`BaseCursor#close(..)` can exhaust the system memory

[Brian Lauber]

Describe the bug

The BaseCursor#close(..) method invokes BaseCursor#nextset(..), which will invoke self.fetchall() under certain circumstances:

• mysqlclient/MySQLdb/cursors.py

  Line 83 in 8f0cbac

   while self.nextset(): 

• mysqlclient/MySQLdb/cursors.py

  Line 134 in 8f0cbac

   self.fetchall() 

Unfortunately, this can cause Python to run out of memory when the cursor yields hundreds of millions of records.

To Reproduce

1. Create a new table
2. Populate it with hundreds of millions of records
3. Create a cursor that will yield all of the records from this table. (Ex: SELECT * FROM HugeTable)
4. Fetch a handful of records from the cursor. (ie: cursor.fetchmany(100) )
5. Close the cursor

Environment

This bug is present regardless of environment; however, here's the environment that I was using:

MySQL Server

• AWS RDS. (Presumably, the use Amazon Linux)
• Server Version 8.0.23

MySQL Client

• OS: Ubuntu Jammy
• Python >= 3.7
• mysqlclient 2.0.3 and mysqlclient 2.1.1

Additional Context

I can probably implement a fix for this once we confirm that it's a legitimate bug. At a high-level, I suspect the call to self.fetchall() probably needs to be replaced with a call to self.fetchmany(..). However, this might be complicated by structural details that I'm not aware of.