MDEV-38942 i_s_dict_fill_sys_tables() aborts when reading INNODB_SYS_TABLES after innodb_force_recovery

[Thirunarayanan]

Problem:

A query on INFORMATION_SCHEMA.INNODB_SYS_TABLES crashes when
SYS_TABLES contains a record that was inserted by a transaction which has not been committed. This can happen after a crash while a CREATE TABLE was in progress, if the server is restarted with innodb_force_recovery=4 or greater, because trx_rollback_recovered() is then skipped and the recovered transaction remains ACTIVE.

dict_sys_tables_rec_read() returns READ_NOT_FOUND for such a record, and dict_load_table_low() returns that as success with no error message and setting *table to nullptr.
i_s_sys_tables_fill_table() checks only the error
message and passes the nullptr table to i_s_dict_fill_sys_tables(), which dereferences it.

Solution:

i_s_sys_tables_fill_table(): Skip the SYS_TABLES record when dict_load_table_low() reports success but returns no table, because such a record is not visible.

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