mmapped_ref_iterator: add iterator over a packed-refs file

Add a new `mmapped_ref_iterator`, which can iterate over the
references in an mmapped `packed-refs` file directly. Use this
iterator from `read_packed_refs()` to fill the packed refs cache.

Note that we are not yet willing to promise that the new iterator
generates its output in order. That doesn't matter for now, because
the packed refs cache doesn't care what order it is filled.

This change adds a lot of boilerplate without providing any obvious
benefits. The benefits will come soon, when we get rid of the
`ref_cache` for packed references altogether.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: mhagger

refs/packed-backend.c

@@ -163,6 +163,141 @@ static NORETURN void die_invalid_line(const char *path,
 
 }
 
+/*
+ * An iterator over a packed-refs file that is currently mmapped.
+ */
+struct mmapped_ref_iterator {
+	struct ref_iterator base;
+
+	struct packed_ref_cache *packed_refs;
+
+	/* The current position in the mmapped file: */
+	const char *pos;
+
+	/* The end of the mmapped file: */
+	const char *eof;
+
+	struct object_id oid, peeled;
+
+	struct strbuf refname_buf;
+};
+
+static int mmapped_ref_iterator_advance(struct ref_iterator *ref_iterator)
+{
+	struct mmapped_ref_iterator *iter =
+		(struct mmapped_ref_iterator *)ref_iterator;
+	const char *p = iter->pos, *eol;
+
+	strbuf_reset(&iter->refname_buf);
+
+	if (iter->pos == iter->eof)
+		return ref_iterator_abort(ref_iterator);
+
+	iter->base.flags = REF_ISPACKED;
+
+	if (iter->eof - p < GIT_SHA1_HEXSZ + 2 ||
+	    parse_oid_hex(p, &iter->oid, &p) ||
+	    !isspace(*p++))
+		die_invalid_line(iter->packed_refs->refs->path,
+				 iter->pos, iter->eof - iter->pos);
+
+	eol = memchr(p, '\n', iter->eof - p);
+	if (!eol)
+		die_unterminated_line(iter->packed_refs->refs->path,
+				      iter->pos, iter->eof - iter->pos);
+
+	strbuf_add(&iter->refname_buf, p, eol - p);
+	iter->base.refname = iter->refname_buf.buf;
+
+	if (check_refname_format(iter->base.refname, REFNAME_ALLOW_ONELEVEL)) {
+		if (!refname_is_safe(iter->base.refname))
+			die("packed refname is dangerous: %s",
+			    iter->base.refname);
+		oidclr(&iter->oid);
+		iter->base.flags |= REF_BAD_NAME | REF_ISBROKEN;
+	}
+	if (iter->packed_refs->peeled == PEELED_FULLY ||
+	    (iter->packed_refs->peeled == PEELED_TAGS &&
+	     starts_with(iter->base.refname, "refs/tags/")))
+		iter->base.flags |= REF_KNOWS_PEELED;
+
+	iter->pos = eol + 1;
+
+	if (iter->pos < iter->eof && *iter->pos == '^') {
+		p = iter->pos + 1;
+		if (iter->eof - p < GIT_SHA1_HEXSZ + 1 ||
+		    parse_oid_hex(p, &iter->peeled, &p) ||
+		    *p++ != '\n')
+			die_invalid_line(iter->packed_refs->refs->path,
+					 iter->pos, iter->eof - iter->pos);
+		iter->pos = p;
+
+		/*
+		 * Regardless of what the file header said, we
+		 * definitely know the value of *this* reference:
+		 */
+		iter->base.flags |= REF_KNOWS_PEELED;
+	} else {
+		oidclr(&iter->peeled);
+	}
+
+	return ITER_OK;
+}
+
+static int mmapped_ref_iterator_peel(struct ref_iterator *ref_iterator,
+				    struct object_id *peeled)
+{
+	struct mmapped_ref_iterator *iter =
+		(struct mmapped_ref_iterator *)ref_iterator;
+
+	if ((iter->base.flags & REF_KNOWS_PEELED)) {
+		oidcpy(peeled, &iter->peeled);
+		return is_null_oid(&iter->peeled) ? -1 : 0;
+	} else if ((iter->base.flags & (REF_ISBROKEN | REF_ISSYMREF))) {
+		return -1;
+	} else {
+		return !!peel_object(iter->oid.hash, peeled->hash);
+	}
+}
+
+static int mmapped_ref_iterator_abort(struct ref_iterator *ref_iterator)
+{
+	struct mmapped_ref_iterator *iter =
+		(struct mmapped_ref_iterator *)ref_iterator;
+
+	release_packed_ref_cache(iter->packed_refs);
+	strbuf_release(&iter->refname_buf);
+	base_ref_iterator_free(ref_iterator);
+	return ITER_DONE;
+}
+
+static struct ref_iterator_vtable mmapped_ref_iterator_vtable = {
+	mmapped_ref_iterator_advance,
+	mmapped_ref_iterator_peel,
+	mmapped_ref_iterator_abort
+};
+
+struct ref_iterator *mmapped_ref_iterator_begin(
+		const char *packed_refs_file,
+		struct packed_ref_cache *packed_refs,
+		const char *pos, const char *eof)
+{
+	struct mmapped_ref_iterator *iter = xcalloc(1, sizeof(*iter));
+	struct ref_iterator *ref_iterator = &iter->base;
+
+	base_ref_iterator_init(ref_iterator, &mmapped_ref_iterator_vtable, 0);
+
+	iter->packed_refs = packed_refs;
+	acquire_packed_ref_cache(iter->packed_refs);
+	iter->pos = pos;
+	iter->eof = eof;
+	strbuf_init(&iter->refname_buf, 0);
+
+	iter->base.oid = &iter->oid;
+
+	return ref_iterator;
+}
+
 /*
  * Read from the `packed-refs` file into a newly-allocated
  * `packed_ref_cache` and return it. The return value will already
@@ -199,9 +334,10 @@ static struct packed_ref_cache *read_packed_refs(struct packed_ref_store *refs)
 	struct stat st;
 	size_t size;
 	char *buf;
-	const char *pos, *eol, *eof;
-	struct strbuf tmp = STRBUF_INIT;
+	const char *pos, *eof;
 	struct ref_dir *dir;
+	struct ref_iterator *iter;
+	int ok;
 
 	packed_refs->refs = refs;
 	acquire_packed_ref_cache(packed_refs);
@@ -235,7 +371,9 @@ static struct packed_ref_cache *read_packed_refs(struct packed_ref_store *refs)
 
 	/* If the file has a header line, process it: */
 	if (pos < eof && *pos == '#') {
+		struct strbuf tmp = STRBUF_INIT;
 		char *p;
+		const char *eol;
 		struct string_list traits = STRING_LIST_INIT_NODUP;
 
 		eol = memchr(pos, '\n', eof - pos);
@@ -259,69 +397,28 @@ static struct packed_ref_cache *read_packed_refs(struct packed_ref_store *refs)
 		pos = eol + 1;
 
 		string_list_clear(&traits, 0);
-		strbuf_reset(&tmp);
+		strbuf_release(&tmp);
 	}
 
 	dir = get_ref_dir(packed_refs->cache->root);
-	while (pos < eof) {
-		const char *p = pos;
-		struct object_id oid;
-		const char *refname;
-		int flag = REF_ISPACKED;
-		struct ref_entry *entry = NULL;
-
-		if (eof - pos < GIT_SHA1_HEXSZ + 2 ||
-		    parse_oid_hex(p, &oid, &p) ||
-		    !isspace(*p++))
-			die_invalid_line(refs->path, pos, eof - pos);
+	iter = mmapped_ref_iterator_begin(refs->path, packed_refs, pos, eof);
+	while ((ok = ref_iterator_advance(iter)) == ITER_OK) {
+		struct ref_entry *entry =
+			create_ref_entry(iter->refname, iter->oid, iter->flags);
 
-		eol = memchr(p, '\n', eof - p);
-		if (!eol)
-			die_unterminated_line(refs->path, pos, eof - pos);
-
-		strbuf_add(&tmp, p, eol - p);
-		refname = tmp.buf;
-
-		if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
-			if (!refname_is_safe(refname))
-				die("packed refname is dangerous: %s", refname);
-			oidclr(&oid);
-			flag |= REF_BAD_NAME | REF_ISBROKEN;
-		}
-		if (packed_refs->peeled == PEELED_FULLY ||
-		    (packed_refs->peeled == PEELED_TAGS &&
-		     starts_with(refname, "refs/tags/")))
-			flag |= REF_KNOWS_PEELED;
-		entry = create_ref_entry(refname, &oid, flag);
+		if ((iter->flags & REF_KNOWS_PEELED))
+			ref_iterator_peel(iter, &entry->u.value.peeled);
 		add_ref_entry(dir, entry);
-
-		pos = eol + 1;
-
-		if (pos < eof && *pos == '^') {
-			p = pos + 1;
-			if (eof - p < GIT_SHA1_HEXSZ + 1 ||
-			    parse_oid_hex(p, &entry->u.value.peeled, &p) ||
-			    *p++ != '\n')
-				die_invalid_line(refs->path, pos, eof - pos);
-
-			/*
-			 * Regardless of what the file header said,
-			 * we definitely know the value of *this*
-			 * reference:
-			 */
-			entry->flag |= REF_KNOWS_PEELED;
-
-			pos = p;
-		}
-
-		strbuf_reset(&tmp);
 	}
 
+	if (ok != ITER_DONE)
+		die("error reading packed-refs file %s", refs->path);
+
 	if (munmap(buf, size))
-		die_errno("error ummapping packed-refs file");
+		die_errno("error ummapping packed-refs file %s", refs->path);
+
 	close(fd);
 
-	strbuf_release(&tmp);
 	return packed_refs;
 }