commit-graph: Support lookups of entries in a commit-graph

This change introduces `git_commit_graph_entry_find()` and
`git_commit_graph_entry_parent()`. These two functions allow a much
faster lookup of commits by ID, since the ODB does not need to be
consulted, the commit object does not need to be inflated, and the
contents of the commit object do not need to be parsed.

Part of: #5757

fuzzers/commit_graph_fuzzer.c

@@ -32,6 +32,7 @@ int LLVMFuzzerInitialize(int *argc, char ***argv)
 int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 {
 	git_commit_graph_file cgraph = {{0}};
+	git_commit_graph_entry e;
 	git_buf commit_graph_buf = GIT_BUF_INIT;
 	git_oid oid = {{0}};
 	bool append_hash = false;
@@ -68,6 +69,10 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 	    < 0)
 		goto cleanup;
 
+	/* Search for any oid, just to exercise that codepath. */
+	if (git_commit_graph_entry_find(&e, &cgraph, &oid, GIT_OID_HEXSZ) < 0)
+		goto cleanup;
+
 cleanup:
 	git_commit_graph_close(&cgraph);
 	git_buf_dispose(&commit_graph_buf);

src/commit_graph.c

@@ -9,6 +9,7 @@
 
 #include "futils.h"
 #include "hash.h"
+#include "pack.h"
 
 #define GIT_COMMIT_GRAPH_MISSING_PARENT 0x70000000
 
@@ -278,6 +279,142 @@ int git_commit_graph_open(git_commit_graph_file **cgraph_out, const char *path)
 	return 0;
 }
 
+static int git_commit_graph_entry_get_byindex(
+		git_commit_graph_entry *e,
+		const git_commit_graph_file *cgraph,
+		size_t pos)
+{
+	const unsigned char *commit_data;
+
+	GIT_ASSERT_ARG(e);
+	GIT_ASSERT_ARG(cgraph);
+
+	if (pos >= cgraph->num_commits) {
+		git_error_set(GIT_ERROR_INVALID, "commit index %zu does not exist", pos);
+		return GIT_ENOTFOUND;
+	}
+
+	commit_data = cgraph->commit_data + pos * (GIT_OID_RAWSZ + 4 * sizeof(uint32_t));
+	git_oid_cpy(&e->tree_oid, (const git_oid *)commit_data);
+	e->parent_indices[0] = ntohl(*((uint32_t *)(commit_data + GIT_OID_RAWSZ)));
+	e->parent_indices[1]
+			= ntohl(*((uint32_t *)(commit_data + GIT_OID_RAWSZ + sizeof(uint32_t))));
+	e->parent_count = (e->parent_indices[0] != GIT_COMMIT_GRAPH_MISSING_PARENT)
+			+ (e->parent_indices[1] != GIT_COMMIT_GRAPH_MISSING_PARENT);
+	e->generation = ntohl(*((uint32_t *)(commit_data + GIT_OID_RAWSZ + 2 * sizeof(uint32_t))));
+	e->commit_time = ntohl(*((uint32_t *)(commit_data + GIT_OID_RAWSZ + 3 * sizeof(uint32_t))));
+
+	e->commit_time |= (e->generation & 0x3ull) << 32ull;
+	e->generation >>= 2u;
+	if (e->parent_indices[1] & 0x80000000u) {
+		uint32_t extra_edge_list_pos = e->parent_indices[1] & 0x7fffffff;
+
+		/* Make sure we're not being sent out of bounds */
+		if (extra_edge_list_pos >= cgraph->num_extra_edge_list) {
+			git_error_set(GIT_ERROR_INVALID,
+				      "commit %u does not exist",
+				      extra_edge_list_pos);
+			return GIT_ENOTFOUND;
+		}
+
+		e->extra_parents_index = extra_edge_list_pos;
+		while (extra_edge_list_pos < cgraph->num_extra_edge_list
+		       && (ntohl(*(
+					   (uint32_t *)(cgraph->extra_edge_list
+							+ extra_edge_list_pos * sizeof(uint32_t))))
+			   & 0x80000000u)
+				       == 0) {
+			extra_edge_list_pos++;
+			e->parent_count++;
+		}
+
+	}
+	git_oid_cpy(&e->sha1, &cgraph->oid_lookup[pos]);
+	return 0;
+}
+
+int git_commit_graph_entry_find(
+		git_commit_graph_entry *e,
+		const git_commit_graph_file *cgraph,
+		const git_oid *short_oid,
+		size_t len)
+{
+	int pos, found = 0;
+	uint32_t hi, lo;
+	const git_oid *current = NULL;
+
+	GIT_ASSERT_ARG(e);
+	GIT_ASSERT_ARG(cgraph);
+	GIT_ASSERT_ARG(short_oid);
+
+	hi = ntohl(cgraph->oid_fanout[(int)short_oid->id[0]]);
+	lo = ((short_oid->id[0] == 0x0) ? 0 : ntohl(cgraph->oid_fanout[(int)short_oid->id[0] - 1]));
+
+	pos = git_pack__lookup_sha1(cgraph->oid_lookup, GIT_OID_RAWSZ, lo, hi, short_oid->id);
+
+	if (pos >= 0) {
+		/* An object matching exactly the oid was found */
+		found = 1;
+		current = cgraph->oid_lookup + pos;
+	} else {
+		/* No object was found */
+		/* pos refers to the object with the "closest" oid to short_oid */
+		pos = -1 - pos;
+		if (pos < (int)cgraph->num_commits) {
+			current = cgraph->oid_lookup + pos;
+
+			if (!git_oid_ncmp(short_oid, current, len))
+				found = 1;
+		}
+	}
+
+	if (found && len != GIT_OID_HEXSZ && pos + 1 < (int)cgraph->num_commits) {
+		/* Check for ambiguousity */
+		const git_oid *next = current + 1;
+
+		if (!git_oid_ncmp(short_oid, next, len)) {
+			found = 2;
+		}
+	}
+
+	if (!found)
+		return git_odb__error_notfound(
+				"failed to find offset for multi-pack index entry", short_oid, len);
+	if (found > 1)
+		return git_odb__error_ambiguous(
+				"found multiple offsets for multi-pack index entry");
+
+	return git_commit_graph_entry_get_byindex(e, cgraph, pos);
+}
+
+int git_commit_graph_entry_parent(
+		git_commit_graph_entry *parent,
+		const git_commit_graph_file *cgraph,
+		const git_commit_graph_entry *entry,
+		size_t n)
+{
+	GIT_ASSERT_ARG(parent);
+	GIT_ASSERT_ARG(cgraph);
+
+	if (n >= entry->parent_count) {
+		git_error_set(GIT_ERROR_INVALID, "parent index %zu does not exist", n);
+		return GIT_ENOTFOUND;
+	}
+
+	if (n == 0 || (n == 1 && entry->parent_count == 2))
+		return git_commit_graph_entry_get_byindex(parent, cgraph, entry->parent_indices[n]);
+
+	return git_commit_graph_entry_get_byindex(
+			parent,
+			cgraph,
+			ntohl(
+					*(uint32_t *)(cgraph->extra_edge_list
+						      + (entry->extra_parents_index + n - 1)
+								      * sizeof(uint32_t)))
+					& 0x7fffffff);
+}
+
+
 int git_commit_graph_close(git_commit_graph_file *cgraph)
 {
 	GIT_ASSERT_ARG(cgraph);

src/commit_graph.h

@@ -57,7 +57,48 @@ typedef struct git_commit_graph_file {
 	git_buf filename;
 } git_commit_graph_file;
 
+/**
+ * An entry in the commit-graph file. Provides a subset of the information that
+ * can be obtained from the commit header.
+ */
+typedef struct git_commit_graph_entry {
+	/* The generation number of the commit within the graph */
+	size_t generation;
+
+	/* Time in seconds from UNIX epoch. */
+	git_time_t commit_time;
+
+	/* The number of parents of the commit. */
+	size_t parent_count;
+
+	/*
+	 * The indices of the parent commits within the Commit Data table. The value
+	 * of `GIT_COMMIT_GRAPH_MISSING_PARENT` indicates that no parent is in that
+	 * position.
+	 */
+	size_t parent_indices[2];
+
+	/* The index within the Extra Edge List of any parent after the first two. */
+	size_t extra_parents_index;
+
+	/* The SHA-1 hash of the root tree of the commit. */
+	git_oid tree_oid;
+
+	/* The SHA-1 hash of the requested commit. */
+	git_oid sha1;
+} git_commit_graph_entry;
+
 int git_commit_graph_open(git_commit_graph_file **cgraph_out, const char *path);
+int git_commit_graph_entry_find(
+		git_commit_graph_entry *e,
+		const git_commit_graph_file *cgraph,
+		const git_oid *short_oid,
+		size_t len);
+int git_commit_graph_entry_parent(
+		git_commit_graph_entry *parent,
+		const git_commit_graph_file *cgraph,
+		const git_commit_graph_entry *entry,
+		size_t n);
 int git_commit_graph_close(git_commit_graph_file *cgraph);
 void git_commit_graph_free(git_commit_graph_file *cgraph);
 

tests/graph/commit_graph.c

@@ -8,12 +8,81 @@ void test_graph_commit_graph__parse(void)
 {
 	git_repository *repo;
 	struct git_commit_graph_file *cgraph;
+	struct git_commit_graph_entry e, parent;
+	git_oid id;
 	git_buf commit_graph_path = GIT_BUF_INIT;
 
 	cl_git_pass(git_repository_open(&repo, cl_fixture("testrepo.git")));
 	cl_git_pass(git_buf_joinpath(&commit_graph_path, git_repository_path(repo), "objects/info/commit-graph"));
 	cl_git_pass(git_commit_graph_open(&cgraph, git_buf_cstr(&commit_graph_path)));
 
+	cl_git_pass(git_oid_fromstr(&id, "5001298e0c09ad9c34e4249bc5801c75e9754fa5"));
+	cl_git_pass(git_commit_graph_entry_find(&e, cgraph, &id, GIT_OID_HEXSZ));
+	cl_assert_equal_oid(&e.sha1, &id);
+	cl_git_pass(git_oid_fromstr(&id, "418382dff1ffb8bdfba833f4d8bbcde58b1e7f47"));
+	cl_assert_equal_oid(&e.tree_oid, &id);
+	cl_assert_equal_i(e.generation, 1);
+	cl_assert_equal_i(e.commit_time, 1273610423ull);
+	cl_assert_equal_i(e.parent_count, 0);
+
+	cl_git_pass(git_oid_fromstr(&id, "be3563ae3f795b2b4353bcce3a527ad0a4f7f644"));
+	cl_git_pass(git_commit_graph_entry_find(&e, cgraph, &id, GIT_OID_HEXSZ));
+	cl_assert_equal_oid(&e.sha1, &id);
+	cl_assert_equal_i(e.generation, 5);
+	cl_assert_equal_i(e.commit_time, 1274813907ull);
+	cl_assert_equal_i(e.parent_count, 2);
+
+	cl_git_pass(git_oid_fromstr(&id, "9fd738e8f7967c078dceed8190330fc8648ee56a"));
+	cl_git_pass(git_commit_graph_entry_parent(&parent, cgraph, &e, 0));
+	cl_assert_equal_oid(&parent.sha1, &id);
+	cl_assert_equal_i(parent.generation, 4);
+
+	cl_git_pass(git_oid_fromstr(&id, "c47800c7266a2be04c571c04d5a6614691ea99bd"));
+	cl_git_pass(git_commit_graph_entry_parent(&parent, cgraph, &e, 1));
+	cl_assert_equal_oid(&parent.sha1, &id);
+	cl_assert_equal_i(parent.generation, 3);
+
+	git_commit_graph_free(cgraph);
+	git_repository_free(repo);
+	git_buf_dispose(&commit_graph_path);
+}
+
+void test_graph_commit_graph__parse_octopus_merge(void)
+{
+	git_repository *repo;
+	struct git_commit_graph_file *cgraph;
+	struct git_commit_graph_entry e, parent;
+	git_oid id;
+	git_buf commit_graph_path = GIT_BUF_INIT;
+
+	cl_git_pass(git_repository_open(&repo, cl_fixture("merge-recursive/.gitted")));
+	cl_git_pass(git_buf_joinpath(&commit_graph_path, git_repository_path(repo), "objects/info/commit-graph"));
+	cl_git_pass(git_commit_graph_open(&cgraph, git_buf_cstr(&commit_graph_path)));
+
+	cl_git_pass(git_oid_fromstr(&id, "d71c24b3b113fd1d1909998c5bfe33b86a65ee03"));
+	cl_git_pass(git_commit_graph_entry_find(&e, cgraph, &id, GIT_OID_HEXSZ));
+	cl_assert_equal_oid(&e.sha1, &id);
+	cl_git_pass(git_oid_fromstr(&id, "348f16ffaeb73f319a75cec5b16a0a47d2d5e27c"));
+	cl_assert_equal_oid(&e.tree_oid, &id);
+	cl_assert_equal_i(e.generation, 7);
+	cl_assert_equal_i(e.commit_time, 1447083009ull);
+	cl_assert_equal_i(e.parent_count, 3);
+
+	cl_git_pass(git_oid_fromstr(&id, "ad2ace9e15f66b3d1138922e6ffdc3ea3f967fa6"));
+	cl_git_pass(git_commit_graph_entry_parent(&parent, cgraph, &e, 0));
+	cl_assert_equal_oid(&parent.sha1, &id);
+	cl_assert_equal_i(parent.generation, 6);
+
+	cl_git_pass(git_oid_fromstr(&id, "483065df53c0f4a02cdc6b2910b05d388fc17ffb"));
+	cl_git_pass(git_commit_graph_entry_parent(&parent, cgraph, &e, 1));
+	cl_assert_equal_oid(&parent.sha1, &id);
+	cl_assert_equal_i(parent.generation, 2);
+
+	cl_git_pass(git_oid_fromstr(&id, "815b5a1c80ca749d705c7aa0cb294a00cbedd340"));
+	cl_git_pass(git_commit_graph_entry_parent(&parent, cgraph, &e, 2));
+	cl_assert_equal_oid(&parent.sha1, &id);
+	cl_assert_equal_i(parent.generation, 6);
+
 	git_commit_graph_free(cgraph);
 	git_repository_free(repo);
 	git_buf_dispose(&commit_graph_path);