commit-graph: Add a way to write commit-graph files

This change adds the git_commit_graph_writer_* functions to allow to
write and create `commit-graph` files from `.idx`/`.pack` files or
`git_revwalk`s.

Part of: #5757

include/git2/sys/commit_graph.h

@@ -40,6 +40,136 @@ GIT_EXTERN(int) git_commit_graph_open(git_commit_graph **cgraph_out, const char
  */
 GIT_EXTERN(void) git_commit_graph_free(git_commit_graph *cgraph);
 
-GIT_END_DECL
+/**
+ * Create a new writer for `commit-graph` files.
+ *
+ * @param out Location to store the writer pointer.
+ * @param objects_info_dir The `objects/info` directory.
+ * The `commit-graph` file will be written in this directory.
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_commit_graph_writer_new(
+		git_commit_graph_writer **out,
+		const char *objects_info_dir);
+
+/**
+ * Free the commit-graph writer and its resources.
+ *
+ * @param w The writer to free. If NULL no action is taken.
+ */
+GIT_EXTERN(void) git_commit_graph_writer_free(git_commit_graph_writer *w);
+
+/**
+ * Add an `.idx` file (associated to a packfile) to the writer.
+ *
+ * @param w The writer.
+ * @param repo The repository that owns the `.idx` file.
+ * @param idx_path The path of an `.idx` file.
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_commit_graph_writer_add_index_file(
+		git_commit_graph_writer *w,
+		git_repository *repo,
+		const char *idx_path);
 
+/**
+ * Add a revwalk to the writer. This will add all the commits from the revwalk
+ * to the commit-graph.
+ *
+ * @param w The writer.
+ * @param walk The git_revwalk.
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_commit_graph_writer_add_revwalk(
+		git_commit_graph_writer *w,
+		git_revwalk *walk);
+
+
+/**
+ * The strategy to use when adding a new set of commits to a pre-existing
+ * commit-graph chain.
+ */
+typedef enum {
+	/**
+	 * Do not split commit-graph files. The other split strategy-related option
+	 * fields are ignored.
+	 */
+	GIT_COMMIT_GRAPH_SPLIT_STRATEGY_SINGLE_FILE = 0,
+} git_commit_graph_split_strategy_t;
+
+/**
+ * Options structure for
+ * `git_commit_graph_writer_commit`/`git_commit_graph_writer_dump`.
+ *
+ * Initialize with `GIT_COMMIT_GRAPH_WRITER_OPTIONS_INIT`. Alternatively, you
+ * can use `git_commit_graph_writer_options_init`.
+ */
+typedef struct {
+	unsigned int version;
+
+	/**
+	 * The strategy to use when adding new commits to a pre-existing commit-graph
+	 * chain.
+	 */
+	git_commit_graph_split_strategy_t split_strategy;
+
+	/**
+	 * The number of commits in level N is less than X times the number of
+	 * commits in level N + 1.
+	 */
+	float size_multiple;
+
+	/**
+	 * The number of commits in level N + 1 is more than C commits.
+	 */
+	size_t max_commits;
+} git_commit_graph_writer_options;
+
+#define GIT_COMMIT_GRAPH_WRITER_OPTIONS_VERSION 1
+#define GIT_COMMIT_GRAPH_WRITER_OPTIONS_INIT                                                       \
+	{                                                                                          \
+		GIT_COMMIT_GRAPH_WRITER_OPTIONS_VERSION,                                           \
+		GIT_COMMIT_GRAPH_SPLIT_STRATEGY_SINGLE_FILE, 2.0f, 64000                           \
+	}
+
+/**
+ * Initialize git_commit_graph_writer_options structure
+ *
+ * Initializes a `git_commit_graph_writer_options` with default values. Equivalent to
+ * creating an instance with `GIT_COMMIT_GRAPH_WRITER_OPTIONS_INIT`.
+ *
+ * @param opts The `git_commit_graph_writer_options` struct to initialize.
+ * @param version The struct version; pass `GIT_COMMIT_GRAPH_WRITER_OPTIONS_VERSION`.
+ * @return Zero on success; -1 on failure.
+ */
+GIT_EXTERN(int) git_commit_graph_writer_options_init(
+	git_commit_graph_writer_options *opts,
+	unsigned int version);
+
+/**
+ * Write a `commit-graph` file to a file.
+ *
+ * @param w The writer
+ * @param opts Pointer to git_commit_graph_writer_options struct.
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_commit_graph_writer_commit(
+		git_commit_graph_writer *w,
+		git_commit_graph_writer_options *opts);
+
+/**
+ * Dump the contents of the `commit-graph` to an in-memory buffer.
+ *
+ * @param buffer Buffer where to store the contents of the `commit-graph`.
+ * @param w The writer.
+ * @param opts Pointer to git_commit_graph_writer_options struct.
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_commit_graph_writer_dump(
+		git_buf *buffer,
+		git_commit_graph_writer *w,
+		git_commit_graph_writer_options *opts);
+
+/** @} */
+GIT_END_DECL
 #endif

include/git2/types.h

@@ -96,6 +96,9 @@ typedef struct git_odb_stream git_odb_stream;
 /** A stream to write a packfile to the ODB */
 typedef struct git_odb_writepack git_odb_writepack;
 
+/** a writer for commit-graph files. */
+typedef struct git_commit_graph_writer git_commit_graph_writer;
+
 /** An open refs database handle. */
 typedef struct git_refdb git_refdb;
 

src/commit_graph.h

@@ -14,6 +14,7 @@
 #include "git2/sys/commit_graph.h"
 
 #include "map.h"
+#include "vector.h"
 
 /**
  * A commit-graph file.
@@ -119,6 +120,20 @@ int git_commit_graph_get_file(git_commit_graph_file **file_out, git_commit_graph
 void git_commit_graph_refresh(git_commit_graph *cgraph);
 
 /*
+ * A writer for `commit-graph` files.
+ */
+struct git_commit_graph_writer {
+	/*
+	 * The path of the `objects/info` directory where the `commit-graph` will be
+	 * stored.
+	 */
+	git_buf objects_info_dir;
+
+	/* The list of packed commits. */
+	git_vector commits;
+};
+
+/*
  * Returns whether the git_commit_graph_file needs to be reloaded since the
  * contents of the commit-graph file have changed on disk.
  */

tests/graph/commit_graph.c

 #include "clar_libgit2.h"
 
 #include <git2.h>
+#include <git2/sys/commit_graph.h>
 
 #include "commit_graph.h"
+#include "futils.h"
 
 void test_graph_commit_graph__parse(void)
 {
@@ -88,3 +90,36 @@ void test_graph_commit_graph__parse_octopus_merge(void)
 	git_repository_free(repo);
 	git_buf_dispose(&commit_graph_path);
 }
+
+void test_graph_commit_graph__writer(void)
+{
+	git_repository *repo;
+	git_commit_graph_writer *w = NULL;
+	git_revwalk *walk;
+	git_commit_graph_writer_options opts = GIT_COMMIT_GRAPH_WRITER_OPTIONS_INIT;
+	git_buf cgraph = GIT_BUF_INIT, expected_cgraph = GIT_BUF_INIT, path = GIT_BUF_INIT;
+
+	cl_git_pass(git_repository_open(&repo, cl_fixture("testrepo.git")));
+
+	cl_git_pass(git_buf_joinpath(&path, git_repository_path(repo), "objects/info"));
+	cl_git_pass(git_commit_graph_writer_new(&w, git_buf_cstr(&path)));
+
+	/* This is equivalent to `git commit-graph write --reachable`. */
+	cl_git_pass(git_revwalk_new(&walk, repo));
+	cl_git_pass(git_revwalk_push_glob(walk, "refs/*"));
+	cl_git_pass(git_commit_graph_writer_add_revwalk(w, walk));
+	git_revwalk_free(walk);
+
+	cl_git_pass(git_commit_graph_writer_dump(&cgraph, w, &opts));
+	cl_git_pass(git_buf_joinpath(&path, git_repository_path(repo), "objects/info/commit-graph"));
+	cl_git_pass(git_futils_readbuffer(&expected_cgraph, git_buf_cstr(&path)));
+
+	cl_assert_equal_i(git_buf_len(&cgraph), git_buf_len(&expected_cgraph));
+	cl_assert_equal_i(memcmp(git_buf_cstr(&cgraph), git_buf_cstr(&expected_cgraph), git_buf_len(&cgraph)), 0);
+
+	git_buf_dispose(&cgraph);
+	git_buf_dispose(&expected_cgraph);
+	git_buf_dispose(&path);
+	git_commit_graph_writer_free(w);
+	git_repository_free(repo);
+}