udev: track remaining buffer size across $links devlinks

FORMAT_SUBST_LINKS passes the same 'l' (remaining size) to strpcpy for
each devlink and discards the returned remaining value, so the cursor
advances while the bound stays at the full size, overflowing dest.
Capture the return value.

Follow-up for f6caab8995

src/udev/test-udev-format.c

@@ -1,9 +1,13 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
+#include "sd-device.h"
+
+#include "device-private.h"
 #include "log.h"
 #include "mountpoint-util.h"
 #include "string-util.h"
 #include "tests.h"
+#include "udev-event.h"
 #include "udev-format.h"
 
 static void test_udev_resolve_subsys_kernel_one(const char *str, bool read_value, int retval, const char *expected) {
@@ -36,6 +40,26 @@ TEST(udev_resolve_subsys_kernel) {
         test_udev_resolve_subsys_kernel_one("[net/lo]/address", true, 0, "00:00:00:00:00:00");
 }
 
+TEST(udev_event_apply_format_links) {
+        _cleanup_(sd_device_unrefp) sd_device *dev = NULL;
+        _cleanup_(udev_event_unrefp) UdevEvent *event = NULL;
+        char dest[64];
+        bool truncated = false;
+
+        ASSERT_OK(sd_device_new_from_syspath(&dev, "/sys/class/net/lo"));
+
+        for (unsigned u = 0; u < 32; u++) {
+                _cleanup_free_ char *l = NULL;
+                ASSERT_OK(asprintf(&l, "/dev/link-that-is-long-%u", u));
+                ASSERT_OK(device_add_devlink(dev, l));
+        }
+
+        ASSERT_NOT_NULL((event = udev_event_new(dev, NULL, EVENT_TEST_SPAWN)));
+
+        udev_event_apply_format(event, "$links", dest, sizeof dest, false, &truncated);
+        ASSERT_TRUE(truncated);
+}
+
 static int intro(void) {
         if (path_is_mount_point("/sys") <= 0)
                 return log_tests_skipped("/sys is not mounted");

src/udev/udev-format.c

@@ -334,9 +334,9 @@ static ssize_t udev_event_subst_format(
         case FORMAT_SUBST_LINKS:
                 FOREACH_DEVICE_DEVLINK(dev, link) {
                         if (s == dest)
-                                strpcpy_full(&s, l, link + STRLEN("/dev/"), &truncated);
+                                l = strpcpy_full(&s, l, link + STRLEN("/dev/"), &truncated);
                         else
-                                strpcpyl_full(&s, l, &truncated, " ", link + STRLEN("/dev/"), NULL);
+                                l = strpcpyl_full(&s, l, &truncated, " ", link + STRLEN("/dev/"), NULL);
                         if (truncated)
                                 break;
                 }