lib/index: Implement new indexer

Implement a new message indexer consisting of a single-threaded scanner
and a multi-threaded indexer.

This allows for a number of optimizations as well as background
indexing, though this initial version should be behave similar to the
old indexer.

lib/index/mu-scanner.hh

@@ -0,0 +1,96 @@
+/*
+** Copyright (C) 2020 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+**
+** This program is free software; you can redistribute it and/or modify it
+** under the terms of the GNU General Public License as published by the
+** Free Software Foundation; either version 3, or (at your option) any
+** later version.
+**
+** This program is distributed in the hope that it will be useful,
+** but WITHOUT ANY WARRANTY; without even the implied warranty of
+** MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+** GNU General Public License for more details.
+**
+** You should have received a copy of the GNU General Public License
+** along with this program; if not, write to the Free Software Foundation,
+** Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+**
+*/
+
+#ifndef MU_SCANNER_HH__
+#define MU_SCANNER_HH__
+
+#include <functional>
+#include <memory>
+
+#include <dirent.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+namespace Mu {
+
+/// @brief Maildir scanner
+///
+/// Scans maildir (trees) recursively, and calls the Handler callback for
+/// directories & files.
+///
+/// It filters out (i.e., does call the handler for):
+///  - files starting with '.'
+///  - files that do not live in a cur / new leaf maildir
+///  - directories '.' and '..'
+///
+class Scanner {
+public:
+        enum struct HandleType { File, EnterDir, LeaveDir };
+
+        /// Prototype for a handler function
+        using Handler     = std::function<bool(const std::string& fullpath,
+                                               struct stat* statbuf,
+                                               HandleType htype)>;
+        /**
+         * Construct a scanner object for scanning a directory, recursively.
+         *
+         * If handler is a directroy
+         *
+         *
+         * @param root_dir root dir to start scanning
+         * @param handler handler function for some direntry
+         */
+        Scanner (const std::string& root_dir, Handler handler);
+
+        /**
+         * DTOR
+         */
+        ~Scanner();
+
+        /**
+         * Start the scan; this is a blocking call than run until
+         * finished or (from another thread) stop() is called.
+         *
+         * @return true if starting worked; false otherwise
+         */
+        bool start();
+
+        /**
+         * Stop the scan
+         *
+         * @return true if stopping worked; false otherwi%sse
+         */
+        bool stop();
+
+        /**
+         * Is a scan currently running?
+         *
+         * @return true or false
+         */
+        bool is_running() const;
+
+private:
+        struct				Private;
+        std::unique_ptr<Private>	priv_;
+};
+
+} // namepace Mu
+
+#endif /* MU_SCANNER_HH__ */