Teach git-commit-graph to read commit graph files and summarize their contents.

Use the --read option to verify the contents of a commit graph file in the
tests.

Signed-off-by: Derrick Stolee <redacted>
---
 Documentation/git-commit-graph.txt |   7 ++
 builtin/commit-graph.c             |  55 +++++++++++++++
 commit-graph.c                     | 138 ++++++++++++++++++++++++++++++++++++-
 commit-graph.h                     |  25 +++++++
 t/t5318-commit-graph.sh            |  28 ++++++--
 5 files changed, 247 insertions(+), 6 deletions(-)

diff --git a/Documentation/git-commit-graph.txt b/Documentation/git-commit-graph.txt
index 3f3790d9a8..09aeaf6c82 100644
--- a/Documentation/git-commit-graph.txt
+++ b/Documentation/git-commit-graph.txt

@@ -10,6 +10,7 @@ SYNOPSIS
 --------
 [verse]
 'git commit-graph' --write <options> [--pack-dir <pack_dir>]
+'git commit-graph' --read <options> [--pack-dir <pack_dir>]

 EXAMPLES
 --------

@@ -20,6 +21,12 @@ EXAMPLES
 $ git commit-graph --write
 ------------------------------------------------
 
+* Read basic information from a graph file.
++
+------------------------------------------------
+$ git commit-graph --read --graph-hash=<hash>
+------------------------------------------------
+
 GIT
 ---
 Part of the linkgit:git[1] suite

diff --git a/builtin/commit-graph.c b/builtin/commit-graph.c
index 7affd512f1..218740b1f8 100644
--- a/builtin/commit-graph.c
+++ b/builtin/commit-graph.c

+		uint64_t chunk_offset = (chunk_offset1 << 32) | chunk_offset2;
+
+		if (chunk_offset > graph_size - GIT_MAX_RAWSZ)
+			die("improper chunk offset %08x%08x", (uint32_t)(chunk_offset >> 32),
+			    (uint32_t)chunk_offset);
+
+		switch (chunk_id) {
+			case GRAPH_CHUNKID_OIDFANOUT:
+				graph->chunk_oid_fanout = data + chunk_offset;
+				break;
+
+			case GRAPH_CHUNKID_OIDLOOKUP:
+				graph->chunk_oid_lookup = data + chunk_offset;
+				break;
+
+			case GRAPH_CHUNKID_DATA:
+				graph->chunk_commit_data = data + chunk_offset;
+				break;
+
+			case GRAPH_CHUNKID_LARGEEDGES:
+				graph->chunk_large_edges = data + chunk_offset;
+				break;
+
+			case 0:
+				break;
+
+			default:
+				free_commit_graph(&graph);
+				die("unrecognized graph chunk id: %08x", chunk_id);
+		}
+
+		if (last_chunk_id == GRAPH_CHUNKID_OIDLOOKUP)
+		{
+			graph->num_commits = (chunk_offset - last_chunk_offset)
+					     / hdr->hash_len;
+		}
+
+		last_chunk_id = chunk_id;
+		last_chunk_offset = chunk_offset;
+	}
+
+	strcpy(graph->pack_dir, pack_dir);
+	return graph;
+}
+
 static void write_graph_chunk_fanout(struct sha1file *f,
 				     struct commit **commits,
 				     int nr_commits)

@@ -361,7 +497,7 @@ struct object_id *construct_commit_graph(const char *pack_dir)
 	sha1close(f, final_hash, CSUM_CLOSE | CSUM_FSYNC);
 
 	f_hash = (struct object_id *)malloc(sizeof(struct object_id));
-	memcpy(f_hash->hash, final_hash, GIT_MAX_RAWSZ);
+	hashcpy(f_hash->hash, final_hash);

 	fname = get_commit_graph_filename_hash(pack_dir, f_hash);
 
 	if (rename(graph_name, fname))

diff --git a/commit-graph.h b/commit-graph.h
index 7b3469a7df..e046ae575c 100644
--- a/commit-graph.h
+++ b/commit-graph.h

@@ -15,6 +15,31 @@ struct commit_graph_header {
 	unsigned char num_chunks;
 };
 
+extern struct commit_graph {
+	int graph_fd;
+
+	const unsigned char *data;
+	size_t data_len;
+
+	const struct commit_graph_header *hdr;
+
+	struct object_id oid;
+
+	uint32_t num_commits;
+
+	const unsigned char *chunk_oid_fanout;
+	const unsigned char *chunk_oid_lookup;
+	const unsigned char *chunk_commit_data;
+	const unsigned char *chunk_large_edges;
+
+	/* something like ".git/objects/pack" */
+	char pack_dir[FLEX_ARRAY]; /* more */
+} *commit_graph;
+
+extern int close_commit_graph(struct commit_graph *g);
+
+extern struct commit_graph *load_commit_graph_one(const char *graph_file, const char *pack_dir);
+
 extern struct object_id *construct_commit_graph(const char *pack_dir);
 
 #endif

diff --git a/t/t5318-commit-graph.sh b/t/t5318-commit-graph.sh
index 6bcd1cc264..da565624e3 100755
--- a/t/t5318-commit-graph.sh
+++ b/t/t5318-commit-graph.sh

@@ -25,11 +25,23 @@ test_expect_success 'create commits and repack' \
      done &&
      git repack'
 
+_graph_read_expect() {
+    cat >expect <<- EOF
+header: 43475048 01 01 14 04
+num_commits: $1
+chunks: oid_fanout oid_lookup commit_metadata large_edges
+pack_dir: $2
+EOF
+}

     'git reset --hard commits/3 &&
      for i in $(test_seq 6 10)
      do

@@ -79,7 +91,10 @@ t_expect_success 'Add more commits' \
 
 test_expect_success 'write graph with merges' \
     'graph2=$(git commit-graph --write) &&
-     test_path_is_file ${packdir}/graph-${graph2}.graph'
+     test_path_is_file ${packdir}/graph-${graph2}.graph &&
+     git commit-graph --read --graph-hash=${graph2} >output &&
+     _graph_read_expect "18" "${packdir}" &&
+     cmp expect output'
 
 test_expect_success 'setup bare repo' \
     'cd .. &&

@@ -87,10 +102,13 @@ test_expect_success 'setup bare repo' \
      cd bare &&
      git config core.graph true &&
      git config pack.threads 1 &&
-     baredir="objects/pack"'
+     baredir="./objects/pack"'