sha1_file: support lazily fetching missing
objects", 2017-12-08), support in index-pack was turned off in the
belief that it accesses the repo only to do hash collision checks.
However, this is not true: it also needs to access the repo to resolve
REF_DELTA bases.

Support for lazy fetching should still generally be turned off in
index-pack because it is used as part of the lazy fetching process
itself (if not, infinite loops may occur), but we do need to fetch the
REF_DELTA bases. (When fetching REF_DELTA bases, it is unlikely that
those are REF_DELTA themselves, because we do not send "have" when
making such fetches.)

To resolve this, prefetch all missing REF_DELTA bases before attempting
to resolve them. This both ensures that all bases are attempted to be
fetched, and ensures that we make only one request per index-pack
invocation, and not one request per missing object.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 61 insertions, 0 deletions

diff --git a/t/t5616-partial-clone.sh b/t/t5616-partial-clone.sh
index 7cc0c71..f1baf83 100755
--- a/t/t5616-partial-clone.sh
+++ b/t/t5616-partial-clone.sh
@@ -339,4 +339,65 @@ test_expect_success 'when partial cloning, tolerate server not sending target of
 	! test -e "$HTTPD_ROOT_PATH/one-time-sed"
 '
 
+test_expect_success 'tolerate server sending REF_DELTA against missing promisor objects' '
+	SERVER="$HTTPD_DOCUMENT_ROOT_PATH/server" &&
+	rm -rf "$SERVER" repo &&
+	test_create_repo "$SERVER" &&
+	test_config -C "$SERVER" uploadpack.allowfilter 1 &&
+	test_config -C "$SERVER" uploadpack.allowanysha1inwant 1 &&
+
+	# Create a commit with a blob to be used as a delta base.
+	for i in $(test_seq 10)
+	do
+		echo "this is a line" >>"$SERVER/foo.txt"
+	done &&
+	git -C "$SERVER" add foo.txt &&
+	git -C "$SERVER" commit -m bar &&
+	git -C "$SERVER" rev-parse HEAD:foo.txt >deltabase &&
+
+	git -c protocol.version=2 clone --no-checkout \
+		--filter=blob:none $HTTPD_URL/one_time_sed/server repo &&
+
+	# Sanity check to ensure that the client does not have that blob.
+	git -C repo rev-list --objects --exclude-promisor-objects \
+		-- $(cat deltabase) >objlist &&
+	test_line_count = 0 objlist &&
+
+	# Another commit. This commit will be fetched by the client.
+	echo "abcdefghijklmnopqrstuvwxyz" >>"$SERVER/foo.txt" &&
+	git -C "$SERVER" add foo.txt &&
+	git -C "$SERVER" commit -m baz &&
+
+	# Pack a thin pack containing, among other things, HEAD:foo.txt
+	# delta-ed against HEAD^:foo.txt.
+	printf "%s\n--not\n%s\n" \
+		$(git -C "$SERVER" rev-parse HEAD) \
+		$(git -C "$SERVER" rev-parse HEAD^) |
+		git -C "$SERVER" pack-objects --thin --stdout >thin.pack &&
+
+	# Ensure that the pack contains one delta against HEAD^:foo.txt. Since
+	# the delta contains at least 26 novel characters, the size cannot be
+	# contained in 4 bits, so the object header will take up 2 bytes. The
+	# most significant nybble of the first byte is 0b1111 (0b1 to indicate
+	# that the header continues, and 0b111 to indicate REF_DELTA), followed
+	# by any 3 nybbles, then the OID of the delta base.
+	git -C "$SERVER" rev-parse HEAD^:foo.txt >deltabase &&
+	printf "f.,..%s" $(intersperse "," <deltabase) >want &&
+	hex_unpack <thin.pack | intersperse "," >have &&
+	grep $(cat want) have &&
+
+	replace_packfile thin.pack &&
+
+	# Use protocol v2 because the sed command looks for the "packfile"
+	# section header.
+	test_config -C "$SERVER" protocol.version 2 &&
+
+	# Fetch the thin pack and ensure that index-pack is able to handle the
+	# REF_DELTA object with a missing promisor delta base.
+	git -C repo -c protocol.version=2 fetch &&
+
+	# Ensure that the one-time-sed script was used.
+	! test -e "$HTTPD_ROOT_PATH/one-time-sed"
+'
+
 test_done