rev-list: add commit object support in `--missing` option

The `--missing` object option in rev-list currently works only with
missing blobs/trees. For missing commits the revision walker fails with
a fatal error.

Let's extend the functionality of `--missing` option to also support
commit objects. This is done by adding a `missing_objects` field to
`rev_info`. This field is an `oidset` to which we'll add the missing
commits as we encounter them. The revision walker will now continue the
traversal and call `show_commit()` even for missing commits. In rev-list
we can then check if the commit is a missing commit and call the
existing code for parsing `--missing` objects.

A scenario where this option would be used is to find the boundary
objects between different object directories. Consider a repository with
a main object directory (GIT_OBJECT_DIRECTORY) and one or more alternate
object directories (GIT_ALTERNATE_OBJECT_DIRECTORIES). In such a
repository, using the `--missing=print` option while disabling the
alternate object directory allows us to find the boundary objects
between the main and alternate object directory.

Helped-by: Patrick Steinhardt <ps@pks.im>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 74 insertions, 0 deletions

diff --git a/t/t6022-rev-list-missing.sh b/t/t6022-rev-list-missing.sh
new file mode 100755
index 0000000000..40265a4f66
--- /dev/null
+++ b/t/t6022-rev-list-missing.sh
@@ -0,0 +1,74 @@
+#!/bin/sh
+
+test_description='handling of missing objects in rev-list'
+
+TEST_PASSES_SANITIZE_LEAK=true
+. ./test-lib.sh
+
+# We setup the repository with two commits, this way HEAD is always
+# available and we can hide commit 1.
+test_expect_success 'create repository and alternate directory' '
+	test_commit 1 &&
+	test_commit 2 &&
+	test_commit 3
+'
+
+for obj in "HEAD~1" "HEAD~1^{tree}" "HEAD:1.t"
+do
+	test_expect_success "rev-list --missing=error fails with missing object $obj" '
+		oid="$(git rev-parse $obj)" &&
+		path=".git/objects/$(test_oid_to_path $oid)" &&
+
+		mv "$path" "$path.hidden" &&
+		test_when_finished "mv $path.hidden $path" &&
+
+		test_must_fail git rev-list --missing=error --objects \
+			--no-object-names HEAD
+	'
+done
+
+for obj in "HEAD~1" "HEAD~1^{tree}" "HEAD:1.t"
+do
+	for action in "allow-any" "print"
+	do
+		test_expect_success "rev-list --missing=$action with missing $obj" '
+			oid="$(git rev-parse $obj)" &&
+			path=".git/objects/$(test_oid_to_path $oid)" &&
+
+			# Before the object is made missing, we use rev-list to
+			# get the expected oids.
+			git rev-list --objects --no-object-names \
+				HEAD ^$obj >expect.raw &&
+
+			# Blobs are shared by all commits, so evethough a commit/tree
+			# might be skipped, its blob must be accounted for.
+			if [ $obj != "HEAD:1.t" ]; then
+				echo $(git rev-parse HEAD:1.t) >>expect.raw &&
+				echo $(git rev-parse HEAD:2.t) >>expect.raw
+			fi &&
+
+			mv "$path" "$path.hidden" &&
+			test_when_finished "mv $path.hidden $path" &&
+
+			git rev-list --missing=$action --objects --no-object-names \
+				HEAD >actual.raw &&
+
+			# When the action is to print, we should also add the missing
+			# oid to the expect list.
+			case $action in
+			allow-any)
+				;;
+			print)
+				grep ?$oid actual.raw &&
+				echo ?$oid >>expect.raw
+				;;
+			esac &&
+
+			sort actual.raw >actual &&
+			sort expect.raw >expect &&
+			test_cmp expect actual
+		'
+	done
+done
+
+test_done