Fix max_keepalive option not being able to be set to 0.

ChangeLog.txt

@@ -1,3 +1,10 @@
+2.0.13 - 2021-xx-xx
+===================
+
+Broker:
+- Fix `max_keepalive` option not being able to be set to 0.
+
+
 2.0.12 - 2021-08-31
 ===================
 

man/mosquitto.conf.5.xml

@@ -599,7 +599,15 @@ log_timestamp_format %Y-%m-%dT%H:%M:%S
  be sent a server keepalive telling them to use
  max_keepalive. This only applies to MQTT v5 clients.
  The maximum value allowable, and default value, is
- 65535. Do not set below 10 seconds.</para>
+ 65535.</para>
+
+ <para>
+ Set to 0 to allow clients to set keepalive = 0, which
+ means no keepalive checks are made and the client will
+ never be disconnected by the broker if no messages are
+ received. You should be very sure this is the behaviour
+ that you want.
+ </para>
 
  <para>
  For MQTT v3.1.1 and v3.1 clients, there is no mechanism

mosquitto.conf

@@ -79,8 +79,13 @@
 # use the new keepalive value. The max_keepalive option allows you to specify
 # that clients may only connect with keepalive less than or equal to this
 # value, otherwise they will be sent a server keepalive telling them to use
-# max_keepalive. This only applies to MQTT v5 clients. The maximum value
-# allowable is 65535. Do not set below 10.
+# max_keepalive. This only applies to MQTT v5 clients. The default, and maximum
+# value allowable, is 65535.
+#
+# Set to 0 to allow clients to set keepalive = 0, which means no keepalive
+# checks are made and the client will never be disconnected by the broker if no
+# messages are received. You should be very sure this is the behaviour that you
+# want.
 #
 # For MQTT v3.1.1 and v3.1 clients, there is no mechanism to tell the client
 # what keepalive value they should use. If an MQTT v3.1.1 or v3.1 client

src/conf.c

@@ -1671,7 +1671,7 @@ static int config__read_file_core(struct mosquitto__config *config, bool reload,
  config->max_inflight_messages = (uint16_t)tmp_int;
  }else if(!strcmp(token, "max_keepalive")){
  if(conf__parse_int(&token, "max_keepalive", &tmp_int, saveptr)) return MOSQ_ERR_INVAL;
- if(tmp_int < 10 || tmp_int > UINT16_MAX){
+ if(tmp_int < 0 || tmp_int > UINT16_MAX){
  log__printf(NULL, MOSQ_LOG_ERR, "Error: Invalid max_keepalive value (%d).", tmp_int);
  return MOSQ_ERR_INVAL;
  }