commit-graph: use config to specify generation type

We have two established generation number versions:

 1: topological levels
 2: corrected commit dates

The corrected commit dates are enabled by default, but they also write
extra data in the GDAT and GDOV chunks. Services that host Git data
might want to have more control over when this feature rolls out than
just updating the Git binaries.

Add a new "commitGraph.generationVersion" config option that specifies
the intended generation number version. If this value is less than 2,
then the GDAT chunk is never written _or read_ from an existing file.

This can replace our use of the GIT_TEST_COMMIT_GRAPH_NO_GDAT
environment variable in the test suite. Remove it.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

7 files changed, 25 insertions, 15 deletions

diff --git a/Documentation/config/commitgraph.txt b/Documentation/config/commitgraph.txt
index 4582c39fc4..30604e4a4c 100644
--- a/Documentation/config/commitgraph.txt
+++ b/Documentation/config/commitgraph.txt
@@ -1,3 +1,9 @@
+commitGraph.generationVersion::
+	Specifies the type of generation number version to use when writing
+	or reading the commit-graph file. If version 1 is specified, then
+	the corrected commit dates will not be written or read. Defaults to
+	2.
+
 commitGraph.maxNewFilters::
 	Specifies the default value for the `--max-new-filters` option of `git
 	commit-graph write` (c.f., linkgit:git-commit-graph[1]).
diff --git a/commit-graph.c b/commit-graph.c
index 8b07a9a0a6..1e9b88c003 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -96,6 +96,13 @@ define_commit_slab(commit_graph_data_slab, struct commit_graph_data);
 static struct commit_graph_data_slab commit_graph_data_slab =
 	COMMIT_SLAB_INIT(1, commit_graph_data_slab);
 
+static int get_configured_generation_version(struct repository *r)
+{
+	int version = 2;
+	repo_config_get_int(r, "commitgraph.generationversion", &version);
+	return version;
+}
+
 uint32_t commit_graph_position(const struct commit *c)
 {
 	struct commit_graph_data *data =
@@ -394,10 +401,13 @@ struct commit_graph *parse_commit_graph(struct repository *r,
 	pair_chunk(cf, GRAPH_CHUNKID_DATA, &graph->chunk_commit_data);
 	pair_chunk(cf, GRAPH_CHUNKID_EXTRAEDGES, &graph->chunk_extra_edges);
 	pair_chunk(cf, GRAPH_CHUNKID_BASE, &graph->chunk_base_graphs);
-	pair_chunk(cf, GRAPH_CHUNKID_GENERATION_DATA,
-		   &graph->chunk_generation_data);
-	pair_chunk(cf, GRAPH_CHUNKID_GENERATION_DATA_OVERFLOW,
-		   &graph->chunk_generation_data_overflow);
+
+	if (get_configured_generation_version(r) >= 2) {
+		pair_chunk(cf, GRAPH_CHUNKID_GENERATION_DATA,
+			&graph->chunk_generation_data);
+		pair_chunk(cf, GRAPH_CHUNKID_GENERATION_DATA_OVERFLOW,
+			&graph->chunk_generation_data_overflow);
+	}
 
 	if (r->settings.commit_graph_read_changed_paths) {
 		pair_chunk(cf, GRAPH_CHUNKID_BLOOMINDEXES,
@@ -1771,8 +1781,6 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 	add_chunk(cf, GRAPH_CHUNKID_DATA, (hashsz + 16) * ctx->commits.nr,
 		  write_graph_chunk_data);
 
-	if (git_env_bool(GIT_TEST_COMMIT_GRAPH_NO_GDAT, 0))
-		ctx->write_generation_data = 0;
 	if (ctx->write_generation_data)
 		add_chunk(cf, GRAPH_CHUNKID_GENERATION_DATA,
 			  sizeof(uint32_t) * ctx->commits.nr,
@@ -2179,7 +2187,7 @@ int write_commit_graph(struct object_directory *odb,
 	ctx->split = flags & COMMIT_GRAPH_WRITE_SPLIT ? 1 : 0;
 	ctx->opts = opts;
 	ctx->total_bloom_filter_data_size = 0;
-	ctx->write_generation_data = 1;
+	ctx->write_generation_data = (get_configured_generation_version(r) == 2);
 	ctx->num_generation_data_overflows = 0;
 
 	bloom_settings.bits_per_entry = git_env_ulong("GIT_TEST_BLOOM_SETTINGS_BITS_PER_ENTRY",
diff --git a/commit-graph.h b/commit-graph.h
index 97f3497c27..96c24fb577 100644
--- a/commit-graph.h
+++ b/commit-graph.h
@@ -6,7 +6,6 @@
 #include "oidset.h"
 
 #define GIT_TEST_COMMIT_GRAPH "GIT_TEST_COMMIT_GRAPH"
-#define GIT_TEST_COMMIT_GRAPH_NO_GDAT "GIT_TEST_COMMIT_GRAPH_NO_GDAT"
 #define GIT_TEST_COMMIT_GRAPH_DIE_ON_PARSE "GIT_TEST_COMMIT_GRAPH_DIE_ON_PARSE"
 #define GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS "GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS"
 
diff --git a/t/README b/t/README
index 8a12148727..c730a70770 100644
--- a/t/README
+++ b/t/README
@@ -393,9 +393,6 @@ GIT_TEST_COMMIT_GRAPH=<boolean>, when true, forces the commit-graph to
 be written after every 'git commit' command, and overrides the
 'core.commitGraph' setting to true.
 
-GIT_TEST_COMMIT_GRAPH_NO_GDAT=<boolean>, when true, forces the
-commit-graph to be written without generation data chunk.
-
 GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS=<boolean>, when true, forces
 commit-graph write to compute and write changed path Bloom filters for
 every 'git commit-graph write', as if the `--changed-paths` option was
diff --git a/t/t5318-commit-graph.sh b/t/t5318-commit-graph.sh
index c7da741284..6c051ca6bc 100755
--- a/t/t5318-commit-graph.sh
+++ b/t/t5318-commit-graph.sh
@@ -454,7 +454,7 @@ test_expect_success 'warn on improper hash version' '
 
 test_expect_success 'git commit-graph verify' '
 	cd "$TRASH_DIRECTORY/full" &&
-	git rev-parse commits/8 | GIT_TEST_COMMIT_GRAPH_NO_GDAT=1 git commit-graph write --stdin-commits &&
+	git rev-parse commits/8 | git -c commitGraph.generationVersion=1 commit-graph write --stdin-commits &&
 	git commit-graph verify >output &&
 	graph_read_expect 9 extra_edges
 '
diff --git a/t/t5324-split-commit-graph.sh b/t/t5324-split-commit-graph.sh
index 8e90f3423b..587226ed10 100755
--- a/t/t5324-split-commit-graph.sh
+++ b/t/t5324-split-commit-graph.sh
@@ -489,7 +489,7 @@ test_expect_success 'setup repo for mixed generation commit-graph-chain' '
 			test_commit $i &&
 			git branch commits/$i || return 1
 		done &&
-		git commit-graph write --reachable --split &&
+		git -c commitGraph.generationVersion=2 commit-graph write --reachable --split &&
 		graph_read_expect $NUM_FIRST_LAYER_COMMITS &&
 		test_line_count = 1 $graphdir/commit-graph-chain &&
 		for i in $(test_seq $SECOND_LAYER_SEQUENCE_START $SECOND_LAYER_SEQUENCE_END)
@@ -497,7 +497,7 @@ test_expect_success 'setup repo for mixed generation commit-graph-chain' '
 			test_commit $i &&
 			git branch commits/$i || return 1
 		done &&
-		GIT_TEST_COMMIT_GRAPH_NO_GDAT=1 git commit-graph write --reachable --split=no-merge &&
+		git -c commitGraph.generationVersion=1 commit-graph write --reachable --split=no-merge &&
 		test_line_count = 2 $graphdir/commit-graph-chain &&
 		test-tool read-graph >output &&
 		cat >expect <<-EOF &&
diff --git a/t/t6600-test-reach.sh b/t/t6600-test-reach.sh
index e2d33a8a4c..3d7a62ddab 100755
--- a/t/t6600-test-reach.sh
+++ b/t/t6600-test-reach.sh
@@ -55,7 +55,7 @@ test_expect_success 'setup' '
 	git show-ref -s commit-5-5 | git commit-graph write --stdin-commits &&
 	mv .git/objects/info/commit-graph commit-graph-half &&
 	chmod u+w commit-graph-half &&
-	GIT_TEST_COMMIT_GRAPH_NO_GDAT=1 git commit-graph write --reachable &&
+	git -c commitGraph.generationVersion=1 commit-graph write --reachable &&
 	mv .git/objects/info/commit-graph commit-graph-no-gdat &&
 	chmod u+w commit-graph-no-gdat &&
 	git config core.commitGraph true