store: expose maildirs() method

This gets the current list of maildirs by asking the scanner to do a
file-system search.

lib/mu-store.cc

@@ -39,6 +39,7 @@
 #include "mu-store.hh"
 #include "mu-query.hh"
 #include "mu-xapian-db.hh"
+#include "index/mu-scanner.hh"
 
 #include "utils/mu-error.hh"
 
@@ -630,3 +631,21 @@ Store::parse_query(const std::string& expr, bool xapian) const
 		return q.parse(expr, xapian);
 	}, std::string{});
 }
+
+
+std::vector<std::string>
+Store::maildirs() const
+{
+	std::vector<std::string> mdirs;
+	const auto prefix_size = root_maildir().size();
+	Scanner::Handler handler = [&](const std::string& path, auto&& _1, auto&& _2) {
+		mdirs.emplace_back(path.substr(prefix_size));
+		return true;
+	};
+
+	Scanner scanner{root_maildir(), handler, Scanner::Mode::MaildirsOnly};
+	scanner.start();
+	std::sort(mdirs.begin(), mdirs.end());
+
+	return mdirs;
+}

lib/mu-store.hh

@@ -426,6 +426,17 @@ public:
 	 */
 	bool empty() const { return xapian_db().empty(); }
 
+
+	/**
+	 * Get the list of maildirs, that is, the list of maildirs
+	 * under root_maildir, without file-system prefix.
+	 *
+	 * This does a file-system scan.
+	 *
+	 * @return list of maildirs
+	 */
+	std::vector<std::string> maildirs() const;
+
 	/*
 	 * _almost_ private
 	 */