static void read_head_pointers(void)
{
	if (read_ref("HEAD", head_sha1))
		die("No HEAD -- no initial commit yet?");
	if (read_ref("MERGE_HEAD", merge_head_sha1)) {
		fprintf(stderr, "Not in the middle of a merge.\n");
		exit(0);
	}
}

static int for_each_replace_name(const char **argv, each_replace_name_fn fn)
{
	const char **p, *full_hex;
	struct strbuf ref = STRBUF_INIT;
	size_t base_len;
	int had_error = 0;
	struct object_id oid;

	strbuf_addstr(&ref, git_replace_ref_base);
	base_len = ref.len;

	for (p = argv; *p; p++) {
		if (get_oid(*p, &oid)) {
			error("Failed to resolve '%s' as a valid ref.", *p);
			had_error = 1;
			continue;
		}

		strbuf_setlen(&ref, base_len);
		strbuf_addstr(&ref, oid_to_hex(&oid));
		full_hex = ref.buf + base_len;

		if (read_ref(ref.buf, &oid)) {
			error("replace ref '%s' not found.", full_hex);
			had_error = 1;
			continue;
		}
		if (fn(full_hex, ref.buf, &oid))
			had_error = 1;
	}
	strbuf_release(&ref);
	return had_error;
}

static int sequencer_continue(struct replay_opts *opts, int skip)
{
	struct commit_list *todo_list = NULL;

	if (!file_exists(git_path(SEQ_TODO_FILE)))
		return continue_single_pick();
	read_populate_opts(&opts);
	read_populate_todo(&todo_list, opts);
	if (opts->action == REPLAY_PICK)
		load_rewritten(&rewritten, git_path(SEQ_REWR_FILE));

	/* Verify that the conflict has been resolved */
	if (file_exists(git_path("CHERRY_PICK_HEAD")) ||
	    file_exists(git_path("REVERT_HEAD"))) {
		int ret = continue_single_pick();
		if (ret)
			return ret;
	}
	if (index_differs_from("HEAD", 0))
		return error_dirty_index(opts);
	if (opts->action == REPLAY_PICK && !skip) {
		unsigned char to[20];
		if (!read_ref("HEAD", to))
			add_rewritten(&rewritten, todo_list->item->object.sha1, to);
	}
	todo_list = todo_list->next;
	return pick_commits(todo_list, opts);
}

static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
{
	static const char *prefix[] = {
		"",
		"refs",
		"refs/tags",
		"refs/heads",
		NULL
	};
	const char **p;

	if (len == 40 && !get_sha1_hex(str, sha1))
		return 0;

	/* Accept only unambiguous ref paths. */
	if (ambiguous_path(str, len))
		return -1;

	for (p = prefix; *p; p++) {
		char *pathname = git_path("%s/%.*s", *p, len, str);
		if (!read_ref(pathname, sha1))
			return 0;
	}
	return -1;
}

void create_notes_commit(struct notes_tree *t, struct commit_list *parents,
			 const char *msg, unsigned char *result_sha1)
{
	unsigned char tree_sha1[20];

	assert(t->initialized);

	if (write_notes_tree(t, tree_sha1))
		die("Failed to write notes tree to database");

	if (!parents) {
		/* Deduce parent commit from t->ref */
		unsigned char parent_sha1[20];
		if (!read_ref(t->ref, parent_sha1)) {
			struct commit *parent = lookup_commit(parent_sha1);
			if (!parent || parse_commit(parent))
				die("Failed to find/parse commit %s", t->ref);
			commit_list_insert(parent, &parents);
		}
		/* else: t->ref points to nothing, assume root/orphan commit */
	}

	if (commit_tree(msg, tree_sha1, parents, result_sha1, NULL))
		die("Failed to commit notes tree to database");
}

int
bibl_read( bibl *b, FILE *fp, char *filename, param *p )
{
	convert_rules r;
	param lp;
	bibl bin;

	if ( !b ) return BIBL_ERR_BADINPUT;
	if ( !fp ) return BIBL_ERR_BADINPUT;
	if ( !p ) return BIBL_ERR_BADINPUT;
	if ( bibl_illegalinmode( p->readformat ) ) return BIBL_ERR_BADINPUT;

	bibl_setreadparams( &lp, p );
	bibl_init( &bin );
	rules_init( &r, p->readformat );
	read_ref( fp, &bin, filename, &r, &lp );
	if ( !lp.output_raw || ( lp.output_raw & BIBL_RAW_WITHCHARCONVERT ) )
		bibl_fixcharsets( &bin, &lp );
	if ( !lp.output_raw )
		convert_ref( &bin, filename, b, &r, &lp );
	else {
		if ( p->verbose > 1 ) bibl_verbose0( &bin );
		bibl_copy( b, &bin );
	}
	if ( !lp.output_raw || ( lp.output_raw & BIBL_RAW_WITHMAKEREFID ) )
		bibl_checkrefid( b, &lp );
	bibl_free( &bin );

	return BIBL_OK;
}

static int notes_cache_match_validity(struct repository *r,
				      const char *ref,
				      const char *validity)
{
	struct object_id oid;
	struct commit *commit;
	struct pretty_print_context pretty_ctx;
	struct strbuf msg = STRBUF_INIT;
	int ret;

	if (read_ref(ref, &oid) < 0)
		return 0;

	commit = lookup_commit_reference_gently(r, &oid, 1);
	if (!commit)
		return 0;

	memset(&pretty_ctx, 0, sizeof(pretty_ctx));
	format_commit_message(commit, "%s", &msg, &pretty_ctx);
	strbuf_trim(&msg);

	ret = !strcmp(msg.buf, validity);
	strbuf_release(&msg);

	return ret;
}

static int delete_pseudoref(const char *pseudoref, const unsigned char *old_sha1)
{
	static struct lock_file lock;
	const char *filename;

	filename = git_path("%s", pseudoref);

	if (old_sha1 && !is_null_sha1(old_sha1)) {
		int fd;
		unsigned char actual_old_sha1[20];

		fd = hold_lock_file_for_update(&lock, filename,
					       LOCK_DIE_ON_ERROR);
		if (fd < 0)
			die_errno(_("Could not open '%s' for writing"), filename);
		if (read_ref(pseudoref, actual_old_sha1))
			die("could not read ref '%s'", pseudoref);
		if (hashcmp(actual_old_sha1, old_sha1)) {
			warning("Unexpected sha1 when deleting %s", pseudoref);
			rollback_lock_file(&lock);
			return -1;
		}

		unlink(filename);
		rollback_lock_file(&lock);
	} else {
		unlink(filename);
	}

	return 0;
}

static int do_fetch(struct transport *transport,
		    struct refspec *refs, int ref_count)
{
	struct ref *ref_map;
	struct ref *rm;
	int autotags = (transport->remote->fetch_tags == 1);
	if (transport->remote->fetch_tags == 2 && tags != TAGS_UNSET)
		tags = TAGS_SET;
	if (transport->remote->fetch_tags == -1)
		tags = TAGS_UNSET;

	if (!transport->get_refs_list || !transport->fetch)
		die("Don't know how to fetch from %s", transport->url);

	/* if not appending, truncate FETCH_HEAD */
	if (!append) {
		char *filename = git_path("FETCH_HEAD");
		FILE *fp = fopen(filename, "w");
		if (!fp)
			return error("cannot open %s: %s\n", filename, strerror(errno));
		fclose(fp);
	}

	ref_map = get_ref_map(transport, refs, ref_count, tags, &autotags);
	if (!update_head_ok)
		check_not_current_branch(ref_map);

	for (rm = ref_map; rm; rm = rm->next) {
		if (rm->peer_ref)
			read_ref(rm->peer_ref->name, rm->peer_ref->old_sha1);
	}

	if (tags == TAGS_DEFAULT && autotags)
		transport_set_option(transport, TRANS_OPT_FOLLOWTAGS, "1");
	if (fetch_refs(transport, ref_map)) {
		free_refs(ref_map);
		return 1;
	}
	free_refs(ref_map);

	/* if neither --no-tags nor --tags was specified, do automated tag
	 * following ... */
	if (tags == TAGS_DEFAULT && autotags) {
		struct ref **tail = &ref_map;
		ref_map = NULL;
		find_non_local_tags(transport, &ref_map, &tail);
		if (ref_map) {
			transport_set_option(transport, TRANS_OPT_FOLLOWTAGS, NULL);
			transport_set_option(transport, TRANS_OPT_DEPTH, "0");
			fetch_refs(transport, ref_map);
		}
		free_refs(ref_map);
	}

	return 0;
}

static int do_reupdate(int ac, const char **av,
		       const char *prefix, int prefix_length)
{
	/* Read HEAD and run update-index on paths that are
	 * merged and already different between index and HEAD.
	 */
	int pos;
	int has_head = 1;
	struct pathspec pathspec;

	parse_pathspec(&pathspec, 0,
		       PATHSPEC_PREFER_CWD,
		       prefix, av + 1);

	if (read_ref("HEAD", &head_oid))
		/* If there is no HEAD, that means it is an initial
		 * commit.  Update everything in the index.
		 */
		has_head = 0;
 redo:
	for (pos = 0; pos < active_nr; pos++) {
		const struct cache_entry *ce = active_cache[pos];
		struct cache_entry *old = NULL;
		int save_nr;
		char *path;

		if (ce_stage(ce) || !ce_path_match(&the_index, ce, &pathspec, NULL))
			continue;
		if (has_head)
			old = read_one_ent(NULL, &head_oid,
					   ce->name, ce_namelen(ce), 0);
		if (old && ce->ce_mode == old->ce_mode &&
		    !oidcmp(&ce->oid, &old->oid)) {
			discard_cache_entry(old);
			continue; /* unchanged */
		}
		/* Be careful.  The working tree may not have the
		 * path anymore, in which case, under 'allow_remove',
		 * or worse yet 'allow_replace', active_nr may decrease.
		 */
		save_nr = active_nr;
		path = xstrdup(ce->name);
		update_one(path);
		free(path);
		discard_cache_entry(old);
		if (save_nr != active_nr)
			goto redo;
	}
	clear_pathspec(&pathspec);
	return 0;
}

static int check_ref_valid(struct object_id *object,
			    struct object_id *prev,
			    struct strbuf *ref,
			    int force)
{
	strbuf_reset(ref);
	strbuf_addf(ref, "%s%s", git_replace_ref_base, oid_to_hex(object));
	if (check_refname_format(ref->buf, 0))
		return error("'%s' is not a valid ref name.", ref->buf);

	if (read_ref(ref->buf, prev))
		oidclr(prev);
	else if (!force)
		return error("replace ref '%s' already exists", ref->buf);
	return 0;
}

static int do_reupdate(int ac, const char **av,
		       const char *prefix, int prefix_length)
{
	/* Read HEAD and run update-index on paths that are
	 * merged and already different between index and HEAD.
	 */
	int pos;
	int has_head = 1;
	const char **paths = get_pathspec(prefix, av + 1);
	struct pathspec pathspec;

	init_pathspec(&pathspec, paths);

	if (read_ref("HEAD", head_sha1))
		/* If there is no HEAD, that means it is an initial
		 * commit.  Update everything in the index.
		 */
		has_head = 0;
 redo:
	for (pos = 0; pos < active_nr; pos++) {
		struct cache_entry *ce = active_cache[pos];
		struct cache_entry *old = NULL;
		int save_nr;

		if (ce_stage(ce) || !ce_path_match(ce, &pathspec))
			continue;
		if (has_head)
			old = read_one_ent(NULL, head_sha1,
					   ce->name, ce_namelen(ce), 0);
		if (old && ce->ce_mode == old->ce_mode &&
		    !hashcmp(ce->sha1, old->sha1)) {
			free(old);
			continue; /* unchanged */
		}
		/* Be careful.  The working tree may not have the
		 * path anymore, in which case, under 'allow_remove',
		 * or worse yet 'allow_replace', active_nr may decrease.
		 */
		save_nr = active_nr;
		update_one(ce->name + prefix_length, prefix, prefix_length);
		if (save_nr != active_nr)
			goto redo;
	}
	free_pathspec(&pathspec);
	return 0;
}

int cmd_show_ref(int argc, const char **argv, const char *prefix)
{
	if (argc == 2 && !strcmp(argv[1], "-h"))
		usage_with_options(show_ref_usage, show_ref_options);

	argc = parse_options(argc, argv, prefix, show_ref_options,
			     show_ref_usage, PARSE_OPT_NO_INTERNAL_HELP);

	if (exclude_arg)
		return exclude_existing(exclude_existing_arg);

	pattern = argv;
	if (!*pattern)
		pattern = NULL;

	if (verify) {
		if (!pattern)
			die("--verify requires a reference");
		while (*pattern) {
			unsigned char sha1[20];

			if (!prefixcmp(*pattern, "refs/") &&
			    !read_ref(*pattern, sha1)) {
				if (!quiet)
					show_one(*pattern, sha1);
			}
			else if (!quiet)
				die("'%s' - not a valid ref", *pattern);
			else
				return 1;
			pattern++;
		}
		return 0;
	}

	if (show_head)
		head_ref(show_ref, NULL);
	for_each_ref(show_ref, NULL);
	if (!found_match) {
		if (verify && !quiet)
			die("No match");
		return 1;
	}
	return 0;
}

static int write_pseudoref(const char *pseudoref, const unsigned char *sha1,
			   const unsigned char *old_sha1, struct strbuf *err)
{
	const char *filename;
	int fd;
	static struct lock_file lock;
	struct strbuf buf = STRBUF_INIT;
	int ret = -1;

	strbuf_addf(&buf, "%s\n", sha1_to_hex(sha1));

	filename = git_path("%s", pseudoref);
	fd = hold_lock_file_for_update(&lock, filename, LOCK_DIE_ON_ERROR);
	if (fd < 0) {
		strbuf_addf(err, "could not open '%s' for writing: %s",
			    filename, strerror(errno));
		return -1;
	}

	if (old_sha1) {
		unsigned char actual_old_sha1[20];

		if (read_ref(pseudoref, actual_old_sha1))
			die("could not read ref '%s'", pseudoref);
		if (hashcmp(actual_old_sha1, old_sha1)) {
			strbuf_addf(err, "unexpected sha1 when writing '%s'", pseudoref);
			rollback_lock_file(&lock);
			goto done;
		}
	}

	if (write_in_full(fd, buf.buf, buf.len) != buf.len) {
		strbuf_addf(err, "could not write to '%s'", filename);
		rollback_lock_file(&lock);
		goto done;
	}

	commit_lock_file(&lock);
	ret = 0;
done:
	strbuf_release(&buf);
	return ret;
}

int cmd_show_ref(int argc, const char **argv, const char *prefix)
{
	git_config(git_default_config, NULL);
	argc = parse_options(argc, argv, prefix, show_ref_options,
			     show_ref_usage, 0);

	if (exclude_arg)
		return exclude_existing(exclude_existing_arg);

	pattern = argv;
	if (!*pattern)
		pattern = NULL;

	if (verify) {
		if (!pattern)
			die("--verify requires a reference");
		while (*pattern) {
			struct object_id oid;

			if ((starts_with(*pattern, "refs/") || !strcmp(*pattern, "HEAD")) &&
			    !read_ref(*pattern, &oid)) {
				show_one(*pattern, &oid);
			}
			else if (!quiet)
				die("'%s' - not a valid ref", *pattern);
			else
				return 1;
			pattern++;
		}
		return 0;
	}

	if (show_head)
		head_ref(show_ref, NULL);
	for_each_ref(show_ref, NULL);
	if (!found_match) {
		if (verify && !quiet)
			die("No match");
		return 1;
	}
	return 0;
}

int commit_notes(struct notes_tree *t, const char *msg)
{
	struct commit_list *parent;
	unsigned char tree_sha1[20], prev_commit[20], new_commit[20];
	struct strbuf buf = STRBUF_INIT;

	if (!t)
		t = &default_notes_tree;
	if (!t->initialized || !t->ref || !*t->ref)
		die("Cannot commit uninitialized/unreferenced notes tree");
	if (!t->dirty)
		return 0; /* don't have to commit an unchanged tree */

	/* Prepare commit message and reflog message */
	strbuf_addstr(&buf, "notes: "); /* commit message starts at index 7 */
	strbuf_addstr(&buf, msg);
	if (buf.buf[buf.len - 1] != '\n')
		strbuf_addch(&buf, '\n'); /* Make sure msg ends with newline */

	/* Convert notes tree to tree object */
	if (write_notes_tree(t, tree_sha1))
		die("Failed to write current notes tree to database");

	/* Create new commit for the tree object */
	if (!read_ref(t->ref, prev_commit)) { /* retrieve parent commit */
		parent = xmalloc(sizeof(*parent));
		parent->item = lookup_commit(prev_commit);
		parent->next = NULL;
	} else {
		hashclr(prev_commit);
		parent = NULL;
	}
	if (commit_tree(buf.buf + 7, tree_sha1, parent, new_commit, NULL))
		die("Failed to commit notes tree to database");

	/* Update notes ref with new commit */
	update_ref(buf.buf, t->ref, new_commit, prev_commit, 0, DIE_ON_ERR);

	strbuf_release(&buf);
	return 0;
}

void init_notes(struct notes_tree *t, const char *notes_ref,
		combine_notes_fn combine_notes, int flags)
{
	unsigned char sha1[20], object_sha1[20];
	unsigned mode;
	struct leaf_node root_tree;

	if (!t)
		t = &default_notes_tree;
	assert(!t->initialized);

	if (!notes_ref)
		notes_ref = getenv(GIT_NOTES_REF_ENVIRONMENT);
	if (!notes_ref)
		notes_ref = notes_ref_name; /* value of core.notesRef config */
	if (!notes_ref)
		notes_ref = GIT_NOTES_DEFAULT_REF;

	if (!combine_notes)
		combine_notes = combine_notes_concatenate;

	t->root = (struct int_node *) xcalloc(sizeof(struct int_node), 1);
	t->first_non_note = NULL;
	t->prev_non_note = NULL;
	t->ref = notes_ref ? xstrdup(notes_ref) : NULL;
	t->combine_notes = combine_notes;
	t->initialized = 1;

	if (flags & NOTES_INIT_EMPTY || !notes_ref ||
	    read_ref(notes_ref, object_sha1))
		return;
	if (get_tree_entry(object_sha1, "", sha1, &mode))
		die("Failed to read notes tree referenced by %s (%s)",
		    notes_ref, object_sha1);

	hashclr(root_tree.key_sha1);
	hashcpy(root_tree.val_sha1, sha1);
	load_subtree(t, &root_tree, t->root, 0);
}

void init_notes(struct notes_tree *t, const char *notes_ref,
		combine_notes_fn combine_notes, int flags)
{
	struct object_id oid, object_oid;
	unsigned mode;
	struct leaf_node root_tree;

	if (!t)
		t = &default_notes_tree;
	assert(!t->initialized);

	if (!notes_ref)
		notes_ref = default_notes_ref();

	if (!combine_notes)
		combine_notes = combine_notes_concatenate;

	t->root = (struct int_node *) xcalloc(1, sizeof(struct int_node));
	t->first_non_note = NULL;
	t->prev_non_note = NULL;
	t->ref = xstrdup_or_null(notes_ref);
	t->update_ref = (flags & NOTES_INIT_WRITABLE) ? t->ref : NULL;
	t->combine_notes = combine_notes;
	t->initialized = 1;
	t->dirty = 0;

	if (flags & NOTES_INIT_EMPTY || !notes_ref ||
	    get_oid_treeish(notes_ref, &object_oid))
		return;
	if (flags & NOTES_INIT_WRITABLE && read_ref(notes_ref, &object_oid))
		die("Cannot use notes ref %s", notes_ref);
	if (get_tree_entry(&object_oid, "", &oid, &mode))
		die("Failed to read notes tree referenced by %s (%s)",
		    notes_ref, oid_to_hex(&object_oid));

	oidclr(&root_tree.key_oid);
	oidcpy(&root_tree.val_oid, &oid);
	load_subtree(t, &root_tree, t->root, 0);
}

void init_notes(struct notes_tree *t, const char *notes_ref,
		combine_notes_fn combine_notes, int flags)
{
	unsigned char sha1[20], object_sha1[20];
	unsigned mode;
	struct leaf_node root_tree;

	if (!t)
		t = &default_notes_tree;
	assert(!t->initialized);

	if (!notes_ref)
		notes_ref = default_notes_ref();

	if (!combine_notes)
		combine_notes = combine_notes_concatenate;

	t->root = (struct int_node *) xcalloc(1, sizeof(struct int_node));
	t->first_non_note = NULL;
	t->prev_non_note = NULL;
	t->ref = xstrdup_or_null(notes_ref);
	t->combine_notes = combine_notes;
	t->initialized = 1;
	t->dirty = 0;

	if (flags & NOTES_INIT_EMPTY || !notes_ref ||
	    read_ref(notes_ref, object_sha1))
		return;
	if (get_tree_entry(object_sha1, "", sha1, &mode))
		die("Failed to read notes tree referenced by %s (%s)",
		    notes_ref, sha1_to_hex(object_sha1));

	hashclr(root_tree.key_sha1);
	hashcpy(root_tree.val_sha1, sha1);
	load_subtree(t, &root_tree, t->root, 0);
}

static int notes_cache_match_validity(const char *ref, const char *validity)
{
	unsigned char sha1[20];
	struct commit *commit;
	struct pretty_print_context pretty_ctx;
	struct strbuf msg = STRBUF_INIT;
	int ret;

	if (read_ref(ref, sha1) < 0)
		return 0;

	commit = lookup_commit_reference_gently(sha1, 1);
	if (!commit)
		return 0;

	memset(&pretty_ctx, 0, sizeof(pretty_ctx));
	format_commit_message(commit, "%s", &msg, &pretty_ctx);
	strbuf_trim(&msg);

	ret = !strcmp(msg.buf, validity);
	strbuf_release(&msg);

	return ret;
}