sysusers: emit a bit more info at debug level when locking fails

This is the first error message when running unprivileged, and the message is
unspecific, so let's at least add some logging at debug level to make this less
confusing.

src/basic/user-util.c

@@ -553,18 +553,18 @@ int take_etc_passwd_lock(const char *root) {
          * awfully racy, and thus we just won't do them. */
 
         if (root)
-                path = prefix_roota(root, "/etc/.pwd.lock");
+                path = prefix_roota(root, ETC_PASSWD_LOCK_PATH);
         else
-                path = "/etc/.pwd.lock";
+                path = ETC_PASSWD_LOCK_PATH;
 
         fd = open(path, O_WRONLY|O_CREAT|O_CLOEXEC|O_NOCTTY|O_NOFOLLOW, 0600);
         if (fd < 0)
-                return -errno;
+                return log_debug_errno(errno, "Cannot open %s: %m", path);
 
         r = fcntl(fd, F_SETLKW, &flock);
         if (r < 0) {
                 safe_close(fd);
-                return -errno;
+                return log_debug_errno(errno, "Locking %s failed: %m", path);
         }
 
         return fd;

src/basic/user-util.h

@@ -63,6 +63,8 @@ int take_etc_passwd_lock(const char *root);
 #define UID_NOBODY ((uid_t) 65534U)
 #define GID_NOBODY ((gid_t) 65534U)
 
+#define ETC_PASSWD_LOCK_PATH "/etc/.pwd.lock"
+
 static inline bool uid_is_dynamic(uid_t uid) {
         return DYNAMIC_UID_MIN <= uid && uid <= DYNAMIC_UID_MAX;
 }

src/sysusers/sysusers.c

@@ -1841,7 +1841,7 @@ int main(int argc, char *argv[]) {
 
         lock = take_etc_passwd_lock(arg_root);
         if (lock < 0) {
-                log_error_errno(lock, "Failed to take lock: %m");
+                log_error_errno(lock, "Failed to take /etc/passwd lock: %m");
                 goto finish;
         }