[PATCH] revision: add `--ignore-missing-links` user option

[Karthik Nayak <karthik.188@gmail.com>]

The revision backend is used by multiple porcelain commands such as
git-rev-list(1) and git-log(1). The backend currently supports ignoring
missing links by setting the `ignore_missing_links` bit. This allows the
revision walk to skip any objects links which are missing.

Currently there is no way to use git-rev-list(1) to traverse the objects
of the main object directory (GIT_OBJECT_DIRECTORY) and print the
boundary objects when moving from the main object directory to the
alternate object directories (GIT_ALTERNATE_OBJECT_DIRECTORIES).

By exposing this new flag `--ignore-missing-links`, users can set the
required env variables (GIT_OBJECT_DIRECTORY and
GIT_ALTERNATE_OBJECT_DIRECTORIES) along with the `--boundary` flag to
find the boundary objects between object directories.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
---
 Documentation/rev-list-options.txt |  5 ++++
 revision.c                         |  2 ++
 t/t6022-rev-list-alternates.sh     | 43 ++++++++++++++++++++++++++++++
 3 files changed, 50 insertions(+)
 create mode 100755 t/t6022-rev-list-alternates.sh

diff --git a/Documentation/rev-list-options.txt b/Documentation/rev-list-options.txt
index a4a0cb93b2..a0b48db8a8 100644
--- a/Documentation/rev-list-options.txt
+++ b/Documentation/rev-list-options.txt
@@ -227,6 +227,11 @@ explicitly.
 	Upon seeing an invalid object name in the input, pretend as if
 	the bad input was not given.
 
+--ignore-missing-links::
+	When an object points to another object that is missing, pretend as if the
+	link did not exist. These missing links are not written to stdout unless
+	the --boundary flag is passed.
+
 ifndef::git-rev-list[]
 --bisect::
 	Pretend as if the bad bisection ref `refs/bisect/bad`
diff --git a/revision.c b/revision.c
index 2f4c53ea20..cbfcbf6e28 100644
--- a/revision.c
+++ b/revision.c
@@ -2595,6 +2595,8 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 		revs->limited = 1;
 	} else if (!strcmp(arg, "--ignore-missing")) {
 		revs->ignore_missing = 1;
+	} else if (!strcmp(arg, "--ignore-missing-links")) {
+		revs->ignore_missing_links = 1;
 	} else if (opt && opt->allow_exclude_promisor_objects &&
 		   !strcmp(arg, "--exclude-promisor-objects")) {
 		if (fetch_if_missing)
diff --git a/t/t6022-rev-list-alternates.sh b/t/t6022-rev-list-alternates.sh
new file mode 100755
index 0000000000..626ebb2dce
--- /dev/null
+++ b/t/t6022-rev-list-alternates.sh
@@ -0,0 +1,43 @@
+#!/bin/sh
+
+test_description='handling of alternates in rev-list'
+
+TEST_PASSES_SANITIZE_LEAK=true
+. ./test-lib.sh
+
+# We create 5 commits and move them to the alt directory and
+# create 5 more commits which will stay in the main odb.
+test_expect_success 'create repository and alternate directory' '
+	git init main &&
+	test_commit_bulk -C main 5 &&
+	mkdir alt &&
+	mv main/.git/objects/* alt &&
+	GIT_ALTERNATE_OBJECT_DIRECTORIES=$PWD/alt test_commit_bulk --start=6 -C main 5
+'
+
+# When the alternate odb is provided, all commits are listed.
+test_expect_success 'rev-list passes with alternate object directory' '
+	GIT_ALTERNATE_OBJECT_DIRECTORIES=$PWD/alt test_stdout_line_count = 10 git -C main rev-list HEAD
+'
+
+# When the alternate odb is not provided, rev-list fails since the 5th commit's
+# parent is not present in the main odb.
+test_expect_success 'rev-list fails without alternate object directory' '
+	test_must_fail git -C main rev-list HEAD
+'
+
+# With `--ignore-missing-links`, we stop the traversal when we encounter a
+# missing link.
+test_expect_success 'rev-list only prints main odb commits with --ignore-missing-links' '
+	test_stdout_line_count = 5 git -C main rev-list --ignore-missing-links HEAD
+'
+
+# With `--ignore-missing-links` and `--boundary`, we can even print those boundary
+# commits.
+test_expect_success 'rev-list prints boundary commit with --ignore-missing-links' '
+	git -C main rev-list --ignore-missing-links --boundary HEAD >list-output &&
+	test_stdout_line_count = 6 cat list-output &&
+	test_stdout_line_count = 1 cat list-output | grep "^-"
+'
+
+test_done
-- 
2.41.0