[PATCH v3 08/21] read_packed_refs(): read references with minimal copying

[Michael Haggerty <mhagger@alum.mit.edu>]

Instead of copying data from the `packed-refs` file one line at time
and then processing it, process the data in place as much as possible.

Also, instead of processing one line per iteration of the main loop,
process a reference line plus its corresponding peeled line (if
present) together.

Note that this change slightly tightens up the parsing of the
`packed-refs` file. Previously, the parser would have accepted
multiple "peeled" lines for a single reference (ignoring all but the
last one). Now it would reject that.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
---
 refs/packed-backend.c | 101 ++++++++++++++++++++------------------------------
 1 file changed, 40 insertions(+), 61 deletions(-)

diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index a45e3ff92f..2b80f244c8 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -134,33 +134,6 @@ static void clear_packed_ref_cache(struct packed_ref_store *refs)
 	}
 }
 
-/* The length of a peeled reference line in packed-refs, including EOL: */
-#define PEELED_LINE_LENGTH 42
-
-/*
- * Parse one line from a packed-refs file.  Write the SHA1 to sha1.
- * Return a pointer to the refname within the line (null-terminated),
- * or NULL if there was a problem.
- */
-static const char *parse_ref_line(struct strbuf *line, struct object_id *oid)
-{
-	const char *ref;
-
-	if (parse_oid_hex(line->buf, oid, &ref) < 0)
-		return NULL;
-	if (!isspace(*ref++))
-		return NULL;
-
-	if (isspace(*ref))
-		return NULL;
-
-	if (line->buf[line->len - 1] != '\n')
-		return NULL;
-	line->buf[--line->len] = 0;
-
-	return ref;
-}
-
 static NORETURN void die_unterminated_line(const char *path,
 					   const char *p, size_t len)
 {
@@ -221,8 +194,7 @@ static struct packed_ref_cache *read_packed_refs(struct packed_ref_store *refs)
 	size_t size;
 	char *buf;
 	const char *pos, *eol, *eof;
-	struct ref_entry *last = NULL;
-	struct strbuf line = STRBUF_INIT;
+	struct strbuf tmp = STRBUF_INIT;
 	enum { PEELED_NONE, PEELED_TAGS, PEELED_FULLY } peeled = PEELED_NONE;
 	struct ref_dir *dir;
 
@@ -264,9 +236,9 @@ static struct packed_ref_cache *read_packed_refs(struct packed_ref_store *refs)
 		if (!eol)
 			die_unterminated_line(refs->path, pos, eof - pos);
 
-		strbuf_add(&line, pos, eol - pos);
+		strbuf_add(&tmp, pos, eol - pos);
 
-		if (!skip_prefix(line.buf, "# pack-refs with:", (const char **)&p))
+		if (!skip_prefix(tmp.buf, "# pack-refs with:", (const char **)&p))
 			die_invalid_line(refs->path, pos, eof - pos);
 
 		string_list_split_in_place(&traits, p, ' ', -1);
@@ -281,61 +253,68 @@ static struct packed_ref_cache *read_packed_refs(struct packed_ref_store *refs)
 		pos = eol + 1;
 
 		string_list_clear(&traits, 0);
-		strbuf_reset(&line);
+		strbuf_reset(&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;
 
-		eol = memchr(pos, '\n', eof - pos);
+		if (eof - pos < GIT_SHA1_HEXSZ + 2 ||
+		    parse_oid_hex(p, &oid, &p) ||
+		    !isspace(*p++))
+			die_invalid_line(refs->path, pos, eof - pos);
+
+		eol = memchr(p, '\n', eof - p);
 		if (!eol)
 			die_unterminated_line(refs->path, pos, eof - pos);
 
-		strbuf_add(&line, pos, eol + 1 - 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 (peeled == PEELED_FULLY ||
+		    (peeled == PEELED_TAGS && starts_with(refname, "refs/tags/")))
+			flag |= REF_KNOWS_PEELED;
+		entry = create_ref_entry(refname, &oid, flag);
+		add_ref_entry(dir, entry);
 
-		refname = parse_ref_line(&line, &oid);
-		if (refname) {
-			int flag = REF_ISPACKED;
+		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);
 
-			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;
-			}
-			last = create_ref_entry(refname, &oid, flag);
-			if (peeled == PEELED_FULLY ||
-			    (peeled == PEELED_TAGS && starts_with(refname, "refs/tags/")))
-				last->flag |= REF_KNOWS_PEELED;
-			add_ref_entry(dir, last);
-		} else if (last &&
-		    line.buf[0] == '^' &&
-		    line.len == PEELED_LINE_LENGTH &&
-		    line.buf[PEELED_LINE_LENGTH - 1] == '\n' &&
-		    !get_oid_hex(line.buf + 1, &oid)) {
-			oidcpy(&last->u.value.peeled, &oid);
 			/*
 			 * Regardless of what the file header said,
 			 * we definitely know the value of *this*
 			 * reference:
 			 */
-			last->flag |= REF_KNOWS_PEELED;
-		} else {
-			die_invalid_line(refs->path, line.buf, line.len);
+			entry->flag |= REF_KNOWS_PEELED;
+
+			pos = p;
 		}
 
-		/* The "+ 1" is for the LF character. */
-		pos = eol + 1;
-		strbuf_reset(&line);
+		strbuf_reset(&tmp);
 	}
 
 	if (munmap(buf, size))
 		die_errno("error ummapping packed-refs file");
 	close(fd);
 
-	strbuf_release(&line);
+	strbuf_release(&tmp);
 	return packed_refs;
 }
 
-- 
2.14.1