MXS-2167: Add support for extra_port

This allows MaxScale to connect to the server even when max_connections
has been reached.

Documentation/Getting-Started/Configuration-Guide.md

@@ -1287,6 +1287,16 @@ The `monitorpw` parameter may be either a plain text password or it may be an
 encrypted password.  See the section on encrypting passwords for use in the
 maxscale.cnf file.
 
+#### `extra_port`
+
+An alternative port used to monitor the server. This allows MaxScale to connect
+even when `max_connections` has been reached on the backend server. If this
+parameter is defined and a connection to the normal port fails, the alternative
+port is used.
+
+For more information, read the
+[extra_port documentation](https://mariadb.com/kb/en/library/thread-pool-system-and-status-variables/#extra_port).
+
 #### `persistpoolmax`
 
 The `persistpoolmax` parameter defaults to zero but can be set to an integer

include/maxscale/config.h

@@ -155,6 +155,7 @@ extern const char CN_PASSIVE[];
 extern const char CN_PASSWORD[];
 extern const char CN_POLL_SLEEP[];
 extern const char CN_PORT[];
+extern const char CN_EXTRA_PORT[];
 extern const char CN_PROTOCOL[];
 extern const char CN_QUERY_CLASSIFIER[];
 extern const char CN_QUERY_CLASSIFIER_ARGS[];

include/maxscale/server.h

@@ -122,6 +122,7 @@ typedef struct server
     char*          name;                            /**< Server config name */
     char           address[MAX_SERVER_ADDRESS_LEN]; /**< Server hostname/IP-address */
     unsigned short port;                            /**< Server port */
+    unsigned short extra_port;                      /**< Server extra_port */
     char*          protocol;                        /**< Backend protocol module name */
     char*          authenticator;                   /**< Authenticator module name */
     // Other settings
@@ -505,6 +506,7 @@ extern DCB*    server_get_persistent(SERVER* server,
                                      int id);
 extern void     server_update_address(SERVER* server, const char* address);
 extern void     server_update_port(SERVER* server, unsigned short port);
+extern void     server_update_extra_port(SERVER* server, unsigned short port);
 extern uint64_t server_map_status(const char* str);
 extern void     server_set_version_string(SERVER* server, const char* version_string);
 extern void     server_set_version(SERVER* server, const char* version_string, uint64_t version);

server/core/config.cc

@@ -136,6 +136,7 @@ const char CN_PASSIVE[] = "passive";
 const char CN_PASSWORD[] = "password";
 const char CN_POLL_SLEEP[] = "poll_sleep";
 const char CN_PORT[] = "port";
+const char CN_EXTRA_PORT[] = "extra_port";
 const char CN_PROTOCOL[] = "protocol";
 const char CN_QUERY_CLASSIFIER[] = "query_classifier";
 const char CN_QUERY_CLASSIFIER_ARGS[] = "query_classifier_args";
@@ -402,6 +403,7 @@ const MXS_MODULE_PARAM config_server_params[] =
     {CN_PROTOCOL,                    MXS_MODULE_PARAM_STRING, NULL,
      MXS_MODULE_OPT_REQUIRED},
     {CN_PORT,                        MXS_MODULE_PARAM_COUNT,  "3306"},
+    {CN_EXTRA_PORT,                  MXS_MODULE_PARAM_COUNT,  "0"},
     {CN_AUTHENTICATOR,               MXS_MODULE_PARAM_STRING},
     {CN_MONITORUSER,                 MXS_MODULE_PARAM_STRING},
     {CN_MONITORPW,                   MXS_MODULE_PARAM_STRING},

server/core/config_runtime.cc

@@ -488,6 +488,10 @@ bool runtime_alter_server(SERVER* server, const char* key, const char* value)
             server_update_port(server, ival);
         }
     }
+    else if (strcmp(key, CN_EXTRA_PORT) == 0)
+    {
+        server_update_extra_port(server, atoi(value));
+    }
     else if (strcmp(key, CN_MONITORUSER) == 0)
     {
         server_update_credentials(server, value, server->monpw);

server/core/mysql_utils.cc

@@ -30,6 +30,7 @@
 #include <maxscale/alloc.h>
 #include <maxscale/config.h>
 #include <maxscale/log.h>
+#include <maxbase/atomic.hh>
 
 namespace
 {
@@ -191,6 +192,13 @@ MYSQL* mxs_mysql_real_connect(MYSQL* con, SERVER* server, const char* user, cons
     }
 
     MYSQL* mysql = mysql_real_connect(con, server->address, user, passwd, NULL, server->port, NULL, 0);
+    auto extra_port = mxb::atomic::load(&server->extra_port, mxb::atomic::RELAXED);
+
+    if (!mysql && extra_port)
+    {
+        mysql = mysql_real_connect(con, server->address, user, passwd, NULL, extra_port, NULL, 0);
+        MXS_WARNING("Could not connect with normal port to server '%s', using extra_port", server->name);
+    }
 
     if (mysql)
     {

server/core/server.cc

@@ -154,6 +154,7 @@ SERVER* server_alloc(const char* name, MXS_CONFIG_PARAMETER* params)
 
     server->name = my_name;
     server->port = config_get_integer(params, CN_PORT);
+    server->extra_port = config_get_integer(params, CN_EXTRA_PORT);
     server->protocol = my_protocol;
     server->authenticator = my_authenticator;
     server->monuser[0] = '\0';
@@ -1034,6 +1035,18 @@ void server_update_port(SERVER* server, unsigned short port)
     }
 }
 
+/*
+ * Update the extra_port value of a specific server
+ *
+ * @param server        The server to update
+ * @param port          The new extra_port value
+ *
+ */
+void server_update_extra_port(SERVER* server, unsigned short port)
+{
+    mxb::atomic::store(&server->extra_port, port, mxb::atomic::RELAXED);
+}
+
 static struct
 {
     const char* str;