Example plugin that forces all messages to be retained.

plugins/examples/CMakeLists.txt

@@ -6,4 +6,5 @@ add_subdirectory(auth-by-ip)
 add_subdirectory(client-properties)
 add_subdirectory(connection-state)
 add_subdirectory(delayed-auth)
+add_subdirectory(force-retain)
 add_subdirectory(payload-modification)

plugins/examples/Makefile

@@ -4,6 +4,7 @@ DIRS= \
 		client-properties \
 		connection-state \
 		delayed-auth \
+		force-retain \
 		message-timestamp \
 		payload-modification
 

plugins/examples/force-retain/CMakeLists.txt

@@ -0,0 +1,24 @@
+add_library(mosquitto_force_retain MODULE
+	mosquitto_force_retain.c
+)
+
+target_include_directories(mosquitto_force_retain PRIVATE
+	"${OPENSSL_INCLUDE_DIR}"
+	"${STDBOOL_H_PATH}"
+	"${STDINT_H_PATH}"
+	"${mosquitto_SOURCE_DIR}"
+	"${mosquitto_SOURCE_DIR}/include"
+)
+
+link_directories(${mosquitto_SOURCE_DIR})
+
+set_target_properties(mosquitto_force_retain PROPERTIES
+	PREFIX ""
+	POSITION_INDEPENDENT_CODE 1
+)
+if(WIN32)
+	target_link_libraries(mosquitto_force_retain mosquitto)
+endif()
+
+# Don't install, these are example plugins only.
+#install(TARGETS mosquitto_force_retain RUNTIME DESTINATION "${CMAKE_INSTALL_BINDIR}" LIBRARY DESTINATION "${CMAKE_INSTALL_LIBDIR}")

plugins/examples/force-retain/Makefile

@@ -0,0 +1,28 @@
+include ../../../config.mk
+
+.PHONY : all binary check clean reallyclean test install uninstall
+
+PLUGIN_NAME=mosquitto_force_retain
+PLUGIN_CFLAGS+=-I../../../include -I../../../
+
+all : binary
+
+binary : ${PLUGIN_NAME}.so
+
+${PLUGIN_NAME}.so : ${PLUGIN_NAME}.c
+	$(CROSS_COMPILE)$(CC) $(PLUGIN_CPPFLAGS) $(PLUGIN_CFLAGS) $(PLUGIN_LDFLAGS) -fPIC -shared $< -o $@
+
+reallyclean : clean
+clean:
+	-rm -f *.o ${PLUGIN_NAME}.so *.gcda *.gcno
+
+check: test
+test:
+
+install: ${PLUGIN_NAME}.so
+	# Don't install, these are examples only.
+	#$(INSTALL) -d "${DESTDIR}$(libdir)"
+	#$(INSTALL) ${STRIP_OPTS} ${PLUGIN_NAME}.so "${DESTDIR}${libdir}/${PLUGIN_NAME}.so"
+
+uninstall :
+	-rm -f "${DESTDIR}${libdir}/${PLUGIN_NAME}.so"

plugins/examples/force-retain/mosquitto_force_retain.c

@@ -0,0 +1,85 @@
+/*
+Copyright (c) 2021 Roger Light <roger@atchoo.org>
+
+All rights reserved. This program and the accompanying materials
+are made available under the terms of the Eclipse Public License 2.0
+and Eclipse Distribution License v1.0 which accompany this distribution.
+ 
+The Eclipse Public License is available at
+   https://www.eclipse.org/legal/epl-2.0/
+and the Eclipse Distribution License is available at
+  http://www.eclipse.org/org/documents/edl-v10.php.
+ 
+SPDX-License-Identifier: EPL-2.0 OR BSD-3-Clause
+
+Contributors:
+   Roger Light - initial implementation and documentation.
+*/
+
+/*
+ * This is an *example* plugin which forces all messages processed by the
+ * broker to be treated as though they had the retained bit set.
+ *
+ * Compile with:
+ *   gcc -I<path to mosquitto-repo/include> -fPIC -shared mosquitto_force_retain.c -o mosquitto_force_retain.so
+ *
+ * Use in config with:
+ *
+ *   plugin /path/to/mosquitto_force_retain.so
+ *
+ * Note that this only works on Mosquitto 2.0 or later.
+ */
+#include <stdio.h>
+#include <string.h>
+
+#include "mosquitto_broker.h"
+#include "mosquitto_plugin.h"
+#include "mosquitto.h"
+#include "mqtt_protocol.h"
+
+#define UNUSED(A) (void)(A)
+
+static mosquitto_plugin_id_t *mosq_pid = NULL;
+
+static int callback_message(int event, void *event_data, void *userdata)
+{
+	struct mosquitto_evt_message *ed = event_data;
+
+	UNUSED(event);
+	UNUSED(userdata);
+
+	ed->retain = true;
+
+	return MOSQ_ERR_SUCCESS;
+}
+
+int mosquitto_plugin_version(int supported_version_count, const int *supported_versions)
+{
+	int i;
+
+	for(i=0; i<supported_version_count; i++){
+		if(supported_versions[i] == 5){
+			return 5;
+		}
+	}
+	return -1;
+}
+
+int mosquitto_plugin_init(mosquitto_plugin_id_t *identifier, void **user_data, struct mosquitto_opt *opts, int opt_count)
+{
+	UNUSED(user_data);
+	UNUSED(opts);
+	UNUSED(opt_count);
+
+	mosq_pid = identifier;
+	return mosquitto_callback_register(mosq_pid, MOSQ_EVT_MESSAGE, callback_message, NULL, NULL);
+}
+
+int mosquitto_plugin_cleanup(void *user_data, struct mosquitto_opt *opts, int opt_count)
+{
+	UNUSED(user_data);
+	UNUSED(opts);
+	UNUSED(opt_count);
+
+	return mosquitto_callback_unregister(mosq_pid, MOSQ_EVT_MESSAGE, callback_message, NULL);
+}

plugins/examples/force-retain/test.conf

@@ -0,0 +1 @@
+plugin ./mosquitto_force_retain.so

plugins/examples/force-retain/test.sh

@@ -0,0 +1 @@
+../../../src/mosquitto -c test.conf -v