[RFC PATCH 02/12] commit-graph: add 'check' subcommand

[Derrick Stolee <dstolee@microsoft.com>]

If the commit-graph file becomes corrupt, we need a way to verify
its contents match the object database. In the manner of 'git fsck'
we will implement a 'git commit-graph check' subcommand to report
all issues with the file.

Add the 'check' subcommand to the 'commit-graph' builtin and its
documentation. Add a simple test that ensures the command returns
a zero error code.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 Documentation/git-commit-graph.txt |  7 +++++-
 builtin/commit-graph.c             | 38 ++++++++++++++++++++++++++++++
 commit-graph.c                     |  5 ++++
 commit-graph.h                     |  2 ++
 t/t5318-commit-graph.sh            |  5 ++++
 5 files changed, 56 insertions(+), 1 deletion(-)

diff --git a/Documentation/git-commit-graph.txt b/Documentation/git-commit-graph.txt
index 4c97b555cc..93c7841ba2 100644
--- a/Documentation/git-commit-graph.txt
+++ b/Documentation/git-commit-graph.txt
@@ -9,10 +9,10 @@ git-commit-graph - Write and verify Git commit graph files
 SYNOPSIS
 --------
 [verse]
+'git commit-graph check' [--object-dir <dir>]
 'git commit-graph read' [--object-dir <dir>]
 'git commit-graph write' <options> [--object-dir <dir>]
 
-
 DESCRIPTION
 -----------
 
@@ -52,6 +52,11 @@ existing commit-graph file.
 Read a graph file given by the commit-graph file and output basic
 details about the graph file. Used for debugging purposes.
 
+'check'::
+
+Read the commit-graph file and verify its contents against the object
+database. Used to check for corrupted data.
+
 
 EXAMPLES
 --------
diff --git a/builtin/commit-graph.c b/builtin/commit-graph.c
index 37420ae0fd..77c1a04932 100644
--- a/builtin/commit-graph.c
+++ b/builtin/commit-graph.c
@@ -7,11 +7,17 @@
 
 static char const * const builtin_commit_graph_usage[] = {
 	N_("git commit-graph [--object-dir <objdir>]"),
+	N_("git commit-graph check [--object-dir <objdir>]"),
 	N_("git commit-graph read [--object-dir <objdir>]"),
 	N_("git commit-graph write [--object-dir <objdir>] [--append] [--stdin-packs|--stdin-commits]"),
 	NULL
 };
 
+static const char * const builtin_commit_graph_check_usage[] = {
+	N_("git commit-graph check [--object-dir <objdir>]"),
+	NULL
+};
+
 static const char * const builtin_commit_graph_read_usage[] = {
 	N_("git commit-graph read [--object-dir <objdir>]"),
 	NULL
@@ -29,6 +35,36 @@ static struct opts_commit_graph {
 	int append;
 } opts;
 
+
+static int graph_check(int argc, const char **argv)
+{
+	struct commit_graph *graph = 0;
+	char *graph_name;
+
+	static struct option builtin_commit_graph_check_options[] = {
+		OPT_STRING(0, "object-dir", &opts.obj_dir,
+			   N_("dir"),
+			   N_("The object directory to store the graph")),
+		OPT_END(),
+	};
+
+	argc = parse_options(argc, argv, NULL,
+			     builtin_commit_graph_check_options,
+			     builtin_commit_graph_check_usage, 0);
+
+	if (!opts.obj_dir)
+		opts.obj_dir = get_object_directory();
+
+	graph_name = get_commit_graph_filename(opts.obj_dir);
+	graph = load_commit_graph_one(graph_name);
+
+	if (!graph)
+		die("graph file %s does not exist", graph_name);
+	FREE_AND_NULL(graph_name);
+
+	return check_commit_graph(graph);
+}
+
 static int graph_read(int argc, const char **argv)
 {
 	struct commit_graph *graph = NULL;
@@ -160,6 +196,8 @@ int cmd_commit_graph(int argc, const char **argv, const char *prefix)
 			     PARSE_OPT_STOP_AT_NON_OPTION);
 
 	if (argc > 0) {
+		if (!strcmp(argv[0], "check"))
+			return graph_check(argc, argv);
 		if (!strcmp(argv[0], "read"))
 			return graph_read(argc, argv);
 		if (!strcmp(argv[0], "write"))
diff --git a/commit-graph.c b/commit-graph.c
index 3f0c142603..cd0634bba0 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -819,3 +819,8 @@ void write_commit_graph(const char *obj_dir,
 	oids.alloc = 0;
 	oids.nr = 0;
 }
+
+int check_commit_graph(struct commit_graph *g)
+{
+	return !g;
+}
diff --git a/commit-graph.h b/commit-graph.h
index 96cccb10f3..e8c8d99dff 100644
--- a/commit-graph.h
+++ b/commit-graph.h
@@ -53,4 +53,6 @@ void write_commit_graph(const char *obj_dir,
 			int nr_commits,
 			int append);
 
+int check_commit_graph(struct commit_graph *g);
+
 #endif
diff --git a/t/t5318-commit-graph.sh b/t/t5318-commit-graph.sh
index 77d85aefe7..e91053271a 100755
--- a/t/t5318-commit-graph.sh
+++ b/t/t5318-commit-graph.sh
@@ -230,4 +230,9 @@ test_expect_success 'perform fast-forward merge in full repo' '
 	test_cmp expect output
 '
 
+test_expect_success 'git commit-graph check' '
+	cd "$TRASH_DIRECTORY/full" &&
+	git commit-graph check >output
+'
+
 test_done
-- 
2.17.0.39.g685157f7fb