When read_only option was enabled, a user without SUPER privilege could
perform CREATE DATABASE and DROP DATABASE operations.
This patch adds a check to make sure this isn't possible. It also attempts to
simplify the logic used to determine if relevant tables are updated,
making it more human readable.
Bug#11733 (COMMITs should not happen if read-only is set)
Bug#22009 (Can write to a read-only server under some circumstances)
See the work log for details
The change consist of
a) acquiring the global read lock in SET GLOBAL READONLY
b) honoring opt_readonly in ha_commit_trans(),
c) honoring opt_readonly in mysql_lock_tables().
a) takes care of the server stability,
b) makes the transactional tables safe (Bug 11733)
c) makes the non transactional tables safe (Bug 22009)
Do not issue a 'read-only' error in case of DROP TEMPORARY TABLE on a non-existing temporary table.
Instead produce the correct "Unknown table" error or warning (in cases when the IF EXISTS clause was specified).
To a documentor: the part of the manual describing the 'read_only' system variable should be clarified to state the following:
"When the read_only variable is set to ON, all operations which create/update/drop tables are rejected with the exceptions for:
1. Any operation performed by the replication thread on a slave server
2. Any operation performed by a user that have the SUPER privilege
3. Any operation that creates/updates/drops only temporary tables"
test "length" first (otherwise when "length" is 0, the *from invalid access still triggers a Valgrind warning).
I wrote to the Valgrind authors in case this is something fixable in Valgrind (normally the
decision to issue a warning is based on the simulated CPU condition code, which should not be undefined here).
the READ_ONLY global variable now allows statements which are to update only temporary tables
(note: if a statement, after parse stage, looks like it will update a non-temp table, it will be rejected,
even if at execution it would have turned out that 0 rows would be updated; for example
UPDATE my_non_tem_table SET a=1 WHERE 1 = 0; will be rejected).
