attr: fix silently splitting up lines longer than 2048 bytes

When reading attributes from a file we use fgets(3P) with a buffer size
of 2048 bytes. This means that as soon as a line exceeds the buffer size
we split it up into multiple parts and parse each of them as a separate
pattern line. This is of course not what the user intended, and even
worse the behaviour is inconsistent with how we read attributes from the
index.

Fix this bug by converting the code to use `strbuf_getline()` instead.
This will indeed read in the whole line, which may theoretically lead to
an out-of-memory situation when the gitattributes file is huge. We're
about to reject any gitattributes files larger than 100MB in the next
commit though, which makes this less of a concern.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

attr.c

@@ -699,21 +699,22 @@ void git_attr_set_direction(enum git_attr_direction new_direction)
 
 static struct attr_stack *read_attr_from_file(const char *path, int macro_ok)
 {
+	struct strbuf buf = STRBUF_INIT;
 	FILE *fp = fopen_or_warn(path, "r");
 	struct attr_stack *res;
-	char buf[2048];
 	int lineno = 0;
 
 	if (!fp)
 		return NULL;
 	res = xcalloc(1, sizeof(*res));
-	while (fgets(buf, sizeof(buf), fp)) {
-		char *bufp = buf;
-		if (!lineno)
-			skip_utf8_bom(&bufp, strlen(bufp));
-		handle_attr_line(res, bufp, path, ++lineno, macro_ok);
+	while (strbuf_getline(&buf, fp) != EOF) {
+		if (!lineno && starts_with(buf.buf, utf8_bom))
+			strbuf_remove(&buf, 0, strlen(utf8_bom));
+		handle_attr_line(res, buf.buf, path, ++lineno, macro_ok);
 	}
+
 	fclose(fp);
+	strbuf_release(&buf);
 	return res;
 }
 

t/t0003-attributes.sh

@@ -339,4 +339,25 @@ test_expect_success 'query binary macro directly' '
 	test_cmp expect actual
 '
 
+test_expect_success 'large attributes line ignores trailing content in tree' '
+	test_when_finished "rm .gitattributes" &&
+	# older versions of Git broke lines at 2048 bytes; the 2045 bytes
+	# of 0-padding here is accounting for the three bytes of "a 1", which
+	# would knock "trailing" to the "next" line, where it would be
+	# erroneously parsed.
+	printf "a %02045dtrailing attribute\n" 1 >.gitattributes &&
+	git check-attr --all trailing >actual 2>err &&
+	test_must_be_empty err &&
+	test_must_be_empty actual
+'
+
+test_expect_success 'large attributes line ignores trailing content in index' '
+	test_when_finished "git update-index --remove .gitattributes" &&
+	blob=$(printf "a %02045dtrailing attribute\n" 1 | git hash-object -w --stdin) &&
+	git update-index --add --cacheinfo 100644,$blob,.gitattributes &&
+	git check-attr --cached --all trailing >actual 2>err &&
+	test_must_be_empty err &&
+	test_must_be_empty actual
+'
+
 test_done