MDEV-33182 Server assertion fails when trying to test the connection with DBeaver

Some connectors (JDBC, Node.js) can sent non-default collation IDs
in the handshake packet. The code in thd_init_client_charset() handling
@@character_set_collations did not expect that and crashed on DBUG_ASSERT.

Changing the code to ignore @@character_set_collations in case of non-default
IDs. This fixes the problem in a backward compatible
(with pre-@@character_set_collations server versions) way for such connectors
sending non-default IDs.

mysql-test/main/connect_debug.result

@@ -18,3 +18,23 @@ connect(localhost,root,,test,MASTER_MYPORT,MYSQL_TMP_DIR/mysqld.1.sock);
 connect con2,localhost,root;
 ERROR HY000: Received malformed packet
 set global debug_dbug=@old_dbug;
+#
+# Start of 11.2 tests
+#
+#
+# MDEV-33182 Server assertion fails when trying to test the connection with DBeaver
+#
+SET global debug_dbug='+d,thd_init_client_charset_utf8mb3_bin';
+connect con1,localhost,root;
+connection con1;
+SHOW VARIABLES LIKE 'collation%';
+Variable_name	Value
+collation_connection	utf8mb3_bin
+collation_database	latin1_swedish_ci
+collation_server	latin1_swedish_ci
+disconnect con1;
+connection default;
+SET global debug_dbug=@old_debug;
+#
+# End of 11.2 tests
+#

mysql-test/main/connect_debug.test

@@ -37,3 +37,24 @@ set global debug_dbug='+d,poison_srv_handshake_scramble_len';
 --error 2027
 connect con2,localhost,root;
 set global debug_dbug=@old_dbug;
+
+
+--echo #
+--echo # Start of 11.2 tests
+--echo #
+
+--echo #
+--echo # MDEV-33182 Server assertion fails when trying to test the connection with DBeaver
+--echo #
+
+SET global debug_dbug='+d,thd_init_client_charset_utf8mb3_bin';
+connect con1,localhost,root;
+connection con1;
+SHOW VARIABLES LIKE 'collation%';
+disconnect con1;
+connection default;
+SET global debug_dbug=@old_debug;
+
+--echo #
+--echo # End of 11.2 tests
+--echo #

sql/sql_connect.cc

@@ -779,6 +779,10 @@ void update_global_user_stats(THD *thd, bool create_user, time_t now)
 bool thd_init_client_charset(THD *thd, uint cs_number)
 {
   CHARSET_INFO *cs;
+
+  // Test a non-default collation ID. See also comments in this function below.
+  DBUG_EXECUTE_IF("thd_init_client_charset_utf8mb3_bin", cs_number= 83;);
+
   /*
    Use server character set and collation if
    - opt_character_set_client_handshake is not set
@@ -801,9 +805,25 @@ bool thd_init_client_charset(THD *thd, uint cs_number)
                cs->cs_name.str);
       return true;
     }
+    /*
+      Some connectors (e.g. JDBC, Node.js) can send non-default collation IDs
+      in the handshake packet, to set @@collation_connection right during
+      handshake. Although this is a non-documenting feature,
+      for better backward compatibility with such connectors let's:
+      a. resolve only default collations according to @@character_set_collations
+      b. preserve non-default collations as is
+
+      Perhaps eventually we should change (b) also to resolve non-default
+      collations accoding to @@character_set_collations. Clients that used to
+      send a non-default collation ID in the handshake packet will have to set
+      @@character_set_collations instead.
+    */
+    if (cs->state & MY_CS_PRIMARY)
+    {
       Sql_used used;
       cs= global_system_variables.character_set_collations.
             get_collation_for_charset(&used, cs);
+    }
     thd->org_charset= cs;
     thd->update_charset(cs,cs,cs);
   }