Add topic modification example plugin.

plugins/README.md

@@ -59,3 +59,12 @@ contents.
 ## Examples / Print IP on publish
 This is an **example** plugin that prints out client ID and IP address of any
 client that publishes on a particular topic.
+
+## Examples / Topic modification
+This is an **example** plugin to demonstrate how it is possible to modify the
+topic of messages after they have been received, and before they are sent on
+to subscribers.
+
+This plugin removes the `/uplink` end part of topics that match the pattern
+`device/+/data/uplink`, so devices publishing to `device/0001/data/uplink` will
+effectively be publishing to `device/0001/data`.

plugins/examples/CMakeLists.txt

@@ -9,3 +9,4 @@ add_subdirectory(delayed-auth)
 add_subdirectory(force-retain)
 add_subdirectory(payload-modification)
 add_subdirectory(print-ip-on-publish)
+add_subdirectory(topic-modification)

plugins/examples/Makefile

@@ -7,7 +7,8 @@ DIRS= \
 		force-retain \
 		message-timestamp \
 		payload-modification \
-		print-ip-on-publish
+		print-ip-on-publish \
+		topic-modification
 
 .PHONY : all binary check clean reallyclean test install uninstall
 

plugins/examples/topic-modification/CMakeLists.txt

@@ -0,0 +1,24 @@
+add_library(mosquitto_topic_modification MODULE
+	mosquitto_topic_modification.c
+)
+
+target_include_directories(mosquitto_topic_modification 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_topic_modification PROPERTIES
+	PREFIX ""
+	POSITION_INDEPENDENT_CODE 1
+)
+if(WIN32)
+	target_link_libraries(mosquitto_topic_modification mosquitto)
+endif()
+
+# Don't install, these are example plugins only.
+#install(TARGETS mosquitto_topic_modification RUNTIME DESTINATION "${CMAKE_INSTALL_BINDIR}" LIBRARY DESTINATION "${CMAKE_INSTALL_LIBDIR}")

plugins/examples/topic-modification/Makefile

@@ -0,0 +1,28 @@
+include ../../../config.mk
+
+.PHONY : all binary check clean reallyclean test install uninstall
+
+PLUGIN_NAME=mosquitto_topic_modification
+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/topic-modification/mosquitto_topic_modification.c

@@ -0,0 +1,99 @@
+/*
+Copyright (c) 2020 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 demonstrates how to modify the topic of
+ * a message after it is received by the broker and before it is sent on to
+ * other clients. 
+ *
+ * You should be very sure of what you are doing before making use of this feature.
+ *
+ * Compile with:
+ *   gcc -I<path to mosquitto-repo/include> -fPIC -shared mosquitto_topic_modification.c -o mosquitto_topic_modification.so
+ *
+ * Use in config with:
+ *
+ *   plugin /path/to/mosquitto_topic_modification.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 PLUGIN_NAME "topic-modification"
+#define PLUGIN_VERSION "1.0"
+
+#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;
+	bool result;
+
+	UNUSED(event);
+	UNUSED(userdata);
+
+	/* This simply removes "/uplink" from the end of every matching topic. You
+	 * can of course do much more complicated message processing if needed. */
+
+	mosquitto_topic_matches_sub("device/+/data/uplink", ed->topic, &result);
+
+	if(result){
+		ed->topic[strlen(ed->topic) - strlen("/uplink")] = '\0';
+	}
+	
+	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/topic-modification/test.conf

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

plugins/examples/topic-modification/test.sh

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+../../../src/mosquitto -c test.conf -v