commit-graph: implement "--additive" option

Teach git-commit-graph to add all commits from the existing commit-graph file to the file about to be written. This should be used when adding new commits without performing garbage collection. Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

Author: derrickstolee

Documentation/git-commit-graph.txt

@@ -43,6 +43,9 @@ With the `--stdin-commits` option, generate the new commit graph by
 walking commits starting at the commits specified in stdin as a list
 of OIDs in hex, one OID per line. (Cannot be combined with
 --stdin-packs.)
++
+With the `--append` option, include all commits that are present in the
+existing commit-graph file.
 
 'read'::
 
@@ -72,6 +75,13 @@ $ echo <pack-index> | git commit-graph write --stdin-packs
 $ git show-ref -s | git commit-graph write --stdin-commits
 ------------------------------------------------
 
+* Write a graph file containing all commits in the current
+* commit-graph file along with those reachable from HEAD.
++
+------------------------------------------------
+$ git rev-parse HEAD | git commit-graph write --stdin-commits --append
+------------------------------------------------
+
 * Read basic information from the commit-graph file.
 +
 ------------------------------------------------

builtin/commit-graph.c

@@ -8,7 +8,7 @@
 static char const * const builtin_commit_graph_usage[] = {
 N_("git commit-graph [--object-dir <objdir>]"),
 N_("git commit-graph read [--object-dir <objdir>]"),
-N_("git commit-graph write [--object-dir <objdir>] [--stdin-packs|--stdin-commits]"),
+N_("git commit-graph write [--object-dir <objdir>] [--append] [--stdin-packs|--stdin-commits]"),
 NULL
 };
 
@@ -18,14 +18,15 @@ static const char * const builtin_commit_graph_read_usage[] = {
 };
 
 static const char * const builtin_commit_graph_write_usage[] = {
-N_("git commit-graph write [--object-dir <objdir>] [--stdin-packs|--stdin-commits]"),
+N_("git commit-graph write [--object-dir <objdir>] [--append] [--stdin-packs|--stdin-commits]"),
 NULL
 };
 
 static struct opts_commit_graph {
 const char *obj_dir;
 int stdin_packs;
 int stdin_commits;
+int append;
 } opts;
 
 static int graph_read(int argc, const char **argv)
@@ -94,6 +95,8 @@ static int graph_write(int argc, const char **argv)
 N_("scan pack-indexes listed by stdin for commits")),
 OPT_BOOL(0, "stdin-commits", &opts.stdin_commits,
 N_("start walk at commits listed by stdin")),
+OPT_BOOL(0, "append", &opts.append,
+N_("include all commits already in the commit-graph file")),
 OPT_END(),
 };
 
@@ -131,7 +134,8 @@ static int graph_write(int argc, const char **argv)
  pack_indexes,
  packs_nr,
  commit_hex,
- commits_nr);
+ commits_nr,
+ opts.append);
 
 return 0;
 }

commit-graph.c

@@ -553,7 +553,8 @@ void write_commit_graph(const char *obj_dir,
 const char **pack_indexes,
 int nr_packs,
 const char **commit_hex,
-int nr_commits)
+int nr_commits,
+int append)
 {
 struct packed_oid_list oids;
 struct packed_commit_list commits;
@@ -571,10 +572,24 @@ void write_commit_graph(const char *obj_dir,
 oids.nr = 0;
 oids.alloc = approximate_object_count() / 4;
 
+if (append) {
+prepare_commit_graph_one(obj_dir);
+if (commit_graph)
+oids.alloc += commit_graph->num_commits;
+}
+
 if (oids.alloc < 1024)
 oids.alloc = 1024;
 ALLOC_ARRAY(oids.list, oids.alloc);
 
+if (append && commit_graph) {
+for (i = 0; i < commit_graph->num_commits; i++) {
+const unsigned char *hash = commit_graph->chunk_oid_lookup +
+commit_graph->hash_len * i;
+hashcpy(oids.list[oids.nr++].hash, hash);
+}
+}
+
 if (pack_indexes) {
 struct strbuf packname = STRBUF_INIT;
 int dirlen;

commit-graph.h

@@ -40,6 +40,7 @@ void write_commit_graph(const char *obj_dir,
 const char **pack_indexes,
 int nr_packs,
 const char **commit_hex,
-int nr_commits);
+int nr_commits,
+int append);
 
 #endif

t/t5318-commit-graph.sh

@@ -190,6 +190,16 @@ test_expect_success 'build graph from commits with closure' '
 graph_git_behavior 'graph from commits, commit 8 vs merge 1' full commits/8 merge/1
 graph_git_behavior 'graph from commits, commit 8 vs merge 2' full commits/8 merge/2
 
+test_expect_success 'build graph from commits with append' '
+cd "$TRASH_DIRECTORY/full" &&
+git rev-parse merge/3 | git commit-graph write --stdin-commits --append &&
+test_path_is_file $objdir/info/commit-graph &&
+graph_read_expect "10" "large_edges"
+'
+
+graph_git_behavior 'append graph, commit 8 vs merge 1' full commits/8 merge/1
+graph_git_behavior 'append graph, commit 8 vs merge 2' full commits/8 merge/2
+
 test_expect_success 'setup bare repo' '
 cd "$TRASH_DIRECTORY" &&
 git clone --bare --no-local full bare &&