Add a backend interface version number for eventual future changes.

Note that the interface is still a work in progress and might change
while still keeping the same interface version. This is going to be true
until the first stable release of this project (non beta).

include/backends/backend_loader.hpp

@@ -42,6 +42,8 @@ namespace dindb {
 		Backend& backend ( void );
 		const Backend& backend ( void ) const;
 		bool is_loaded ( void ) const;
+		int backend_interface_version ( void ) const;
+		int max_supported_interface_version ( void ) const;
 
 		BackendPlugin& operator= ( BackendPlugin&& ) = default;
 
@@ -51,6 +53,7 @@ namespace dindb {
 		SoHandle m_lib;
 		BackendPtr m_backend;
 		boost::string_ref m_name;
+		int m_iface_ver;
 	};
 
 	std::string backend_name ( const std::string& parSOPath );

include/backends/backend_version.hpp

@@ -0,0 +1,25 @@
+/* Copyright 2015, 2016, Michele Santullo
+ * This file is part of "dindexer".
+ *
+ * "dindexer" 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 of the License, or
+ * (at your option) any later version.
+ *
+ * "dindexer" 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 "dindexer".  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef idAA27B58429DB41C2AF53204CC5010E94
+#define idAA27B58429DB41C2AF53204CC5010E94
+
+namespace dindb {
+	constexpr const int g_current_iface_version = 1;
+} //namespace dindb
+
+#endif

include/backends/exposed_functions.hpp

@@ -29,5 +29,6 @@ namespace dindb {
 extern "C" dindb::Backend* dindexer_create_backend ( const YAML::Node* parConfig );
 extern "C" void dindexer_destroy_backend ( dindb::Backend* parDele );
 extern "C" const char* dindexer_backend_name ( void );
+extern "C" int dindexer_backend_iface_version ( void );
 
 #endif

src/backends/backend_loader.cpp

@@ -20,6 +20,7 @@
 
 #include "backends/backend_loader.hpp"
 #include "backends/exposed_functions.hpp"
+#include "backends/backend_version.hpp"
 #include <dlfcn.h>
 #include <cassert>
 #include <functional>
@@ -50,6 +51,13 @@ namespace dindb {
 
 		void nop_destroy (Backend*) {
 		}
+
+		int backend_iface_version (void* parSOHandle) {
+			typedef decltype(&dindexer_backend_iface_version) GetVersionFun;
+
+			auto get_version = reinterpret_cast<GetVersionFun>(dlsym(parSOHandle, "dindexer_backend_iface_version"));
+			return get_version();
+		}
 	} //unnamed namespace
 
 	std::string backend_name (const std::string& parSOPath) {
@@ -70,8 +78,13 @@ namespace dindb {
 	BackendPlugin::BackendPlugin (const std::string& parSOPath, const YAML::Node* parConfig) :
 		m_lib(dlopen(parSOPath.c_str(), RTLD_LAZY), &dlclose),
 		m_backend(load_backend(m_lib.get(), parConfig)),
-		m_name(backend_name(m_lib.get()))
+		m_name(backend_name(m_lib.get())),
+		m_iface_ver(backend_iface_version(m_lib.get()))
 	{
+		if (g_current_iface_version != m_iface_ver) {
+			m_backend.reset();
+			m_lib.reset();
+		}
 	}
 
 	BackendPlugin::~BackendPlugin() noexcept {
@@ -104,4 +117,12 @@ namespace dindb {
 	bool BackendPlugin::is_loaded() const {
 		return static_cast<bool>(m_backend);
 	}
+
+	int BackendPlugin::backend_interface_version() const {
+		return m_iface_ver;
+	}
+
+	int BackendPlugin::max_supported_interface_version() const {
+		return g_current_iface_version;
+	}
 } //namespace dindb

src/backends/postgresql/backend_postgresql.cpp

@@ -17,6 +17,7 @@
 
 #include "backend_postgresql.hpp"
 #include "backends/exposed_functions.hpp"
+#include "backends/backend_version.hpp"
 #include "tag.hpp"
 #include "delete.hpp"
 #include "scan.hpp"
@@ -179,3 +180,7 @@ extern "C" void dindexer_destroy_backend (dindb::Backend* parDele) {
 extern "C" const char* dindexer_backend_name() {
 	return "postgresql";
 }
+
+extern "C" int dindexer_backend_iface_version() {
+	return dindb::g_current_iface_version;
+}