Bug #109134 rseg->trx_ref_count is incorrect when truncating undo log

Description:
An undo log can not be truncated. Some variables are as follows:
show global variables like '%_undo_%'
innodb_max_undo_log_size    1073741824
innodb_undo_directory   /data/mysql/
innodb_undo_log_truncate    ON
innodb_undo_logs    128
innodb_undo_tablespaces 3

/data/mysql# du -h -c undo00*
2.1G    undo001
200G    undo002
201G    undo003

Some memory information of mysqld which is collected by gdb as follows:
1. one rseg blocks the truncate operation of the undo log due to the value of rseg->trx_ref_count is 1.

2. the history length in all undo logs is 0

3. rw_trx_list has no trx in it.

4. mysql_trx_list has only one trx, it has not be started and has no rsegs assigned.

Maybe some 'rseg->trx_ref_count--' losts.

How to repeat:
no repeate

Hi Mr. wang,

Thank you for your bug report.

However, your report is about a release that is very , very old.

Please try 8.0.31.

Unsupported.

Please, also let us know where do we describe truncation of the undo log in our Reference Manual.

That operation was not supported in 5.7.