cache-tree: prefetch in partial clone read-tree

"git read-tree" checks the existence of the blobs referenced by the
given tree, but does not bulk prefetch them. Add a bulk prefetch.

The lack of prefetch here was noticed at $DAYJOB during a merge
involving some specific commits, but I couldn't find a minimal merge
that didn't also trigger the prefetch in check_updates() in
unpack-trees.c (and in all these cases, the lack of prefetch in
cache-tree.c didn't matter because all the relevant blobs would have
already been prefetched by then). This is why I used read-tree here to
exercise this code path.

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

1 files changed, 33 insertions, 0 deletions

diff --git a/t/t1022-read-tree-partial-clone.sh b/t/t1022-read-tree-partial-clone.sh
new file mode 100755
index 0000000000..a763e27c7d
--- /dev/null
+++ b/t/t1022-read-tree-partial-clone.sh
@@ -0,0 +1,33 @@
+#!/bin/sh
+
+test_description='git read-tree in partial clones'
+
+TEST_NO_CREATE_REPO=1
+
+GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
+export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
+
+. ./test-lib.sh
+
+test_expect_success 'read-tree in partial clone prefetches in one batch' '
+	test_when_finished "rm -rf server client trace" &&
+
+	git init server &&
+	echo foo >server/one &&
+	echo bar >server/two &&
+	git -C server add one two &&
+	git -C server commit -m "initial commit" &&
+	TREE=$(git -C server rev-parse HEAD^{tree}) &&
+
+	git -C server config uploadpack.allowfilter 1 &&
+	git -C server config uploadpack.allowanysha1inwant 1 &&
+	git clone --bare --filter=blob:none "file://$(pwd)/server" client &&
+	GIT_TRACE_PACKET="$(pwd)/trace" git -C client read-tree $TREE &&
+
+	# "done" marks the end of negotiation (once per fetch). Expect that
+	# only one fetch occurs.
+	grep "fetch> done" trace >donelines &&
+	test_line_count = 1 donelines
+'
+
+test_done