Limit number of server-initiated instant failovers followed by client

Set the limit to 8 for now. This will prevent the client from getting stuck
in redirect loops.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services, Inc.

libmariadb/mariadb_lib.c

@@ -85,6 +85,7 @@
 #define MA_RPL_VERSION_HACK "5.5.5-"
 
 #define CHARSET_NAME_LEN 64
+#define INSTANT_FAILOVER_LIMIT 8
 
 #undef max_allowed_packet
 #undef net_buffer_length
@@ -1589,7 +1590,7 @@ MYSQL *mthd_my_real_connect(MYSQL *mysql, const char *host, const char *user,
   my_bool is_multi= 0;
   char *host_copy= NULL;
   struct st_host *host_list= NULL;
-  int connect_attempts= 0;
+  int connect_attempts= 0, instant_failovers=0;
 
   if (!mysql->methods)
     mysql->methods= &MARIADB_DEFAULT_METHODS;
@@ -2002,6 +2003,13 @@ MYSQL *mthd_my_real_connect(MYSQL *mysql, const char *host, const char *user,
         /* Client has disabled instant failover. Fall through and treat this
          * as a "normal" error. */
       }
+      else if (instant_failovers >= INSTANT_FAILOVER_LIMIT)
+      {
+        /* Too many instant failovers */
+        my_set_error(mysql, ER_INSTANT_FAILOVER, SQLSTATE_UNKNOWN,
+                     "Too many instant failovers (>= %d)",
+                     INSTANT_FAILOVER_LIMIT);
+      }
       else
       {
         char *p= mysql->net.last_error; /* Should look like '|message|host[:port]' */
@@ -2020,6 +2028,7 @@ MYSQL *mthd_my_real_connect(MYSQL *mysql, const char *host, const char *user,
             port= 0;
           }
         fprintf(stderr, "Got instant failover to '%s' (port %d)\n", host, port);
+          ++instant_failovers;
           goto tcp_redirect;
         }
       }