read_packed_refs(): make parsing of the header line more robust

The old code parsed the traits in the `packed-refs` header by looking
for the string " trait " (i.e., the name of the trait with a space on
either side) in the header line. This is fragile, because if any other
implementation of Git forgets to write the trailing space, the last
trait would silently be ignored (and the error might never be
noticed).

So instead, use `string_list_split_in_place()` to split the traits
into tokens then use `unsorted_string_list_has_string()` to look for
the tokens we are interested in. This means that we can read the
traits correctly even if the header line is missing a trailing
space (or indeed, if it is missing the space after the colon, or if it
has multiple spaces somewhere).

However, older Git clients (and perhaps other Git implementations)
still require the surrounding spaces, so we still have to output the
header with a trailing space.

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

refs/packed-backend.c

@@ -257,25 +257,30 @@ 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 == '#') {
-		const char *traits;
+		char *p;
+		struct string_list traits = STRING_LIST_INIT_NODUP;
 
 		eol = memchr(pos, '\n', eof - pos);
 		if (!eol)
 			die_unterminated_line(refs->path, pos, eof - pos);
 
-		strbuf_add(&line, pos, eol + 1 - pos);
+		strbuf_add(&line, pos, eol - pos);
 
-		if (!skip_prefix(line.buf, "# pack-refs with:", &traits))
+		if (!skip_prefix(line.buf, "# pack-refs with:", (const char **)&p))
 			die_invalid_line(refs->path, pos, eof - pos);
 
-		if (strstr(traits, " fully-peeled "))
+		string_list_split_in_place(&traits, p, ' ', -1);
+
+		if (unsorted_string_list_has_string(&traits, "fully-peeled"))
 			peeled = PEELED_FULLY;
-		else if (strstr(traits, " peeled "))
+		else if (unsorted_string_list_has_string(&traits, "peeled"))
 			peeled = PEELED_TAGS;
 		/* perhaps other traits later as well */
 
 		/* The "+ 1" is for the LF character. */
 		pos = eol + 1;
+
+		string_list_clear(&traits, 0);
 		strbuf_reset(&line);
 	}
 
@@ -610,7 +615,11 @@ int packed_refs_is_locked(struct ref_store *ref_store)
 
 /*
  * The packed-refs header line that we write out.  Perhaps other
- * traits will be added later.  The trailing space is required.
+ * traits will be added later.
+ *
+ * Note that earlier versions of Git used to parse these traits by
+ * looking for " trait " in the line. For this reason, the space after
+ * the colon and the trailing space are required.
  */
 static const char PACKED_REFS_HEADER[] =
 	"# pack-refs with: peeled fully-peeled \n";