lib: create mu-xapian-db, mu-config

XapianDb is a fairly thing wrapper around Xapian, which handles locking,
exception handling and some tracking.

On top of that, Config add a configuration database for type /
introspectable configuration info.

lib/mu-xapian-db.cc

@@ -0,0 +1,136 @@
+/*
+** Copyright (C) 2023 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.
+**
+*/
+
+
+#include "mu-xapian-db.hh"
+#include "utils/mu-utils.hh"
+#include <inttypes.h>
+
+#include <mutex>
+
+using namespace Mu;
+
+struct XapianDb::Private {
+	using DbType = std::variant<Xapian::Database, Xapian::WritableDatabase>;
+
+	Private(Xapian::Database&& db, const std::string& path):
+		db_{std::move(db)}, path_{path} {}
+	Private(Xapian::WritableDatabase&& wdb, const std::string& path):
+		db_{std::move(wdb)}, path_{path} {}
+
+	DbType			db_;
+	const std::string	path_;
+	mutable std::mutex	lock_;
+};
+
+
+XapianDb::XapianDb(Xapian::Database&& db, const std::string& path):
+	priv_{std::make_unique<Private>(std::move(db), path)}
+{}
+XapianDb::XapianDb(Xapian::WritableDatabase&& wdb, const std::string& path):
+	priv_{std::make_unique<Private>(std::move(wdb), path)}
+{}
+
+XapianDb::XapianDb(XapianDb&& rhs) = default;
+XapianDb::~XapianDb() = default;
+
+
+const Xapian::Database&
+XapianDb::db() const
+{
+	if (std::holds_alternative<Xapian::WritableDatabase>(priv_->db_))
+		return std::get<Xapian::WritableDatabase>(priv_->db_);
+	else
+		return std::get<Xapian::Database>(priv_->db_);
+}
+
+Xapian::WritableDatabase&
+XapianDb::wdb()
+{
+	if (read_only())
+		throw std::runtime_error("database is read-only");
+	return std::get<Xapian::WritableDatabase>(priv_->db_);
+}
+
+bool
+XapianDb::read_only() const
+{
+	return !std::holds_alternative<Xapian::WritableDatabase>(priv_->db_);
+}
+
+const std::string&
+XapianDb::path() const
+{
+	return priv_->path_;
+}
+
+std::mutex&
+XapianDb::lock() const
+{
+	return priv_->lock_;
+}
+
+void
+XapianDb::set_timestamp(const std::string_view key)
+{
+	wdb().set_metadata(std::string{key},
+			   format("%" PRIi64, static_cast<int64_t>(::time({}))));
+}
+
+Result<XapianDb>
+XapianDb::make(const std::string& db_path, Flavor flavor) noexcept try {
+
+	if (flavor != Flavor::ReadOnly) {
+		/* we do our own flushing, set Xapian's internal one as
+		 * the backstop*/
+		g_setenv("XAPIAN_FLUSH_THRESHOLD", "500000", 1);
+		/* create path if needed */
+		if (g_mkdir_with_parents(db_path.c_str(), 0700) != 0)
+			return Err(Error::Code::File, "failed to create database dir %s: %s",
+				   db_path.c_str(), ::strerror(errno));
+	}
+
+	switch (flavor) {
+
+	case Flavor::ReadOnly:
+		return Ok(XapianDb(Xapian::Database(db_path), db_path));
+	case Flavor::Open:
+		return Ok(XapianDb(Xapian::WritableDatabase(
+					   db_path, Xapian::DB_OPEN), db_path));
+	case Flavor::CreateOverwrite: {
+		auto&& xdb{XapianDb(Xapian::WritableDatabase(
+					    db_path,
+					    Xapian::DB_CREATE_OR_OVERWRITE), db_path)};
+		xdb.set_timestamp(MetadataIface::created_key);
+		return Ok(std::move(xdb));
+	}
+	default:
+		return Err(Error::Code::Internal, "invalid xapian options");
+	}
+
+} catch (const Xapian::DatabaseLockError& xde) {
+	return Err(Error::Code::StoreLock, "%s", xde.get_msg().c_str());
+} catch (const Xapian::DatabaseError& xde) {
+	return Err(Error::Code::Store, "%s", xde.get_msg().c_str());
+} catch (const Mu::Error& me) {
+	return Err(me);
+} catch (...) {
+	return Err(Error::Code::Internal,
+		   "something went wrong when opening store @ %s", db_path.c_str());
+}