[PATCH v2 3/3] builtin/commit-graph.c: support '--input=none'

[Taylor Blau <me@ttaylorr.com>]

In the previous commit, we introduced '--split=<no-merge|merge-all>',
and alluded to the fact that '--split=merge-all' would be useful for
callers who wish to always trigger a merge of an incremental chain.

There is a problem with the above approach, which is that there is no
way to specify to the commit-graph builtin that a caller only wants to
include commits already in the graph. One can specify '--input=append'
to include all commits in the existing graphs, but the absence of
'--input=stdin-{commits,packs}' causes the builtin to call
'fill_oids_from_all_packs()'.

Passing '--input=reachable' (as in 'git commit-graph write
--split=merge-all --input=reachable --input=append') works around this
issue by making '--input=reachable' effectively a no-op, but this can be
prohibitively expensive in large repositories, making it an undesirable
choice for some users.

Teach '--input=none' as an option to behave as if '--input=append' were
given, but to consider no other sources in addition.

This, in conjunction with the option introduced in the previous patch
offers the convenient way to force the commit-graph machinery to
condense a chain of incrementals without requiring any new commits:

  $ git commit-graph write --split=merge-all --input=none

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 Documentation/git-commit-graph.txt |  8 +++++++-
 builtin/commit-graph.c             | 11 ++++++++---
 commit-graph.c                     |  6 ++++--
 commit-graph.h                     |  3 ++-
 t/t5324-split-commit-graph.sh      | 26 ++++++++++++++++++++++++++
 5 files changed, 47 insertions(+), 7 deletions(-)

diff --git a/Documentation/git-commit-graph.txt b/Documentation/git-commit-graph.txt
index 2ae9de679a..633cfbe023 100644
--- a/Documentation/git-commit-graph.txt
+++ b/Documentation/git-commit-graph.txt
@@ -39,7 +39,7 @@ COMMANDS
 --------
 'write'::
 
-Write a commit-graph file based on the commits found in packfiles.
+Write a commit-graph file based on the specified sources of input:
 +
 With the `--input=stdin-packs` option, generate the new commit graph by
 walking objects only in the specified pack-indexes. (Cannot be combined
@@ -57,6 +57,12 @@ walking commits starting at all refs. (Cannot be combined with
 With the `--input=append` option, include all commits that are present
 in the existing commit-graph file.
 +
+With the `--input=none` option, behave as if `--input=append` were
+given, but do not walk other packs to find additional commits.
+
+If none of the above options are given, then generate the new
+commit-graph by walking over all pack-indexes.
++
 With the `--split[=<strategy>]` option, write the commit-graph as a
 chain of multiple commit-graph files stored in
 `<dir>/info/commit-graphs`. Commit-graph layers are merged based on the
diff --git a/builtin/commit-graph.c b/builtin/commit-graph.c
index 0ff25896d0..a71af88815 100644
--- a/builtin/commit-graph.c
+++ b/builtin/commit-graph.c
@@ -11,7 +11,7 @@ static char const * const builtin_commit_graph_usage[] = {
 	N_("git commit-graph verify [--object-dir <objdir>] [--shallow] [--[no-]progress]"),
 	N_("git commit-graph write [--object-dir <objdir>] "
 	   "[--split[=<strategy>]] "
-	   "[--input=<reachable|stdin-packs|stdin-commits|append>] "
+	   "[--input=<reachable|stdin-packs|stdin-commits|append|none>] "
 	   "[--[no-]progress] <split options>"),
 	NULL
 };
@@ -24,7 +24,7 @@ static const char * const builtin_commit_graph_verify_usage[] = {
 static const char * const builtin_commit_graph_write_usage[] = {
 	N_("git commit-graph write [--object-dir <objdir>] "
 	   "[--split[=<strategy>]] "
-	   "[--input=<reachable|stdin-packs|stdin-commits|append>] "
+	   "[--input=<reachable|stdin-packs|stdin-commits|append|none>] "
 	   "[--[no-]progress] <split options>"),
 	NULL
 };
@@ -33,7 +33,8 @@ enum commit_graph_input {
 	COMMIT_GRAPH_INPUT_REACHABLE     = (1 << 1),
 	COMMIT_GRAPH_INPUT_STDIN_PACKS   = (1 << 2),
 	COMMIT_GRAPH_INPUT_STDIN_COMMITS = (1 << 3),
-	COMMIT_GRAPH_INPUT_APPEND        = (1 << 4)
+	COMMIT_GRAPH_INPUT_APPEND        = (1 << 4),
+	COMMIT_GRAPH_INPUT_NONE          = (1 << 5)
 };
 
 static struct opts_commit_graph {
@@ -80,6 +81,8 @@ static int option_parse_input(const struct option *opt, const char *arg,
 		*to |= COMMIT_GRAPH_INPUT_STDIN_COMMITS;
 	else if (!strcmp(arg, "append"))
 		*to |= COMMIT_GRAPH_INPUT_APPEND;
+	else if (!strcmp(arg, "none"))
+		*to |= (COMMIT_GRAPH_INPUT_APPEND | COMMIT_GRAPH_INPUT_NONE);
 	else
 		die(_("unrecognized --input source, %s"), arg);
 	return 0;
@@ -225,6 +228,8 @@ static int graph_write(int argc, const char **argv)
 		opts.obj_dir = get_object_directory();
 	if (opts.input & COMMIT_GRAPH_INPUT_APPEND)
 		flags |= COMMIT_GRAPH_WRITE_APPEND;
+	if (opts.input & COMMIT_GRAPH_INPUT_NONE)
+		flags |= COMMIT_GRAPH_WRITE_NO_INPUT;
 	if (opts.split)
 		flags |= COMMIT_GRAPH_WRITE_SPLIT;
 	if (opts.progress)
diff --git a/commit-graph.c b/commit-graph.c
index 3a5cb23cd7..417b7eac9c 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -788,7 +788,8 @@ struct write_commit_graph_context {
 	unsigned append:1,
 		 report_progress:1,
 		 split:1,
-		 check_oids:1;
+		 check_oids:1,
+		 no_input:1;
 
 	const struct split_commit_graph_opts *split_opts;
 };
@@ -1785,6 +1786,7 @@ int write_commit_graph(struct object_directory *odb,
 	ctx->split = flags & COMMIT_GRAPH_WRITE_SPLIT ? 1 : 0;
 	ctx->check_oids = flags & COMMIT_GRAPH_WRITE_CHECK_OIDS ? 1 : 0;
 	ctx->split_opts = split_opts;
+	ctx->no_input = flags & COMMIT_GRAPH_WRITE_NO_INPUT ? 1 : 0;
 
 	if (ctx->split) {
 		struct commit_graph *g;
@@ -1843,7 +1845,7 @@ int write_commit_graph(struct object_directory *odb,
 			goto cleanup;
 	}
 
-	if (!pack_indexes && !commit_hex)
+	if (!ctx->no_input && !pack_indexes && !commit_hex)
 		fill_oids_from_all_packs(ctx);
 
 	close_reachable(ctx);
diff --git a/commit-graph.h b/commit-graph.h
index 65a7d2edae..df7f3f5961 100644
--- a/commit-graph.h
+++ b/commit-graph.h
@@ -79,7 +79,8 @@ enum commit_graph_write_flags {
 	COMMIT_GRAPH_WRITE_PROGRESS   = (1 << 1),
 	COMMIT_GRAPH_WRITE_SPLIT      = (1 << 2),
 	/* Make sure that each OID in the input is a valid commit OID. */
-	COMMIT_GRAPH_WRITE_CHECK_OIDS = (1 << 3)
+	COMMIT_GRAPH_WRITE_CHECK_OIDS = (1 << 3),
+	COMMIT_GRAPH_WRITE_NO_INPUT   = (1 << 4)
 };
 
 enum commit_graph_split_flags {
diff --git a/t/t5324-split-commit-graph.sh b/t/t5324-split-commit-graph.sh
index 353523eca4..e3f317a1f4 100755
--- a/t/t5324-split-commit-graph.sh
+++ b/t/t5324-split-commit-graph.sh
@@ -369,4 +369,30 @@ test_expect_success '--split=no-merge always writes an incremental' '
 	test_line_count = 2 $graphdir/commit-graph-chain
 '
 
+test_expect_success '--split=no-merge, --input=none writes nothing' '
+	test_when_finished rm -rf a graphs.before graphs.after &&
+	rm -rf $graphdir &&
+	git reset --hard commits/2 &&
+	git rev-list -1 HEAD~1 >a &&
+	git commit-graph write --split=no-merge --input=stdin-commits <a &&
+	ls $graphdir/graph-*.graph >graphs.before &&
+	test_line_count = 1 $graphdir/commit-graph-chain &&
+	git commit-graph write --split --input=none &&
+	ls $graphdir/graph-*.graph >graphs.after &&
+	test_cmp graphs.before graphs.after
+'
+
+test_expect_success '--split=merge-all, --input=none merges the chain' '
+	test_when_finished rm -rf a b &&
+	rm -rf $graphdir &&
+	git reset --hard commits/2 &&
+	git rev-list -1 HEAD~1 >a &&
+	git rev-list -1 HEAD >b &&
+	git commit-graph write --split=no-merge --input=stdin-commits <a &&
+	git commit-graph write --split=no-merge --input=stdin-commits <b &&
+	test_line_count = 2 $graphdir/commit-graph-chain &&
+	git commit-graph write --split=merge-all --input=none &&
+	test_line_count = 1 $graphdir/commit-graph-chain
+'
+
 test_done
-- 
2.25.0.119.gaa12b7378b