Forbid running with persistence true and a plugin at the same time.

Author: ralight

ChangeLog.txt

@@ -1,6 +1,10 @@
 2.1.2 - 2026-02-xx
 ==================
 
+Broker:
+- Forbid running with `persistence true` and with a persistence plugin at the
+  same time.
+
 Build:
 - Build fixes for OpenBSD. Closes #3474.
 - Add missing libedit to docker builds. Closes #3476.

man/mosquitto.conf.5.xml

@@ -419,13 +419,15 @@
 			<varlistentry>
 				<term><option>autosave_interval</option> <replaceable>seconds</replaceable></term>
 				<listitem>
-					<para>The number of seconds that mosquitto will wait
+					<para>
+						The number of seconds that mosquitto will wait
 						between each time it saves the in-memory database to
 						disk. If set to 0, the in-memory database will only be
 						saved when mosquitto exits or when receiving the
 						SIGUSR1 signal. Note that this setting only has an
-						effect if persistence is enabled. Defaults to 1800
-						seconds (30 minutes).</para>
+						effect if the built-in persistence is enabled. Defaults
+						to 1800 seconds (30 minutes).
+					</para>
 
 					<para>This option applies globally.</para>
 
@@ -445,6 +447,7 @@
 						<option>autosave_interval</option> as a time in
 						seconds.</para>
 
+					<para>Applies to built-in persistence only.</para>
 					<para>This option applies globally.</para>
 
 					<para>Reloaded on reload signal.</para>
@@ -1003,18 +1006,25 @@ log_timestamp_format %Y-%m-%dT%H:%M:%S
 			<varlistentry>
 				<term><option>persistence</option> [ true | false ]</term>
 				<listitem>
-					<para>If <replaceable>true</replaceable>, connection,
-						subscription and message data will be written to the
-						disk in mosquitto.db at the location dictated by
-						persistence_location. When mosquitto is restarted, it
-						will reload the information stored in mosquitto.db. The
-						data will be written to disk when mosquitto closes and
-						also at periodic intervals as defined by
-						autosave_interval. Writing of the persistence database
-						may also be forced by sending mosquitto the SIGUSR1
-						signal. If <replaceable>false</replaceable>, the data
-						will be stored in memory only. Defaults to
-						<replaceable>false</replaceable>.</para>
+					<para>
+						If <replaceable>true</replaceable>, then built-in persistence
+						is enabled. It is recommended that a plugin based persistence
+						is used instead.
+					</para>
+
+					<para>
+						If enabled, connection, subscription and message data
+						will be written to disk in mosquitto.db at the location
+						dictated by persistence_location. When mosquitto is
+						restarted, it will reload the information stored in
+						mosquitto.db. The data will be written to disk when
+						mosquitto closes and also at periodic intervals as
+						defined by autosave_interval. Writing of the persistence
+						database may also be forced by sending mosquitto the
+						SIGUSR1 signal. If <replaceable>false</replaceable>,
+						the data will be stored in memory only. Defaults to
+						<replaceable>false</replaceable>.
+					</para>
 					<para>The persistence file may change its format in a new
 						version. The broker can currently read all old formats,
 						but will only save in the latest format. It should always
@@ -1031,8 +1041,10 @@ log_timestamp_format %Y-%m-%dT%H:%M:%S
 			<varlistentry>
 				<term><option>persistence_file</option> <replaceable>file name</replaceable></term>
 				<listitem>
-					<para>The filename to use for the persistent database.
-						Defaults to mosquitto.db.</para>
+					<para>
+						The filename to use for the built-in persistent database.
+						Defaults to mosquitto.db.
+					</para>
 
 					<para>This option applies globally.</para>
 
@@ -1042,8 +1054,12 @@ log_timestamp_format %Y-%m-%dT%H:%M:%S
 			<varlistentry>
 				<term><option>persistence_location</option> <replaceable>path</replaceable></term>
 				<listitem>
-					<para>The path where the persistence database should be
-						stored. If not given, then the current directory is used.</para>
+					<para>
+						The path where plugins should store any persistence
+						data, and the path where the built-in persistence will
+						store its data. If not given, then the current directory
+						is used.
+					</para>
 
 					<para>This option applies globally.</para>
 

src/plugin_callbacks.c

@@ -216,6 +216,26 @@ BROKER_EXPORT int mosquitto_callback_register(
 		return MOSQ_ERR_INVAL;
 	}
 
+	if(db.config->persistence && (event == MOSQ_EVT_PERSIST_RESTORE
+			|| event == MOSQ_EVT_PERSIST_BASE_MSG_ADD
+			|| event == MOSQ_EVT_PERSIST_BASE_MSG_DELETE
+			|| event == MOSQ_EVT_PERSIST_RETAIN_MSG_SET
+			|| event == MOSQ_EVT_PERSIST_RETAIN_MSG_DELETE
+			|| event == MOSQ_EVT_PERSIST_CLIENT_ADD
+			|| event == MOSQ_EVT_PERSIST_CLIENT_DELETE
+			|| event == MOSQ_EVT_PERSIST_CLIENT_UPDATE
+			|| event == MOSQ_EVT_PERSIST_SUBSCRIPTION_ADD
+			|| event == MOSQ_EVT_PERSIST_SUBSCRIPTION_DELETE
+			|| event == MOSQ_EVT_PERSIST_CLIENT_MSG_ADD
+			|| event == MOSQ_EVT_PERSIST_CLIENT_MSG_DELETE
+			|| event == MOSQ_EVT_PERSIST_CLIENT_MSG_UPDATE
+			|| event == MOSQ_EVT_PERSIST_WILL_ADD
+			|| event == MOSQ_EVT_PERSIST_WILL_DELETE
+			)){
+		log__printf(NULL, MOSQ_LOG_ERR, "Error: `persistence true` cannot be used with a persistence plugin.");
+		return MOSQ_ERR_INVAL;
+	}
+
 	if(event == MOSQ_EVT_CONTROL){
 		return control__register_callback(identifier, cb_func, event_data, userdata);
 	}

src/retain.c

@@ -181,6 +181,11 @@ int retain__store(const char *topic, struct mosquitto__base_msg *base_msg, char
 #endif
 
 	if(retainhier->retained){
+		if(retainhier->retained == base_msg){
+			/* This may occur if multiple persistence providers are used */
+			return MOSQ_ERR_SUCCESS;
+		}
+
 		if(persist && retainhier->retained->data.topic[0] != '$' && base_msg->data.payloadlen == 0){
 			/* Only delete if another retained message isn't replacing this one */
 			plugin_persist__handle_retain_msg_delete(retainhier->retained);