Add example payload modification plugin.

plugins/CMakeLists.txt

@@ -1 +1,2 @@
 add_subdirectory(message-timestamp)
+add_subdirectory(payload-modification)

plugins/Makefile

@@ -1,5 +1,6 @@
 DIRS= \
-		message-timestamp
+		message-timestamp \
+		payload-modification
 
 .PHONY : all binary check clean reallyclean test install uninstall
 

plugins/README.md

@@ -0,0 +1,15 @@
+# Plugins
+
+This directory contains plugins for use with Mosquitto.
+
+## Message timestamp
+This is an **example** plugin to demonstrate how it is possible to attach MQTT v5 properties to messages after they have been received, and before they are sent on to subscribers.
+
+This plugin attaches a user-property property to each message which contains the ISO-8601 timestamp of the time the message was received by the broker. This means it is possible for MQTT v5 clients to see how old a retained message is, for example.
+
+## Payload modification
+This is an **example** plugin to demonstrate how it is possible to modify the payload of messages after they have been received, and before they are sent on to subscribers.
+
+If you are considering using this feature, you should be very certain you have verified the payload is the correct format before modifying it.
+
+This plugin adds the text string "hello " to the beginning of each payload, so with anything other than simple plain text messages it will corrupt the payload contents.

plugins/payload-modification/CMakeLists.txt

@@ -0,0 +1,10 @@
+include_directories(${mosquitto_SOURCE_DIR} ${mosquitto_SOURCE_DIR}/include
+			${STDBOOL_H_PATH} ${STDINT_H_PATH})
+
+add_library(mosquitto_payload_modification SHARED mosquitto_payload_modification.c)
+set_target_properties(mosquitto_payload_modification PROPERTIES
+	POSITION_INDEPENDENT_CODE 1
+)
+set_target_properties(mosquitto_payload_modification PROPERTIES PREFIX "")
+
+install(TARGETS mosquitto_payload_modification RUNTIME DESTINATION "${CMAKE_INSTALL_BINDIR}" LIBRARY DESTINATION "${CMAKE_INSTALL_LIBDIR}")

plugins/payload-modification/Makefile

@@ -0,0 +1,26 @@
+include ../../config.mk
+
+.PHONY : all binary check clean reallyclean test install uninstall
+
+PLUGIN_NAME=mosquitto_payload_modification
+
+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
+	$(INSTALL) -d "${DESTDIR}$(prefix)/lib"
+	$(INSTALL) ${STRIP_OPTS} ${PLUGIN_NAME}.so "${DESTDIR}${prefix}/lib/${PLUGIN_NAME}.so"
+
+uninstall :
+	-rm -f "${DESTDIR}${prefix}/lib/${PLUGIN_NAME}.so"

plugins/payload-modification/mosquitto_payload_modification.c

@@ -0,0 +1,92 @@
+/*
+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 v1.0
+and Eclipse Distribution License v1.0 which accompany this distribution.
+ 
+The Eclipse Public License is available at
+   http://www.eclipse.org/legal/epl-v10.html
+and the Eclipse Distribution License is available at
+  http://www.eclipse.org/org/documents/edl-v10.php.
+ 
+Contributors:
+   Roger Light - initial implementation and documentation.
+*/
+
+/*
+ * This is an *example* plugin which demonstrates how to modify the payload 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_payload_modification.c -o mosquitto_payload_modification.so
+ *
+ * Use in config with:
+ *
+ *   plugin /path/to/mosquitto_payload_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"
+
+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;
+	char *new_payload;
+	int new_payloadlen;
+
+	/* This simply adds "hello " to the front of every payload. You can of
+	 * course do much more complicated message processing if needed. */
+
+	/* Calculate the length of our new payload */
+	new_payloadlen = ed->payloadlen + strlen("hello ")+1;
+
+	/* Allocate some memory - use
+	 * mosquitto_calloc/mosquitto_malloc/mosquitto_strdup when allocating, to
+	 * allow the broker to track memory usage */
+	new_payload = mosquitto_calloc(1, new_payloadlen);
+	if(new_payload == NULL){
+		return MOSQ_ERR_NOMEM;
+	}
+
+	/* Print "hello " to the payload */
+	snprintf(new_payload, new_payloadlen, "hello ");
+	memcpy(new_payload+strlen("hello "), ed->payload, ed->payloadlen);
+
+	/* Assign the new payload and payloadlen to the event data structure. You
+	 * must *not* free the original payload, it will be handled by the
+	 * broker. */
+	ed->payload = new_payload;
+	ed->payloadlen = new_payloadlen;
+	
+	return MOSQ_ERR_SUCCESS;
+}
+
+int mosquitto_plugin_version(void)
+{
+	return 5;
+}
+
+int mosquitto_plugin_init(mosquitto_plugin_id_t *identifier, void **user_data, struct mosquitto_opt *opts, int 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)
+{
+	return mosquitto_callback_unregister(mosq_pid, MOSQ_EVT_MESSAGE, callback_message, NULL);
+}