MDEV-13520 InnoDB attempts UPDATE with DB_TRX_ID=0 if innodb_force_recovery=3

trx_set_rw_mode(): Check the flag high_level_read_only instead
of testing srv_force_recovery (innodb_force_recovery) directly.
There is no need to prevent the creation of read-write transactions
if innodb_force_recovery=3 is used. Yes, in that mode any recovered
incomplete transactions will not be rolled back, but these transactions
will continue to hold locks on the records that they have modified.
If the new read-write transactions hit conflicts with already existing
(possibly recovered) transactions, the lock wait timeout mechanism
will work just fine.

mysql-test/suite/innodb/r/read_only_recovery.result

 connect con1, localhost, root;
 CREATE TABLE t(a INT PRIMARY KEY) ENGINE=InnoDB;
+INSERT INTO t VALUES(1);
 BEGIN;
-INSERT INTO t VALUES(1),(2);
+INSERT INTO t VALUES(2);
 DELETE FROM t WHERE a=2;
 connection default;
 # Normal MariaDB shutdown would roll back the above transaction.
@@ -16,20 +17,23 @@ ROLLBACK;
 disconnect con1;
 SELECT * FROM t;
 a
+1
 SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
 SELECT * FROM t;
 a
 1
+UPDATE t SET a=3 WHERE a=1;
 # Starting with MariaDB 10.2, innodb_read_only implies READ UNCOMMITTED.
 # In earlier versions, this would return the last committed version
 # (empty table)!
 SELECT * FROM t;
 a
-1
+3
 SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
 SELECT * FROM t;
 a
-1
+3
 SELECT * FROM t;
 a
+3
 DROP TABLE t;

mysql-test/suite/innodb/t/read_only_recovery.test

@@ -4,9 +4,10 @@
 
 --connect(con1, localhost, root)
 CREATE TABLE t(a INT PRIMARY KEY) ENGINE=InnoDB;
+INSERT INTO t VALUES(1);
 BEGIN;
 # Generate insert_undo log.
-INSERT INTO t VALUES(1),(2);
+INSERT INTO t VALUES(2);
 # Generate update_undo log.
 DELETE FROM t WHERE a=2;
 --connection default
@@ -27,6 +28,7 @@ ROLLBACK;
 SELECT * FROM t;
 SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
 SELECT * FROM t;
+UPDATE t SET a=3 WHERE a=1;
 --let $restart_parameters= --innodb-read-only
 --source include/restart_mysqld.inc
 --echo # Starting with MariaDB 10.2, innodb_read_only implies READ UNCOMMITTED.

storage/innobase/trx/trx0trx.cc

@@ -3091,7 +3091,7 @@ trx_set_rw_mode(
 	ut_ad(!trx->in_rw_trx_list);
 	ut_ad(!trx_is_autocommit_non_locking(trx));
 
-	if (srv_force_recovery >= SRV_FORCE_NO_TRX_UNDO) {
+	if (high_level_read_only) {
 		return;
 	}