[PATCH v3 6/6] Reftable support for git-core

["Han-Wen Nienhuys via GitGitGadget" <gitgitgadget@gmail.com>]

From: Han-Wen Nienhuys <hanwen@google.com>

For background, see the previous commit introducing the library.

TODO:

 * Make CI on gitgitgadget compile.
 * Redo interaction between repo config, repo storage version
 * Resolve the design problem with reflog expiry.
 * Resolve spots marked with XXX

Example use:

  $ ~/vc/git/git init --reftable
  warning: templates not found in /usr/local/google/home/hanwen/share/git-core/templates
  Initialized empty Git repository in /tmp/qz/.git/
  $ echo q > a
  $ ~/vc/git/git add a
  $ ~/vc/git/git commit -mx
  fatal: not a git repository (or any of the parent directories): .git
  [master (root-commit) 373d969] x
   1 file changed, 1 insertion(+)
   create mode 100644 a
  $ ~/vc/git/git show-ref
  373d96972fca9b63595740bba3898a762778ba20 HEAD
  373d96972fca9b63595740bba3898a762778ba20 refs/heads/master
  $ ls -l .git/reftable/
  total 12
  -rw------- 1 hanwen primarygroup  126 Jan 23 20:08 000000000001-000000000001.ref
  -rw------- 1 hanwen primarygroup 4277 Jan 23 20:08 000000000002-000000000002.ref
  $ go run ~/vc/reftable/cmd/dump.go  -table /tmp/qz/.git/reftable/000000000002-000000000002.ref
  ** DEBUG **
  name /tmp/qz/.git/reftable/000000000002-000000000002.ref, sz 4209: 'r' reftable.readerOffsets{Present:true, Offset:0x0, IndexOffset:0x0}, 'o' reftable.readerOffsets{Present:false, Offset:0x0, IndexOffset:0x0} 'g' reftable.readerOffsets{Present:true, Offset:0x1000, IndexOffset:0x0}
  ** REFS **
  reftable.RefRecord{RefName:"refs/heads/master", UpdateIndex:0x2, Value:[]uint8{0x37, 0x3d, 0x96, 0x97, 0x2f, 0xca, 0x9b, 0x63, 0x59, 0x57, 0x40, 0xbb, 0xa3, 0x89, 0x8a, 0x76, 0x27, 0x78, 0xba, 0x20}, TargetValue:[]uint8(nil), Target:""}
  ** LOGS **
  reftable.LogRecord{RefName:"HEAD", UpdateIndex:0x2, New:[]uint8{0x37, 0x3d, 0x96, 0x97, 0x2f, 0xca, 0x9b, 0x63, 0x59, 0x57, 0x40, 0xbb, 0xa3, 0x89, 0x8a, 0x76, 0x27, 0x78, 0xba, 0x20}, Old:[]uint8{0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}, Name:"Han-Wen Nienhuys", Email:"hanwen@google.com", Time:0x5e29ef27, TZOffset:100, Message:"commit (initial): x\n"}

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
---
 Makefile                |  24 +-
 builtin/init-db.c       |  40 +-
 cache.h                 |   2 +
 refs.c                  |  22 +-
 refs/refs-internal.h    |   1 +
 refs/reftable-backend.c | 880 ++++++++++++++++++++++++++++++++++++++++
 repository.c            |   4 +
 repository.h            |   3 +
 setup.c                 |   7 +-
 9 files changed, 959 insertions(+), 24 deletions(-)
 create mode 100644 refs/reftable-backend.c

diff --git a/Makefile b/Makefile
index 6134104ae6..706bb0a981 100644
--- a/Makefile
+++ b/Makefile
@@ -814,6 +814,7 @@ TEST_SHELL_PATH = $(SHELL_PATH)
 LIB_FILE = libgit.a
 XDIFF_LIB = xdiff/lib.a
 VCSSVN_LIB = vcs-svn/lib.a
+REFTABLE_LIB = reftable/libreftable.a
 
 GENERATED_H += command-list.h
 
@@ -959,6 +960,7 @@ LIB_OBJS += rebase-interactive.o
 LIB_OBJS += reflog-walk.o
 LIB_OBJS += refs.o
 LIB_OBJS += refs/files-backend.o
+LIB_OBJS += refs/reftable-backend.o
 LIB_OBJS += refs/iterator.o
 LIB_OBJS += refs/packed-backend.o
 LIB_OBJS += refs/ref-cache.o
@@ -1163,7 +1165,7 @@ THIRD_PARTY_SOURCES += compat/regex/%
 THIRD_PARTY_SOURCES += sha1collisiondetection/%
 THIRD_PARTY_SOURCES += sha1dc/%
 
-GITLIBS = common-main.o $(LIB_FILE) $(XDIFF_LIB)
+GITLIBS = common-main.o $(LIB_FILE) $(XDIFF_LIB) $(REFTABLE_LIB)
 EXTLIBS =
 
 GIT_USER_AGENT = git/$(GIT_VERSION)
@@ -2347,11 +2349,28 @@ VCSSVN_OBJS += vcs-svn/fast_export.o
 VCSSVN_OBJS += vcs-svn/svndiff.o
 VCSSVN_OBJS += vcs-svn/svndump.o
 
+REFTABLE_OBJS += reftable/basics.o
+REFTABLE_OBJS += reftable/block.o
+REFTABLE_OBJS += reftable/bytes.o
+REFTABLE_OBJS += reftable/file.o
+REFTABLE_OBJS += reftable/iter.o
+REFTABLE_OBJS += reftable/merged.o
+REFTABLE_OBJS += reftable/pq.o
+REFTABLE_OBJS += reftable/reader.o
+REFTABLE_OBJS += reftable/record.o
+REFTABLE_OBJS += reftable/slice.o
+REFTABLE_OBJS += reftable/stack.o
+REFTABLE_OBJS += reftable/tree.o
+REFTABLE_OBJS += reftable/writer.o
+REFTABLE_OBJS += reftable/zlib-compat.o
+
+
 TEST_OBJS := $(patsubst %$X,%.o,$(TEST_PROGRAMS)) $(patsubst %,t/helper/%,$(TEST_BUILTINS_OBJS))
 OBJECTS := $(LIB_OBJS) $(BUILTIN_OBJS) $(PROGRAM_OBJS) $(TEST_OBJS) \
 	$(XDIFF_OBJS) \
 	$(VCSSVN_OBJS) \
 	$(FUZZ_OBJS) \
+	$(REFTABLE_OBJS) \
 	common-main.o \
 	git.o
 ifndef NO_CURL
@@ -2488,6 +2507,9 @@ $(XDIFF_LIB): $(XDIFF_OBJS)
 $(VCSSVN_LIB): $(VCSSVN_OBJS)
 	$(QUIET_AR)$(RM) $@ && $(AR) $(ARFLAGS) $@ $^
 
+$(REFTABLE_LIB): $(REFTABLE_OBJS)
+	$(QUIET_AR)$(RM) $@ && $(AR) $(ARFLAGS) $@ $^
+
 export DEFAULT_EDITOR DEFAULT_PAGER
 
 Documentation/GIT-EXCLUDED-PROGRAMS: FORCE
diff --git a/builtin/init-db.c b/builtin/init-db.c
index 45bdea0589..dcea74610c 100644
--- a/builtin/init-db.c
+++ b/builtin/init-db.c
@@ -177,7 +177,7 @@ static int needs_work_tree_config(const char *git_dir, const char *work_tree)
 }
 
 static int create_default_files(const char *template_path,
-				const char *original_git_dir)
+				const char *original_git_dir, int flags)
 {
 	struct stat st1;
 	struct strbuf buf = STRBUF_INIT;
@@ -234,17 +234,24 @@ static int create_default_files(const char *template_path,
 	 * Create the default symlink from ".git/HEAD" to the "master"
 	 * branch, if it does not exist yet.
 	 */
-	path = git_path_buf(&buf, "HEAD");
-	reinit = (!access(path, R_OK)
-		  || readlink(path, junk, sizeof(junk)-1) != -1);
+	if (flags & INIT_DB_REFTABLE) {
+		reinit = 0; /* XXX - how do we recognize a reinit,
+			     * and what should we do? */
+	} else {
+		path = git_path_buf(&buf, "HEAD");
+		reinit = (!access(path, R_OK) ||
+			  readlink(path, junk, sizeof(junk) - 1) != -1);
+	}
+
 	if (!reinit) {
 		if (create_symref("HEAD", "refs/heads/master", NULL) < 0)
 			exit(1);
 	}
 
 	/* This forces creation of new config file */
-	xsnprintf(repo_version_string, sizeof(repo_version_string),
-		  "%d", GIT_REPO_VERSION);
+	xsnprintf(repo_version_string, sizeof(repo_version_string), "%d",
+		  flags & INIT_DB_REFTABLE ? GIT_REPO_VERSION_READ :
+					     GIT_REPO_VERSION);
 	git_config_set("core.repositoryformatversion", repo_version_string);
 
 	/* Check filemode trustability */
@@ -378,7 +385,7 @@ int init_db(const char *git_dir, const char *real_git_dir,
 	 */
 	check_repository_format();
 
-	reinit = create_default_files(template_dir, original_git_dir);
+	reinit = create_default_files(template_dir, original_git_dir, flags);
 
 	create_object_directory();
 
@@ -403,6 +410,10 @@ int init_db(const char *git_dir, const char *real_git_dir,
 		git_config_set("receive.denyNonFastforwards", "true");
 	}
 
+	if (flags & INIT_DB_REFTABLE) {
+		git_config_set("extensions.refStorage", "reftable");
+	}
+
 	if (!(flags & INIT_DB_QUIET)) {
 		int len = strlen(git_dir);
 
@@ -481,15 +492,18 @@ int cmd_init_db(int argc, const char **argv, const char *prefix)
 	const char *template_dir = NULL;
 	unsigned int flags = 0;
 	const struct option init_db_options[] = {
-		OPT_STRING(0, "template", &template_dir, N_("template-directory"),
-				N_("directory from which templates will be used")),
+		OPT_STRING(0, "template", &template_dir,
+			   N_("template-directory"),
+			   N_("directory from which templates will be used")),
 		OPT_SET_INT(0, "bare", &is_bare_repository_cfg,
-				N_("create a bare repository"), 1),
+			    N_("create a bare repository"), 1),
 		{ OPTION_CALLBACK, 0, "shared", &init_shared_repository,
-			N_("permissions"),
-			N_("specify that the git repository is to be shared amongst several users"),
-			PARSE_OPT_OPTARG | PARSE_OPT_NONEG, shared_callback, 0},
+		  N_("permissions"),
+		  N_("specify that the git repository is to be shared amongst several users"),
+		  PARSE_OPT_OPTARG | PARSE_OPT_NONEG, shared_callback, 0 },
 		OPT_BIT('q', "quiet", &flags, N_("be quiet"), INIT_DB_QUIET),
+		OPT_BIT(0, "reftable", &flags, N_("use reftable"),
+			INIT_DB_REFTABLE),
 		OPT_STRING(0, "separate-git-dir", &real_git_dir, N_("gitdir"),
 			   N_("separate git dir from working tree")),
 		OPT_END()
diff --git a/cache.h b/cache.h
index cbfaead23a..929a61e861 100644
--- a/cache.h
+++ b/cache.h
@@ -625,6 +625,7 @@ int path_inside_repo(const char *prefix, const char *path);
 
 #define INIT_DB_QUIET 0x0001
 #define INIT_DB_EXIST_OK 0x0002
+#define INIT_DB_REFTABLE 0x0004
 
 int init_db(const char *git_dir, const char *real_git_dir,
 	    const char *template_dir, unsigned int flags);
@@ -1041,6 +1042,7 @@ struct repository_format {
 	int is_bare;
 	int hash_algo;
 	char *work_tree;
+	char *ref_storage;
 	struct string_list unknown_extensions;
 };
 
diff --git a/refs.c b/refs.c
index 1ab0bb54d3..493d3fb673 100644
--- a/refs.c
+++ b/refs.c
@@ -20,7 +20,7 @@
 /*
  * List of all available backends
  */
-static struct ref_storage_be *refs_backends = &refs_be_files;
+static struct ref_storage_be *refs_backends = &refs_be_reftable;
 
 static struct ref_storage_be *find_ref_storage_backend(const char *name)
 {
@@ -1836,13 +1836,13 @@ static struct ref_store *lookup_ref_store_map(struct hashmap *map,
  * Create, record, and return a ref_store instance for the specified
  * gitdir.
  */
-static struct ref_store *ref_store_init(const char *gitdir,
+static struct ref_store *ref_store_init(const char *gitdir, const char *be_name,
 					unsigned int flags)
 {
-	const char *be_name = "files";
-	struct ref_storage_be *be = find_ref_storage_backend(be_name);
+	struct ref_storage_be *be;
 	struct ref_store *refs;
 
+	be = find_ref_storage_backend(be_name);
 	if (!be)
 		BUG("reference backend %s is unknown", be_name);
 
@@ -1858,7 +1858,12 @@ struct ref_store *get_main_ref_store(struct repository *r)
 	if (!r->gitdir)
 		BUG("attempting to get main_ref_store outside of repository");
 
-	r->refs = ref_store_init(r->gitdir, REF_STORE_ALL_CAPS);
+	r->refs = ref_store_init(r->gitdir,
+				 /* XXX r->ref_storage_format == NULL. Where
+				  * should the config file be parsed out? */
+				 r->ref_storage_format ? r->ref_storage_format :
+							 "reftable",
+				 REF_STORE_ALL_CAPS);
 	return r->refs;
 }
 
@@ -1913,7 +1918,7 @@ struct ref_store *get_submodule_ref_store(const char *submodule)
 		goto done;
 
 	/* assume that add_submodule_odb() has been called */
-	refs = ref_store_init(submodule_sb.buf,
+	refs = ref_store_init(submodule_sb.buf, "files", /* XXX */
 			      REF_STORE_READ | REF_STORE_ODB);
 	register_ref_store_map(&submodule_ref_stores, "submodule",
 			       refs, submodule);
@@ -1927,6 +1932,7 @@ struct ref_store *get_submodule_ref_store(const char *submodule)
 
 struct ref_store *get_worktree_ref_store(const struct worktree *wt)
 {
+	const char *format = "files"; /* XXX */
 	struct ref_store *refs;
 	const char *id;
 
@@ -1940,9 +1946,9 @@ struct ref_store *get_worktree_ref_store(const struct worktree *wt)
 
 	if (wt->id)
 		refs = ref_store_init(git_common_path("worktrees/%s", wt->id),
-				      REF_STORE_ALL_CAPS);
+				      format, REF_STORE_ALL_CAPS);
 	else
-		refs = ref_store_init(get_git_common_dir(),
+		refs = ref_store_init(get_git_common_dir(), format,
 				      REF_STORE_ALL_CAPS);
 
 	if (refs)
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index 1d7a485220..d87e9cbdec 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -661,6 +661,7 @@ struct ref_storage_be {
 };
 
 extern struct ref_storage_be refs_be_files;
+extern struct ref_storage_be refs_be_reftable;
 extern struct ref_storage_be refs_be_packed;
 
 /*
diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
new file mode 100644
index 0000000000..9394389c0a
--- /dev/null
+++ b/refs/reftable-backend.c
@@ -0,0 +1,880 @@
+#include "../cache.h"
+#include "../config.h"
+#include "../refs.h"
+#include "refs-internal.h"
+#include "../iterator.h"
+#include "../lockfile.h"
+#include "../chdir-notify.h"
+
+#include "../reftable/reftable.h"
+
+/*
+  The reftable v1 spec only supports 20-byte binary SHA1s. A new format version
+  will be needed to support SHA256.
+ */
+
+extern struct ref_storage_be refs_be_reftable;
+
+struct reftable_ref_store {
+	struct ref_store base;
+	unsigned int store_flags;
+
+	int err;
+	char *reftable_dir;
+	char *table_list_file;
+	struct stack *stack;
+};
+
+static void clear_log_record(struct log_record *log)
+{
+	log->old_hash = NULL;
+	log->new_hash = NULL;
+	log->message = NULL;
+	log->ref_name = NULL;
+	log_record_clear(log);
+}
+
+static void fill_log_record(struct log_record *log)
+{
+	const char *info = git_committer_info(0);
+	struct ident_split split = {};
+	int result = split_ident_line(&split, info, strlen(info));
+	int sign = 1;
+	assert(0 == result);
+
+	log_record_clear(log);
+	log->name =
+		xstrndup(split.name_begin, split.name_end - split.name_begin);
+	log->email =
+		xstrndup(split.mail_begin, split.mail_end - split.mail_begin);
+	log->time = atol(split.date_begin);
+	if (*split.tz_begin == '-') {
+		sign = -1;
+		split.tz_begin++;
+	}
+	if (*split.tz_begin == '+') {
+		sign = 1;
+		split.tz_begin++;
+	}
+
+	log->tz_offset = sign * atoi(split.tz_begin);
+}
+
+static struct ref_store *reftable_ref_store_create(const char *path,
+						   unsigned int store_flags)
+{
+	struct reftable_ref_store *refs = xcalloc(1, sizeof(*refs));
+	struct ref_store *ref_store = (struct ref_store *)refs;
+	struct write_options cfg = {
+		.block_size = 4096,
+	};
+	struct strbuf sb = STRBUF_INIT;
+
+	base_ref_store_init(ref_store, &refs_be_reftable);
+	refs->store_flags = store_flags;
+
+	strbuf_addf(&sb, "%s/reftable", path);
+	refs->reftable_dir = xstrdup(sb.buf);
+	strbuf_reset(&sb);
+
+	strbuf_addf(&sb, "%s/reftable/tables.list", path);
+	refs->table_list_file = xstrdup(sb.buf);
+	strbuf_reset(&sb);
+
+	strbuf_addf(&sb, "%s/refs", path);
+	safe_create_dir(sb.buf, 1);
+	strbuf_reset(&sb);
+
+	strbuf_addf(&sb, "%s/HEAD", path);
+	write_file(sb.buf, "ref: refs/.invalid");
+	strbuf_reset(&sb);
+
+	strbuf_addf(&sb, "%s/refs/heads", path);
+	write_file(sb.buf, "this repository uses the reftable format");
+
+	refs->err = new_stack(&refs->stack, refs->reftable_dir,
+			      refs->table_list_file, cfg);
+	strbuf_release(&sb);
+	return ref_store;
+}
+
+static int reftable_init_db(struct ref_store *ref_store, struct strbuf *err)
+{
+	struct reftable_ref_store *refs =
+		(struct reftable_ref_store *)ref_store;
+
+	safe_create_dir(refs->reftable_dir, 1);
+	FILE *f = fopen(refs->table_list_file, "a");
+	if (f == NULL) {
+		return -1;
+	}
+	fclose(f);
+	return 0;
+}
+
+struct reftable_iterator {
+	struct ref_iterator base;
+	struct iterator iter;
+	struct ref_record ref;
+	struct object_id oid;
+	struct ref_store *ref_store;
+	unsigned int flags;
+	int err;
+	char *prefix;
+};
+
+static int reftable_ref_iterator_advance(struct ref_iterator *ref_iterator)
+{
+	struct reftable_iterator *ri = (struct reftable_iterator *)ref_iterator;
+	while (ri->err == 0) {
+		ri->err = iterator_next_ref(ri->iter, &ri->ref);
+		if (ri->err) {
+			break;
+		}
+
+		ri->base.refname = ri->ref.ref_name;
+		if (ri->prefix != NULL &&
+		    strncmp(ri->prefix, ri->ref.ref_name, strlen(ri->prefix))) {
+			ri->err = 1;
+			break;
+		}
+		if (ri->flags & DO_FOR_EACH_PER_WORKTREE_ONLY &&
+		    ref_type(ri->base.refname) != REF_TYPE_PER_WORKTREE)
+			continue;
+
+		ri->base.flags = 0;
+		if (ri->ref.value != NULL) {
+			hashcpy(ri->oid.hash, ri->ref.value);
+		} else if (ri->ref.target != NULL) {
+			int out_flags = 0;
+			const char *resolved = refs_resolve_ref_unsafe(
+				ri->ref_store, ri->ref.ref_name,
+				RESOLVE_REF_READING, &ri->oid, &out_flags);
+			ri->base.flags = out_flags;
+			if (resolved == NULL &&
+			    !(ri->flags & DO_FOR_EACH_INCLUDE_BROKEN) &&
+			    (ri->base.flags & REF_ISBROKEN)) {
+				continue;
+			}
+		}
+
+		ri->base.oid = &ri->oid;
+		if (!(ri->flags & DO_FOR_EACH_INCLUDE_BROKEN) &&
+		    !ref_resolves_to_object(ri->base.refname, ri->base.oid,
+					    ri->base.flags)) {
+			continue;
+		}
+
+		break;
+	}
+
+	if (ri->err > 0) {
+		return ITER_DONE;
+	}
+	if (ri->err < 0) {
+		return ITER_ERROR;
+	}
+
+	return ITER_OK;
+}
+
+static int reftable_ref_iterator_peel(struct ref_iterator *ref_iterator,
+				      struct object_id *peeled)
+{
+	struct reftable_iterator *ri = (struct reftable_iterator *)ref_iterator;
+	if (ri->ref.target_value != NULL) {
+		hashcpy(peeled->hash, ri->ref.target_value);
+		return 0;
+	}
+
+	return -1;
+}
+
+static int reftable_ref_iterator_abort(struct ref_iterator *ref_iterator)
+{
+	struct reftable_iterator *ri = (struct reftable_iterator *)ref_iterator;
+	ref_record_clear(&ri->ref);
+	iterator_destroy(&ri->iter);
+	return 0;
+}
+
+static struct ref_iterator_vtable reftable_ref_iterator_vtable = {
+	reftable_ref_iterator_advance, reftable_ref_iterator_peel,
+	reftable_ref_iterator_abort
+};
+
+static struct ref_iterator *
+reftable_ref_iterator_begin(struct ref_store *ref_store, const char *prefix,
+			    unsigned int flags)
+{
+	struct reftable_ref_store *refs =
+		(struct reftable_ref_store *)ref_store;
+	struct reftable_iterator *ri = xcalloc(1, sizeof(*ri));
+	struct merged_table *mt = NULL;
+
+	mt = stack_merged_table(refs->stack);
+	ri->err = refs->err;
+	if (ri->err == 0) {
+		ri->err = merged_table_seek_ref(mt, &ri->iter, prefix);
+	}
+	base_ref_iterator_init(&ri->base, &reftable_ref_iterator_vtable, 1);
+	ri->base.oid = &ri->oid;
+	ri->flags = flags;
+	ri->ref_store = ref_store;
+	return &ri->base;
+}
+
+static int reftable_transaction_prepare(struct ref_store *ref_store,
+					struct ref_transaction *transaction,
+					struct strbuf *err)
+{
+	return 0;
+}
+
+static int reftable_transaction_abort(struct ref_store *ref_store,
+				      struct ref_transaction *transaction,
+				      struct strbuf *err)
+{
+	struct reftable_ref_store *refs =
+		(struct reftable_ref_store *)ref_store;
+	(void)refs;
+	return 0;
+}
+
+static int reftable_check_old_oid(struct ref_store *refs, const char *refname,
+				  struct object_id *want_oid)
+{
+	struct object_id out_oid = {};
+	int out_flags = 0;
+	const char *resolved = refs_resolve_ref_unsafe(
+		refs, refname, RESOLVE_REF_READING, &out_oid, &out_flags);
+	if (is_null_oid(want_oid) != (resolved == NULL)) {
+		return LOCK_ERROR;
+	}
+
+	if (resolved != NULL && !oideq(&out_oid, want_oid)) {
+		return LOCK_ERROR;
+	}
+
+	return 0;
+}
+
+static int ref_update_cmp(const void *a, const void *b)
+{
+	return strcmp(((struct ref_update *)a)->refname,
+		      ((struct ref_update *)b)->refname);
+}
+
+static int write_transaction_table(struct writer *writer, void *arg)
+{
+	struct ref_transaction *transaction = (struct ref_transaction *)arg;
+	struct reftable_ref_store *refs =
+		(struct reftable_ref_store *)transaction->ref_store;
+	uint64_t ts = stack_next_update_index(refs->stack);
+	int err = 0;
+	struct ref_update **sorted =
+		malloc(transaction->nr * sizeof(struct ref_update *));
+	COPY_ARRAY(sorted, transaction->updates, transaction->nr);
+	QSORT(sorted, transaction->nr, ref_update_cmp);
+	writer_set_limits(writer, ts, ts);
+
+	for (int i = 0; i < transaction->nr; i++) {
+		struct ref_update *u = sorted[i];
+		if (u->flags & REF_HAVE_OLD) {
+			err = reftable_check_old_oid(transaction->ref_store,
+						     u->refname, &u->old_oid);
+			if (err < 0) {
+				goto exit;
+			}
+		}
+	}
+
+	for (int i = 0; i < transaction->nr; i++) {
+		struct ref_update *u = sorted[i];
+		if (u->flags & REF_HAVE_NEW) {
+			struct object_id out_oid = {};
+			int out_flags = 0;
+			/* Memory owned by refs_resolve_ref_unsafe, no need to
+			 * free(). */
+			const char *resolved = refs_resolve_ref_unsafe(
+				transaction->ref_store, u->refname, 0, &out_oid,
+				&out_flags);
+			struct ref_record ref = {};
+			ref.ref_name =
+				(char *)(resolved ? resolved : u->refname);
+			ref.value = u->new_oid.hash;
+			ref.update_index = ts;
+			err = writer_add_ref(writer, &ref);
+			if (err < 0) {
+				goto exit;
+			}
+		}
+	}
+
+	for (int i = 0; i < transaction->nr; i++) {
+		struct ref_update *u = sorted[i];
+		struct log_record log = {};
+		fill_log_record(&log);
+
+		log.ref_name = (char *)u->refname;
+		log.old_hash = u->old_oid.hash;
+		log.new_hash = u->new_oid.hash;
+		log.update_index = ts;
+		log.message = u->msg;
+
+		err = writer_add_log(writer, &log);
+		clear_log_record(&log);
+		if (err < 0) {
+			return err;
+		}
+	}
+exit:
+	free(sorted);
+	return err;
+}
+
+static int reftable_transaction_commit(struct ref_store *ref_store,
+				       struct ref_transaction *transaction,
+				       struct strbuf *errmsg)
+{
+	struct reftable_ref_store *refs =
+		(struct reftable_ref_store *)ref_store;
+	int err = stack_add(refs->stack, &write_transaction_table, transaction);
+	if (err < 0) {
+		strbuf_addf(errmsg, "reftable: transaction failure %s",
+			    error_str(err));
+		return -1;
+	}
+
+	return 0;
+}
+
+static int reftable_transaction_finish(struct ref_store *ref_store,
+				       struct ref_transaction *transaction,
+				       struct strbuf *err)
+{
+	return reftable_transaction_commit(ref_store, transaction, err);
+}
+
+struct write_delete_refs_arg {
+	struct stack *stack;
+	struct string_list *refnames;
+	const char *logmsg;
+	unsigned int flags;
+};
+
+static int write_delete_refs_table(struct writer *writer, void *argv)
+{
+	struct write_delete_refs_arg *arg =
+		(struct write_delete_refs_arg *)argv;
+	uint64_t ts = stack_next_update_index(arg->stack);
+	int err = 0;
+
+	writer_set_limits(writer, ts, ts);
+	for (int i = 0; i < arg->refnames->nr; i++) {
+		struct ref_record ref = {
+			.ref_name = (char *)arg->refnames->items[i].string,
+			.update_index = ts,
+		};
+		err = writer_add_ref(writer, &ref);
+		if (err < 0) {
+			return err;
+		}
+	}
+
+	for (int i = 0; i < arg->refnames->nr; i++) {
+		struct log_record log = {};
+		struct ref_record current = {};
+		fill_log_record(&log);
+		log.message = xstrdup(arg->logmsg);
+		log.new_hash = NULL;
+		log.old_hash = NULL;
+		log.update_index = ts;
+		log.ref_name = (char *)arg->refnames->items[i].string;
+
+		if (stack_read_ref(arg->stack, log.ref_name, &current) == 0) {
+			log.old_hash = current.value;
+		}
+		err = writer_add_log(writer, &log);
+		log.old_hash = NULL;
+		ref_record_clear(&current);
+
+		clear_log_record(&log);
+		if (err < 0) {
+			return err;
+		}
+	}
+	return 0;
+}
+
+static int reftable_delete_refs(struct ref_store *ref_store, const char *msg,
+				struct string_list *refnames,
+				unsigned int flags)
+{
+	struct reftable_ref_store *refs =
+		(struct reftable_ref_store *)ref_store;
+	struct write_delete_refs_arg arg = {
+		.stack = refs->stack,
+		.refnames = refnames,
+		.logmsg = msg,
+		.flags = flags,
+	};
+	return stack_add(refs->stack, &write_delete_refs_table, &arg);
+}
+
+static int reftable_pack_refs(struct ref_store *ref_store, unsigned int flags)
+{
+	struct reftable_ref_store *refs =
+		(struct reftable_ref_store *)ref_store;
+	return stack_compact_all(refs->stack, NULL);
+}
+
+struct write_create_symref_arg {
+	struct reftable_ref_store *refs;
+	const char *refname;
+	const char *target;
+	const char *logmsg;
+};
+
+static int write_create_symref_table(struct writer *writer, void *arg)
+{
+	struct write_create_symref_arg *create =
+		(struct write_create_symref_arg *)arg;
+	uint64_t ts = stack_next_update_index(create->refs->stack);
+	int err = 0;
+
+	struct ref_record ref = {
+		.ref_name = (char *)create->refname,
+		.target = (char *)create->target,
+		.update_index = ts,
+	};
+	writer_set_limits(writer, ts, ts);
+	err = writer_add_ref(writer, &ref);
+	if (err < 0) {
+		return err;
+	}
+
+	{
+		struct log_record log = {};
+		struct object_id new_oid = {};
+		struct object_id old_oid = {};
+		struct ref_record current = {};
+		stack_read_ref(create->refs->stack, create->refname, &current);
+
+		fill_log_record(&log);
+		log.ref_name = current.ref_name;
+		if (refs_resolve_ref_unsafe(
+			    (struct ref_store *)create->refs, create->refname,
+			    RESOLVE_REF_READING, &old_oid, NULL) != NULL) {
+			log.old_hash = old_oid.hash;
+		}
+
+		/* XXX should the resolution be done relative or absolute? */
+		if (refs_resolve_ref_unsafe((struct ref_store *)create->refs,
+					    create->target, RESOLVE_REF_READING,
+					    &new_oid, NULL) != NULL) {
+			log.new_hash = new_oid.hash;
+		}
+
+		if (log.old_hash != NULL || log.new_hash != NULL) {
+			writer_add_log(writer, &log);
+		}
+		log.ref_name = NULL;
+		log.old_hash = NULL;
+		log.new_hash = NULL;
+		clear_log_record(&log);
+	}
+	return 0;
+}
+
+static int reftable_create_symref(struct ref_store *ref_store,
+				  const char *refname, const char *target,
+				  const char *logmsg)
+{
+	struct reftable_ref_store *refs =
+		(struct reftable_ref_store *)ref_store;
+	struct write_create_symref_arg arg = { .refs = refs,
+					       .refname = refname,
+					       .target = target,
+					       .logmsg = logmsg };
+	return stack_add(refs->stack, &write_create_symref_table, &arg);
+}
+
+struct write_rename_arg {
+	struct stack *stack;
+	const char *oldname;
+	const char *newname;
+	const char *logmsg;
+};
+
+static int write_rename_table(struct writer *writer, void *argv)
+{
+	struct write_rename_arg *arg = (struct write_rename_arg *)argv;
+	uint64_t ts = stack_next_update_index(arg->stack);
+	struct ref_record ref = {};
+	int err = stack_read_ref(arg->stack, arg->oldname, &ref);
+	if (err) {
+		goto exit;
+	}
+
+	/* XXX do ref renames overwrite the target? */
+	if (stack_read_ref(arg->stack, arg->newname, &ref) == 0) {
+		goto exit;
+	}
+
+	free(ref.ref_name);
+	ref.ref_name = strdup(arg->newname);
+	writer_set_limits(writer, ts, ts);
+	ref.update_index = ts;
+
+	{
+		struct ref_record todo[2] = {};
+		todo[0].ref_name = (char *)arg->oldname;
+		todo[0].update_index = ts;
+		/* leave todo[0] empty */
+		todo[1] = ref;
+		todo[1].update_index = ts;
+
+		err = writer_add_refs(writer, todo, 2);
+		if (err < 0) {
+			goto exit;
+		}
+	}
+
+	if (ref.value != NULL) {
+		struct log_record todo[2] = {};
+		fill_log_record(&todo[0]);
+		fill_log_record(&todo[1]);
+
+		todo[0].ref_name = (char *)arg->oldname;
+		todo[0].update_index = ts;
+		todo[0].message = (char *)arg->logmsg;
+		todo[0].old_hash = ref.value;
+		todo[0].new_hash = NULL;
+
+		todo[1].ref_name = (char *)arg->newname;
+		todo[1].update_index = ts;
+		todo[1].old_hash = NULL;
+		todo[1].new_hash = ref.value;
+		todo[1].message = (char *)arg->logmsg;
+
+		err = writer_add_logs(writer, todo, 2);
+
+		clear_log_record(&todo[0]);
+		clear_log_record(&todo[1]);
+
+		if (err < 0) {
+			goto exit;
+		}
+
+	} else {
+		/* XXX symrefs? */
+	}
+
+exit:
+	ref_record_clear(&ref);
+	return err;
+}
+
+static int reftable_rename_ref(struct ref_store *ref_store,
+			       const char *oldrefname, const char *newrefname,
+			       const char *logmsg)
+{
+	struct reftable_ref_store *refs =
+		(struct reftable_ref_store *)ref_store;
+	struct write_rename_arg arg = {
+		.stack = refs->stack,
+		.oldname = oldrefname,
+		.newname = newrefname,
+		.logmsg = logmsg,
+	};
+	return stack_add(refs->stack, &write_rename_table, &arg);
+}
+
+static int reftable_copy_ref(struct ref_store *ref_store,
+			     const char *oldrefname, const char *newrefname,
+			     const char *logmsg)
+{
+	BUG("reftable reference store does not support copying references");
+}
+
+struct reftable_reflog_ref_iterator {
+	struct ref_iterator base;
+	struct iterator iter;
+	struct log_record log;
+	struct object_id oid;
+	char *last_name;
+};
+
+static int
+reftable_reflog_ref_iterator_advance(struct ref_iterator *ref_iterator)
+{
+	struct reftable_reflog_ref_iterator *ri =
+		(struct reftable_reflog_ref_iterator *)ref_iterator;
+
+	while (1) {
+		int err = iterator_next_log(ri->iter, &ri->log);
+		if (err > 0) {
+			return ITER_DONE;
+		}
+		if (err < 0) {
+			return ITER_ERROR;
+		}
+
+		ri->base.refname = ri->log.ref_name;
+		if (ri->last_name != NULL &&
+		    !strcmp(ri->log.ref_name, ri->last_name)) {
+			continue;
+		}
+
+		free(ri->last_name);
+		ri->last_name = xstrdup(ri->log.ref_name);
+		hashcpy(ri->oid.hash, ri->log.new_hash);
+		return ITER_OK;
+	}
+}
+
+static int reftable_reflog_ref_iterator_peel(struct ref_iterator *ref_iterator,
+					     struct object_id *peeled)
+{
+	BUG("not supported.");
+	return -1;
+}
+
+static int reftable_reflog_ref_iterator_abort(struct ref_iterator *ref_iterator)
+{
+	struct reftable_reflog_ref_iterator *ri =
+		(struct reftable_reflog_ref_iterator *)ref_iterator;
+	log_record_clear(&ri->log);
+	iterator_destroy(&ri->iter);
+	return 0;
+}
+
+static struct ref_iterator_vtable reftable_reflog_ref_iterator_vtable = {
+	reftable_reflog_ref_iterator_advance, reftable_reflog_ref_iterator_peel,
+	reftable_reflog_ref_iterator_abort
+};
+
+static struct ref_iterator *
+reftable_reflog_iterator_begin(struct ref_store *ref_store)
+{
+	struct reftable_reflog_ref_iterator *ri = xcalloc(sizeof(*ri), 1);
+	struct reftable_ref_store *refs =
+		(struct reftable_ref_store *)ref_store;
+
+	struct merged_table *mt = stack_merged_table(refs->stack);
+	int err = merged_table_seek_log(mt, &ri->iter, "");
+	if (err < 0) {
+		free(ri);
+		return NULL;
+	}
+
+	base_ref_iterator_init(&ri->base, &reftable_reflog_ref_iterator_vtable,
+			       1);
+	ri->base.oid = &ri->oid;
+
+	return empty_ref_iterator_begin();
+}
+
+static int
+reftable_for_each_reflog_ent_newest_first(struct ref_store *ref_store,
+					  const char *refname,
+					  each_reflog_ent_fn fn, void *cb_data)
+{
+	struct iterator it = {};
+	struct reftable_ref_store *refs =
+		(struct reftable_ref_store *)ref_store;
+	struct merged_table *mt = stack_merged_table(refs->stack);
+	int err = merged_table_seek_log(mt, &it, refname);
+	struct log_record log = {};
+
+	while (err == 0) {
+		err = iterator_next_log(it, &log);
+		if (err != 0) {
+			break;
+		}
+
+		if (strcmp(log.ref_name, refname)) {
+			break;
+		}
+
+		{
+			struct object_id old_oid = {};
+			struct object_id new_oid = {};
+			const char *full_committer = "";
+
+			hashcpy(old_oid.hash, log.old_hash);
+			hashcpy(new_oid.hash, log.new_hash);
+
+			full_committer = fmt_ident(log.name, log.email,
+						   WANT_COMMITTER_IDENT,
+						   /*date*/ NULL,
+						   IDENT_NO_DATE);
+			if (fn(&old_oid, &new_oid, full_committer, log.time,
+			       log.tz_offset, log.message, cb_data)) {
+				err = -1;
+				break;
+			}
+		}
+	}
+
+	log_record_clear(&log);
+	iterator_destroy(&it);
+	if (err > 0) {
+		err = 0;
+	}
+	return err;
+}
+
+static int
+reftable_for_each_reflog_ent_oldest_first(struct ref_store *ref_store,
+					  const char *refname,
+					  each_reflog_ent_fn fn, void *cb_data)
+{
+	struct iterator it = {};
+	struct reftable_ref_store *refs =
+		(struct reftable_ref_store *)ref_store;
+	struct merged_table *mt = stack_merged_table(refs->stack);
+	int err = merged_table_seek_log(mt, &it, refname);
+
+	struct log_record *logs = NULL;
+	int cap = 0;
+	int len = 0;
+
+	while (err == 0) {
+		struct log_record log = {};
+		err = iterator_next_log(it, &log);
+		if (err != 0) {
+			break;
+		}
+
+		if (strcmp(log.ref_name, refname)) {
+			break;
+		}
+
+		if (len == cap) {
+			cap = 2 * cap + 1;
+			logs = realloc(logs, cap * sizeof(*logs));
+		}
+
+		logs[len++] = log;
+	}
+
+	for (int i = len; i--;) {
+		struct log_record *log = &logs[i];
+		struct object_id old_oid = {};
+		struct object_id new_oid = {};
+		const char *full_committer = "";
+
+		hashcpy(old_oid.hash, log->old_hash);
+		hashcpy(new_oid.hash, log->new_hash);
+
+		full_committer = fmt_ident(log->name, log->email,
+					   WANT_COMMITTER_IDENT, NULL,
+					   IDENT_NO_DATE);
+		if (!fn(&old_oid, &new_oid, full_committer, log->time,
+			log->tz_offset, log->message, cb_data)) {
+			err = -1;
+			break;
+		}
+	}
+
+	for (int i = 0; i < len; i++) {
+		log_record_clear(&logs[i]);
+	}
+	free(logs);
+
+	iterator_destroy(&it);
+	if (err > 0) {
+		err = 0;
+	}
+	return err;
+}
+
+static int reftable_reflog_exists(struct ref_store *ref_store,
+				  const char *refname)
+{
+	/* always exists. */
+	return 1;
+}
+
+static int reftable_create_reflog(struct ref_store *ref_store,
+				  const char *refname, int force_create,
+				  struct strbuf *err)
+{
+	return 0;
+}
+
+static int reftable_delete_reflog(struct ref_store *ref_store,
+				  const char *refname)
+{
+	return 0;
+}
+
+static int reftable_reflog_expire(struct ref_store *ref_store,
+				  const char *refname,
+				  const struct object_id *oid,
+				  unsigned int flags,
+				  reflog_expiry_prepare_fn prepare_fn,
+				  reflog_expiry_should_prune_fn should_prune_fn,
+				  reflog_expiry_cleanup_fn cleanup_fn,
+				  void *policy_cb_data)
+{
+	BUG("per ref reflog expiry is not implemented for reftable backend.");
+	/*
+	  TODO: should iterate over the reflog, and write tombstones. The
+	  tombstones will be removed on compaction.
+	 */
+}
+
+static int reftable_read_raw_ref(struct ref_store *ref_store,
+				 const char *refname, struct object_id *oid,
+				 struct strbuf *referent, unsigned int *type)
+{
+	struct reftable_ref_store *refs =
+		(struct reftable_ref_store *)ref_store;
+	struct ref_record ref = {};
+	int err = stack_read_ref(refs->stack, refname, &ref);
+	if (err) {
+		goto exit;
+	}
+	if (ref.target != NULL) {
+		/* XXX recurse? */
+		strbuf_reset(referent);
+		strbuf_addstr(referent, ref.target);
+		*type |= REF_ISSYMREF;
+	} else {
+		hashcpy(oid->hash, ref.value);
+	}
+exit:
+	ref_record_clear(&ref);
+	return err;
+}
+
+struct ref_storage_be refs_be_reftable = {
+	&refs_be_files,
+	"reftable",
+	reftable_ref_store_create,
+	reftable_init_db,
+	reftable_transaction_prepare,
+	reftable_transaction_finish,
+	reftable_transaction_abort,
+	reftable_transaction_commit,
+
+	reftable_pack_refs,
+	reftable_create_symref,
+	reftable_delete_refs,
+	reftable_rename_ref,
+	reftable_copy_ref,
+
+	reftable_ref_iterator_begin,
+	reftable_read_raw_ref,
+
+	reftable_reflog_iterator_begin,
+	reftable_for_each_reflog_ent_newest_first,
+	reftable_for_each_reflog_ent_oldest_first,
+	reftable_reflog_exists,
+	reftable_create_reflog,
+	reftable_delete_reflog,
+	reftable_reflog_expire
+};
diff --git a/repository.c b/repository.c
index a4174ddb06..aff47302c9 100644
--- a/repository.c
+++ b/repository.c
@@ -174,6 +174,10 @@ int repo_init(struct repository *repo,
 	if (worktree)
 		repo_set_worktree(repo, worktree);
 
+	repo->ref_storage_format = format.ref_storage != NULL ?
+					   xstrdup(format.ref_storage) :
+					   "files"; /* XXX */
+
 	clear_repository_format(&format);
 	return 0;
 
diff --git a/repository.h b/repository.h
index 040057dea6..198d4aa090 100644
--- a/repository.h
+++ b/repository.h
@@ -70,6 +70,9 @@ struct repository {
 	/* The store in which the refs are held. */
 	struct ref_store *refs;
 
+	/* The format to use for the ref database. */
+	char *ref_storage_format;
+
 	/*
 	 * Contains path to often used file names.
 	 */
diff --git a/setup.c b/setup.c
index e7c7dca701..a339186d83 100644
--- a/setup.c
+++ b/setup.c
@@ -448,9 +448,11 @@ static int check_repo_format(const char *var, const char *value, void *vdata)
 			if (!value)
 				return config_error_nonbool(var);
 			data->partial_clone = xstrdup(value);
-		} else if (!strcmp(ext, "worktreeconfig"))
+		} else if (!strcmp(ext, "worktreeconfig")) {
 			data->worktree_config = git_config_bool(var, value);
-		else
+		} else if (!strcmp(ext, "refStorage")) {
+			data->ref_storage = xstrdup(value);
+		} else
 			string_list_append(&data->unknown_extensions, ext);
 	}
 
@@ -539,6 +541,7 @@ void clear_repository_format(struct repository_format *format)
 	string_list_clear(&format->unknown_extensions, 0);
 	free(format->work_tree);
 	free(format->partial_clone);
+	free(format->ref_storage);
 	init_repository_format(format);
 }
 
-- 
gitgitgadget