commit-graph: prepare for commit-graph chains

To prepare for a chain of commit-graph files, augment the
commit_graph struct to point to a base commit_graph. As we load
commits from the graph, we may actually want to read from a base
file according to the graph position.

The "graph position" of a commit is given by concatenating the
lexicographic commit orders from each of the commit-graph files in
the chain. This means that we must distinguish two values:

 * lexicographic index : the position within the lexicographic
   order in a single commit-graph file.

 * graph position: the position within the concatenated order
   of multiple commit-graph files

Given the lexicographic index of a commit in a graph, we can
compute the graph position by adding the number of commits in
the lower-level graphs. To find the lexicographic index of
a commit, we subtract the number of commits in lower-level graphs.

While here, change insert_parent_or_die() to take a uint32_t
position, as that is the type used by its only caller and that
makes more sense with the limits in the commit-graph format.

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

commit-graph.c

@@ -359,9 +359,18 @@ int generation_numbers_enabled(struct repository *r)
 	return !!first_generation;
 }
 
+static void close_commit_graph_one(struct commit_graph *g)
+{
+	if (!g)
+		return;
+
+	close_commit_graph_one(g->base_graph);
+	free_commit_graph(g);
+}
+
 void close_commit_graph(struct raw_object_store *o)
 {
-	free_commit_graph(o->commit_graph);
+	close_commit_graph_one(o->commit_graph);
 	o->commit_graph = NULL;
 }
 
@@ -371,18 +380,38 @@ static int bsearch_graph(struct commit_graph *g, struct object_id *oid, uint32_t
 			    g->chunk_oid_lookup, g->hash_len, pos);
 }
 
+static void load_oid_from_graph(struct commit_graph *g,
+				uint32_t pos,
+				struct object_id *oid)
+{
+	uint32_t lex_index;
+
+	while (g && pos < g->num_commits_in_base)
+		g = g->base_graph;
+
+	if (!g)
+		BUG("NULL commit-graph");
+
+	if (pos >= g->num_commits + g->num_commits_in_base)
+		die(_("invalid commit position. commit-graph is likely corrupt"));
+
+	lex_index = pos - g->num_commits_in_base;
+
+	hashcpy(oid->hash, g->chunk_oid_lookup + g->hash_len * lex_index);
+}
+
 static struct commit_list **insert_parent_or_die(struct repository *r,
 						 struct commit_graph *g,
-						 uint64_t pos,
+						 uint32_t pos,
 						 struct commit_list **pptr)
 {
 	struct commit *c;
 	struct object_id oid;
 
-	if (pos >= g->num_commits)
-		die("invalid parent position %"PRIu64, pos);
+	if (pos >= g->num_commits + g->num_commits_in_base)
+		die("invalid parent position %"PRIu32, pos);
 
-	hashcpy(oid.hash, g->chunk_oid_lookup + g->hash_len * pos);
+	load_oid_from_graph(g, pos, &oid);
 	c = lookup_commit(r, &oid);
 	if (!c)
 		die(_("could not find commit %s"), oid_to_hex(&oid));
@@ -392,7 +421,14 @@ static struct commit_list **insert_parent_or_die(struct repository *r,
 
 static void fill_commit_graph_info(struct commit *item, struct commit_graph *g, uint32_t pos)
 {
-	const unsigned char *commit_data = g->chunk_commit_data + GRAPH_DATA_WIDTH * pos;
+	const unsigned char *commit_data;
+	uint32_t lex_index;
+
+	while (pos < g->num_commits_in_base)
+		g = g->base_graph;
+
+	lex_index = pos - g->num_commits_in_base;
+	commit_data = g->chunk_commit_data + GRAPH_DATA_WIDTH * lex_index;
 	item->graph_pos = pos;
 	item->generation = get_be32(commit_data + g->hash_len + 8) >> 2;
 }
@@ -405,10 +441,25 @@ static int fill_commit_in_graph(struct repository *r,
 	uint32_t *parent_data_ptr;
 	uint64_t date_low, date_high;
 	struct commit_list **pptr;
-	const unsigned char *commit_data = g->chunk_commit_data + (g->hash_len + 16) * pos;
+	const unsigned char *commit_data;
+	uint32_t lex_index;
 
-	item->object.parsed = 1;
+	while (pos < g->num_commits_in_base)
+		g = g->base_graph;
+
+	if (pos >= g->num_commits + g->num_commits_in_base)
+		die(_("invalid commit position. commit-graph is likely corrupt"));
+
+	/*
+	 * Store the "full" position, but then use the
+	 * "local" position for the rest of the calculation.
+	 */
 	item->graph_pos = pos;
+	lex_index = pos - g->num_commits_in_base;
+
+	commit_data = g->chunk_commit_data + (g->hash_len + 16) * lex_index;
+
+	item->object.parsed = 1;
 
 	item->maybe_tree = NULL;
 
@@ -452,7 +503,18 @@ static int find_commit_in_graph(struct commit *item, struct commit_graph *g, uin
 		*pos = item->graph_pos;
 		return 1;
 	} else {
-		return bsearch_graph(g, &(item->object.oid), pos);
+		struct commit_graph *cur_g = g;
+		uint32_t lex_index;
+
+		while (cur_g && !bsearch_graph(cur_g, &(item->object.oid), &lex_index))
+			cur_g = cur_g->base_graph;
+
+		if (cur_g) {
+			*pos = lex_index + cur_g->num_commits_in_base;
+			return 1;
+		}
+
+		return 0;
 	}
 }
 
@@ -492,8 +554,13 @@ static struct tree *load_tree_for_commit(struct repository *r,
 					 struct commit *c)
 {
 	struct object_id oid;
-	const unsigned char *commit_data = g->chunk_commit_data +
-					   GRAPH_DATA_WIDTH * (c->graph_pos);
+	const unsigned char *commit_data;
+
+	while (c->graph_pos < g->num_commits_in_base)
+		g = g->base_graph;
+
+	commit_data = g->chunk_commit_data +
+			GRAPH_DATA_WIDTH * (c->graph_pos - g->num_commits_in_base);
 
 	hashcpy(oid.hash, commit_data);
 	c->maybe_tree = lookup_tree(r, &oid);

commit-graph.h

@@ -48,6 +48,9 @@ struct commit_graph {
 	uint32_t num_commits;
 	struct object_id oid;
 
+	uint32_t num_commits_in_base;
+	struct commit_graph *base_graph;
+
 	const uint32_t *chunk_oid_fanout;
 	const unsigned char *chunk_oid_lookup;
 	const unsigned char *chunk_commit_data;