MySQL Bugs: #79304: MySQL audit plugin receives bad row counts

Bug #79304	MySQL audit plugin receives bad row counts
Submitted:	17 Nov 2015 6:29	Modified:	24 May 2018 7:28
Reporter:	Manuel Ung	Email Updates:
Status:	Analyzing	Impact on me:	None
Category:	MySQL Server: Security: Audit	Severity:	S3 (Non-critical)
Version:	5.6 5.7 8.0	OS:	Any
Assigned to:	Marek Szymczak	CPU Architecture:	Any

Description:
The audit plugin receives row count from using this:

  row= thd->get_stmt_da()->current_row_for_warning();

This does not always give an accurate count of number of rows sent to client.

Also, as far as I can tell, this will always give a value >= 1 and will never return 0, contrary to what documentation says.
(https://dev.mysql.com/doc/refman/5.6/en/writing-audit-plugins.html)

How to repeat:
create table t (i int, primary key (i));
insert into t values (1);

select * from t where i = 0;
=> audit plugin receives row count of 1 (this is correct)

select * from t where i = 1;
=> audit plugin receives row count of 1 (this is incorrect)

Suggested fix:
Something that uses thd->get_sent_row_count() would be more reasonable?

Hello Manuel Ung,

Thank you for the bug report.
I could not see any issue in the user manual. Probably docs are changed recently.

Thanks,
Chiranjeevi.

Quoting from the documentation:

general_rows: For general query log events, zero. For error log events, the row number at which an error occurred. For result events, the number of rows in the result plus one. For statements that produce no result set, the value is 0. This encoding enables statements that produce no result set to be distinguished from those that produce an empty result set. For example, for a DELETE statement, this value is 0. For a SELECT, the result is always 1 or more, where 1 represents an empty result set.

From reading the code, this value is never 0, contrary to the documentation. You can also try this yourself by running a DELETE statement.

Also, this is what the expected output should be:
select * from t where i = 0;
=> audit plugin receives row count of 1 (expected: 1)

select * from t where i = 1;
=> audit plugin receives row count of 1 (expected: 2)

This appears to be a problem in 8.0 still. Although documentation has removed mentions of general_rows (https://dev.mysql.com/doc/refman/8.0/en/writing-audit-plugins.html)

I'm not sure what the intended use case of general_rows is, if queries that return 0 rows and 1 row both set general_rows to a value of 1?