fetch: add fetch.writeCommitGraph config setting

The commit-graph feature is now on by default, and is being
written during 'git gc' by default. Typically, Git only writes
a commit-graph when a 'git gc --auto' command passes the gc.auto
setting to actualy do work. This means that a commit-graph will
typically fall behind the commits that are being used every day.

To stay updated with the latest commits, add a step to 'git
fetch' to write a commit-graph after fetching new objects. The
fetch.writeCommitGraph config setting enables writing a split
commit-graph, so on average the cost of writing this file is
very small. Occasionally, the commit-graph chain will collapse
to a single level, and this could be slow for very large repos.

For additional use, adjust the default to be true when
feature.experimental is enabled.

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

6 files changed, 51 insertions, 0 deletions

diff --git a/Documentation/config/feature.txt b/Documentation/config/feature.txt
index 545522f306..875f8c8a66 100644
--- a/Documentation/config/feature.txt
+++ b/Documentation/config/feature.txt
@@ -17,6 +17,14 @@ which can improve `git push` performance in repos with many files.
 +
 * `fetch.negotiationAlgorithm=skipping` may improve fetch negotiation times by
 skipping more commits at a time, reducing the number of round trips.
++
+* `fetch.writeCommitGraph=true` writes a commit-graph after every `git fetch`
+command that downloads a pack-file from a remote. Using the `--split` option,
+most executions will create a very small commit-graph file on top of the
+existing commit-graph file(s). Occasionally, these files will merge and the
+write may take longer. Having an updated commit-graph file helps performance
+of many Git commands, including `git merge-base`, `git push -f`, and
+`git log --graph`.
 
 feature.manyFiles::
 	Enable config options that optimize for repos with many files in the
diff --git a/Documentation/config/fetch.txt b/Documentation/config/fetch.txt
index d402110638..e8cb20547c 100644
--- a/Documentation/config/fetch.txt
+++ b/Documentation/config/fetch.txt
@@ -69,3 +69,13 @@ fetch.showForcedUpdates::
 	Set to false to enable `--no-show-forced-updates` in
 	linkgit:git-fetch[1] and linkgit:git-pull[1] commands.
 	Defaults to true.
+
+fetch.writeCommitGraph::
+	Set to true to write a commit-graph after every `git fetch` command
+	that downloads a pack-file from a remote. Using the `--split` option,
+	most executions will create a very small commit-graph file on top of
+	the existing commit-graph file(s). Occasionally, these files will
+	merge and the write may take longer. Having an updated commit-graph
+	file helps performance of many Git commands, including `git merge-base`,
+	`git push -f`, and `git log --graph`. Defaults to false, unless
+	`feature.experimental` is true.
diff --git a/builtin/fetch.c b/builtin/fetch.c
index 53ce99d2bb..d36a403859 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -23,6 +23,7 @@
 #include "packfile.h"
 #include "list-objects-filter-options.h"
 #include "commit-reach.h"
+#include "commit-graph.h"
 
 #define FORCED_UPDATES_DELAY_WARNING_IN_MS (10 * 1000)
 
@@ -1715,6 +1716,20 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 
 	string_list_clear(&list, 0);
 
+	prepare_repo_settings(the_repository);
+	if (the_repository->settings.fetch_write_commit_graph) {
+		int commit_graph_flags = COMMIT_GRAPH_SPLIT;
+		struct split_commit_graph_opts split_opts;
+		memset(&split_opts, 0, sizeof(struct split_commit_graph_opts));
+
+		if (progress)
+			commit_graph_flags |= COMMIT_GRAPH_PROGRESS;
+
+		write_commit_graph_reachable(get_object_directory(),
+					     commit_graph_flags,
+					     &split_opts);
+	}
+
 	close_object_store(the_repository->objects);
 
 	if (enable_auto_gc) {
diff --git a/repo-settings.c b/repo-settings.c
index 3779b85c17..05546db98e 100644
--- a/repo-settings.c
+++ b/repo-settings.c
@@ -49,10 +49,14 @@ void prepare_repo_settings(struct repository *r)
 		UPDATE_DEFAULT_BOOL(r->settings.index_version, 4);
 		UPDATE_DEFAULT_BOOL(r->settings.core_untracked_cache, UNTRACKED_CACHE_WRITE);
 	}
+	if (!repo_config_get_bool(r, "fetch.writecommitgraph", &value))
+		r->settings.fetch_write_commit_graph = value;
 	if (!repo_config_get_bool(r, "feature.experimental", &value) && value) {
 		UPDATE_DEFAULT_BOOL(r->settings.pack_use_sparse, 1);
 		UPDATE_DEFAULT_BOOL(r->settings.fetch_negotiation_algorithm, FETCH_NEGOTIATION_SKIPPING);
+		UPDATE_DEFAULT_BOOL(r->settings.fetch_write_commit_graph, 1);
 	}
+	UPDATE_DEFAULT_BOOL(r->settings.fetch_write_commit_graph, 0);
 
 	/* Hack for test programs like test-dump-untracked-cache */
 	if (ignore_untracked_cache_config)
diff --git a/repository.h b/repository.h
index 4da275e73f..fe0b5f5dc6 100644
--- a/repository.h
+++ b/repository.h
@@ -30,6 +30,7 @@ struct repo_settings {
 
 	int core_commit_graph;
 	int gc_write_commit_graph;
+	int fetch_write_commit_graph;
 
 	int index_version;
 	enum untracked_cache_setting core_untracked_cache;
diff --git a/t/t5510-fetch.sh b/t/t5510-fetch.sh
index 139f7106f7..91ede622f0 100755
--- a/t/t5510-fetch.sh
+++ b/t/t5510-fetch.sh
@@ -570,6 +570,19 @@ test_expect_success 'LHS of refspec follows ref disambiguation rules' '
 	)
 '
 
+test_expect_success 'fetch.writeCommitGraph' '
+	git clone three write &&
+	(
+		cd three &&
+		test_commit new
+	) &&
+	(
+		cd write &&
+		git -c fetch.writeCommitGraph fetch origin &&
+		test_path_is_file .git/objects/info/commit-graphs/commit-graph-chain
+	)
+'
+
 # configured prune tests
 
 set_config_tristate () {