scanner: don't skip directory entries with type DT_UNKNOWN

According to the readdir(2) man-page, not all file-systems support returning the
entry's file-type in `d_type`.  For example, the reprotest reproducibility tool,
uses the disorderfs FUSE file-system to shuffle the order in which directory
entries are returned, and this does not set `d_type`.  Therefore, in addition to
entries with type `DT_DIR` and `DT_LNK`, also process entries with type
`DT_UNKNOWN`.

Signed-off-by: Jeremy Sowden <azazel@debian.org>

lib/mu-scanner.cc

@@ -215,9 +215,13 @@ Scanner::Private::process_dir(const std::string& path, bool is_maildir)
 	while (running_) {
 		errno = 0;
 		if (const auto& dentry{::readdir(dir)}; dentry) {
-#if HAVE_DIRENT_D_TYPE /* opttimization: filter out non-dirs early */
+#if HAVE_DIRENT_D_TYPE /* optimization: filter out non-dirs early.  NB not all file-systems support
+			* returning the file-type in `d_type`, so don't skip `DT_UNKNOWN`.
+			*/
 			if (maildirs_only_mode() &&
-			    dentry->d_type != DT_DIR && dentry->d_type != DT_LNK)
+			    dentry->d_type != DT_DIR &&
+			    dentry->d_type != DT_LNK &&
+			    dentry->d_type != DT_UNKNOWN)
 				continue;
 #endif /*HAVE_DIRENT_D_TYPE*/
 			dir_entries.emplace_back(dentry);