At higher levels of innodb_force_recovery, the InnoDB transaction
subsystem will not be set up at all.
At slightly lower levels, recovered transactions will not be rolled back,
and DDL operations could hang due to locks being held at all.
Let us consistently refuse all writes if the predicate
high_level_read_only holds. We failed to refuse DROP TABLE
and DROP DATABASE. (Refusing DROP TABLE is a partial backport
from MDEV-19570 in the 10.5 branch.)
While the primary purpose of innodb_force_recovery is to allow
data to be rescued from an InnoDB instance that would crash due
to some data corruption, the settings 1, 2, or 3 are relatively
safe to use and there is no need to prevent write transactions
in these modes.
The setting innodb_force_recovery=4 and above can cause database
corruption. For those modes, we already set the flag
high_level_read_only to disable modifications, except DROP TABLE.
MODIFICATIONS_NOT_ALLOWED_MSG_FORCE_RECOVERY: Remove. There is no
need to spam the error log for each refused DML operation. It suffices
to return an error to the client. There will be messages at startup
if innodb_read_only or innodb_force_recovery are preventing writes.
