gc: automatically write commit-graph files

The commit-graph file is a very helpful feature for speeding up git
operations. In order to make it more useful, make it possible to
write the commit-graph file during standard garbage collection
operations.

Add a 'gc.commitGraph' config setting that triggers writing a
commit-graph file after any non-trivial 'git gc' command. Defaults to
false while the commit-graph feature matures. We specifically do not
want to have this on by default until the commit-graph feature is fully
integrated with history-modifying features like shallow clones.

Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -904,9 +904,12 @@ core.notesRef::
 This setting defaults to "refs/notes/commits", and it can be overridden by
 the `GIT_NOTES_REF` environment variable.  See linkgit:git-notes[1].
 
-core.commitGraph::
+gc.commitGraph::
-	Enable git commit graph feature. Allows reading from the
+	If true, then gc will rewrite the commit-graph file when
-	commit-graph file.
+	linkgit:git-gc[1] is run. When using linkgit:git-gc[1]
+	'--auto' the commit-graph will be updated if housekeeping is
+	required. Default is false. See linkgit:git-commit-graph[1]
+	for details.
 
 core.sparseCheckout::
 	Enable "sparse checkout" feature. See section "Sparse checkout" in

Documentation/git-gc.txt

@@ -136,6 +136,10 @@ The optional configuration variable `gc.packRefs` determines if
 it within all non-bare repos or it can be set to a boolean value.
 This defaults to true.
 
+The optional configuration variable `gc.commitGraph` determines if
+'git gc' should run 'git commit-graph write'. This can be set to a
+boolean value. This defaults to false.
+
 The optional configuration variable `gc.aggressiveWindow` controls how
 much time is spent optimizing the delta compression of the objects in
 the repository when the --aggressive option is specified.  The larger

builtin/gc.c

@@ -20,6 +20,7 @@
 #include "sigchain.h"
 #include "argv-array.h"
 #include "commit.h"
+#include "commit-graph.h"
 #include "packfile.h"
 #include "object-store.h"
 #include "pack.h"
@@ -40,6 +41,7 @@ static int aggressive_depth = 50;
 static int aggressive_window = 250;
 static int gc_auto_threshold = 6700;
 static int gc_auto_pack_limit = 50;
+static int gc_write_commit_graph;
 static int detach_auto = 1;
 static timestamp_t gc_log_expire_time;
 static const char *gc_log_expire = "1.day.ago";
@@ -129,6 +131,7 @@ static void gc_config(void)
 	git_config_get_int("gc.aggressivedepth", &aggressive_depth);
 	git_config_get_int("gc.auto", &gc_auto_threshold);
 	git_config_get_int("gc.autopacklimit", &gc_auto_pack_limit);
+	git_config_get_bool("gc.writecommitgraph", &gc_write_commit_graph);
 	git_config_get_bool("gc.autodetach", &detach_auto);
 	git_config_get_expiry("gc.pruneexpire", &prune_expire);
 	git_config_get_expiry("gc.worktreepruneexpire", &prune_worktrees_expire);
@@ -641,6 +644,9 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 	if (pack_garbage.nr > 0)
 		clean_pack_garbage();
 
+	if (gc_write_commit_graph)
+		write_commit_graph_reachable(get_object_directory(), 0);
+
 	if (auto_gc && too_many_loose_objects())
 		warning(_("There are too many unreachable loose objects; "
 			"run 'git prune' to remove them."));

t/t5318-commit-graph.sh

@@ -245,6 +245,20 @@ test_expect_success 'perform fast-forward merge in full repo' '
 	test_cmp expect output
 '
 
+test_expect_success 'check that gc computes commit-graph' '
+	cd "$TRASH_DIRECTORY/full" &&
+	git commit --allow-empty -m "blank" &&
+	git commit-graph write --reachable &&
+	cp $objdir/info/commit-graph commit-graph-before-gc &&
+	git reset --hard HEAD~1 &&
+	git config gc.writeCommitGraph true &&
+	git gc &&
+	cp $objdir/info/commit-graph commit-graph-after-gc &&
+	! test_cmp commit-graph-before-gc commit-graph-after-gc &&
+	git commit-graph write --reachable &&
+	test_cmp commit-graph-after-gc $objdir/info/commit-graph
+'
+
 # the verify tests below expect the commit-graph to contain
 # exactly the commits reachable from the commits/8 branch.
 # If the file changes the set of commits in the list, then the