builtin/commit-graph.c: introduce split strategy 'no-merge'

In the previous commit, we laid the groundwork for supporting different
splitting strategies. In this commit, we introduce the first splitting
strategy: 'no-merge'.

Passing '--split=no-merge' is useful for callers which wish to write a
new incremental commit-graph, but do not want to spend effort condensing
the incremental chain [1]. Previously, this was possible by passing
'--size-multiple=0', but this no longer the case following 63020f175f
(commit-graph: prefer default size_mult when given zero, 2020-01-02).

When '--split=no-merge' is given, the commit-graph machinery will never
condense an existing chain, and it will always write a new incremental.

[1]: This might occur when, for example, a server administrator running
some program after each push may want to ensure that each job runs
proportional in time to the size of the push, and does not "jump" when
the commit-graph machinery decides to trigger a merge.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

5 files changed, 36 insertions, 9 deletions

diff --git a/Documentation/git-commit-graph.txt b/Documentation/git-commit-graph.txt
index 10d757c5cc..a4c4a641e5 100644
--- a/Documentation/git-commit-graph.txt
+++ b/Documentation/git-commit-graph.txt
@@ -63,6 +63,11 @@ chain of multiple commit-graph files stored in
 strategy and other splitting options. The new commits not already in the
 commit-graph are added in a new "tip" file. This file is merged with the
 existing file if the following merge conditions are met:
+* If `--split=no-merge` is specified, a merge is never performed, and
+the remaining options are ignored. A bare `--split` defers to the
+remaining options. (Note that merging a chain of commit graphs replaces
+the existing chain with a length-1 chain where the first and only
+incremental holds the entire graph).
 +
 * If `--size-multiple=<X>` is not specified, let `X` equal 2. If the new
 tip file would have `N` commits and the previous tip has `M` commits and
diff --git a/builtin/commit-graph.c b/builtin/commit-graph.c
index 345fd97c61..9234b95ecf 100644
--- a/builtin/commit-graph.c
+++ b/builtin/commit-graph.c
@@ -118,11 +118,16 @@ static struct split_commit_graph_opts split_opts;
 static int write_option_parse_split(const struct option *opt, const char *arg,
 				    int unset)
 {
+	enum commit_graph_split_flags *flags = opt->value;
+
 	opts.split = 1;
 	if (!arg)
 		return 0;
 
-	die(_("unrecognized --split argument, %s"), arg);
+	if (!strcmp(arg, "no-merge"))
+		*flags = COMMIT_GRAPH_SPLIT_MERGE_PROHIBITED;
+	else
+		die(_("unrecognized --split argument, %s"), arg);
 
 	return 0;
 }
diff --git a/commit-graph.c b/commit-graph.c
index 656dd647d5..71f488839c 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -1529,6 +1529,7 @@ static void split_graph_merge_strategy(struct write_commit_graph_context *ctx)
 {
 	struct commit_graph *g;
 	uint32_t num_commits;
+	enum commit_graph_split_flags flags = COMMIT_GRAPH_SPLIT_UNSPECIFIED;
 	uint32_t i;
 
 	int max_commits = 0;
@@ -1539,21 +1540,25 @@ static void split_graph_merge_strategy(struct write_commit_graph_context *ctx)
 
 		if (ctx->split_opts->size_multiple)
 			size_mult = ctx->split_opts->size_multiple;
+
+		flags = ctx->split_opts->flags;
 	}
 
 	g = ctx->r->objects->commit_graph;
 	num_commits = ctx->commits.nr;
 	ctx->num_commit_graphs_after = ctx->num_commit_graphs_before + 1;
 
-	while (g && (g->num_commits <= size_mult * num_commits ||
-		    (max_commits && num_commits > max_commits))) {
-		if (g->odb != ctx->odb)
-			break;
+	if (flags != COMMIT_GRAPH_SPLIT_MERGE_PROHIBITED) {
+		while (g && (g->num_commits <= size_mult * num_commits ||
+			    (max_commits && num_commits > max_commits))) {
+			if (g->odb != ctx->odb)
+				break;
 
-		num_commits += g->num_commits;
-		g = g->base_graph;
+			num_commits += g->num_commits;
+			g = g->base_graph;
 
-		ctx->num_commit_graphs_after--;
+			ctx->num_commit_graphs_after--;
+		}
 	}
 
 	ctx->new_base_graph = g;
diff --git a/commit-graph.h b/commit-graph.h
index e799008ff4..8752afb88d 100644
--- a/commit-graph.h
+++ b/commit-graph.h
@@ -83,7 +83,8 @@ enum commit_graph_write_flags {
 };
 
 enum commit_graph_split_flags {
-	COMMIT_GRAPH_SPLIT_UNSPECIFIED = 0
+	COMMIT_GRAPH_SPLIT_UNSPECIFIED      = 0,
+	COMMIT_GRAPH_SPLIT_MERGE_PROHIBITED = 1
 };
 
 struct split_commit_graph_opts {
diff --git a/t/t5324-split-commit-graph.sh b/t/t5324-split-commit-graph.sh
index c24823431f..29de47e834 100755
--- a/t/t5324-split-commit-graph.sh
+++ b/t/t5324-split-commit-graph.sh
@@ -344,4 +344,15 @@ test_expect_success 'split across alternate where alternate is not split' '
 	test_cmp commit-graph .git/objects/info/commit-graph
 '
 
+test_expect_success '--split=no-merge always writes an incremental' '
+	test_when_finished rm -rf a b &&
+	rm -rf $graphdir $infodir/commit-graph &&
+	git reset --hard commits/2 &&
+	git rev-list HEAD~1 >a &&
+	git rev-list HEAD >b &&
+	git commit-graph write --split --stdin-commits <a &&
+	git commit-graph write --split=no-merge --stdin-commits <b &&
+	test_line_count = 2 $graphdir/commit-graph-chain
+'
+
 test_done