[MDEV-15935] Follow instant failover error packets

If the server responds to our initial connection with an error packet having
error number `ER_INSTANT_FAILOVER`, and an error string formatted as
`|Human-readable message|host[:port]`, then redirect accordingly.

This redirection is performed in `mthd_my_real_connect`, which appears to be
the most appropriate location given that it already contains code for
retrying/repeating a new server connection.

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.

include/mysqld_error.h

@@ -1236,5 +1236,6 @@
 #define ER_JSON_HISTOGRAM_PARSE_FAILED 4186
 #define ER_SF_OUT_INOUT_ARG_NOT_ALLOWED 4187
 #define ER_INCONSISTENT_SLAVE_TEMP_TABLE 4188
-#define ER_ERROR_LAST 4188
+#define ER_INSTANT_FAILOVER 4200
+#define ER_ERROR_LAST 4200
 #endif /* ER_ERROR_FIRST */

libmariadb/mariadb_lib.c

@@ -1727,6 +1727,7 @@ MYSQL *mthd_my_real_connect(MYSQL *mysql, const char *host, const char *user,
   else
 #endif
   {
+  tcp_redirect:
     cinfo.unix_socket=0;				/* This is not used */
     if (!port)
       port=mysql_port;
@@ -1991,7 +1992,30 @@ MYSQL *mthd_my_real_connect(MYSQL *mysql, const char *host, const char *user,
 
   if (run_plugin_auth(mysql, scramble_data, scramble_len,
                              scramble_plugin, db))
+  {
+    if (mysql->net.last_errno == ER_INSTANT_FAILOVER)
+    {
+      char *p= mysql->net.last_error; /* Should look like '|message|host[:port]' */
+      if (p && p[0] == '|')
+        p= strchr(p + 1, '|') ? : NULL;
+      if (p && *++p) {
+        host= p;
+        p= strchr(p, ':') ? : NULL;
+        if (p) {
+          *p++ = '\0';
+          port= atoi(p);
+        }
+        else
+        {
+          /* Restore to the default port, rather than reusing our current one */
+          port= 0;
+        }
+        fprintf(stderr, "Got instant failover to '%s' (port %d)\n", host, port);
+        goto tcp_redirect;
+      }
+    }
     goto error;
+  }
 
   if (mysql->client_flag & CLIENT_COMPRESS ||
       mysql->client_flag & CLIENT_ZSTD_COMPRESSION)