MDEV-31813 SET GLOBAL innodb_max_purge_lag_wait hangs if innodb_read_only

innodb_max_purge_lag_wait_update(): Return immediately if we are
in high_level_read_only mode.

srv_wake_purge_thread_if_not_active(): Relax a debug assertion.
If srv_read_only_mode holds, purge_sys.enabled() will not hold
and this function will do nothing.

trx_t::commit_in_memory(): Remove a redundant condition before
invoking srv_wake_purge_thread_if_not_active().

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

@@ -35,6 +35,7 @@ SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
 SELECT * FROM t;
 a
 3
+SET GLOBAL innodb_max_purge_lag_wait=0;
 # restart
 SELECT * FROM t;
 a

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

@@ -37,6 +37,7 @@ UPDATE t SET a=3 WHERE a=1;
 SELECT * FROM t;
 SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
 SELECT * FROM t;
+SET GLOBAL innodb_max_purge_lag_wait=0;
 --let $restart_parameters=
 --source include/restart_mysqld.inc
 SELECT * FROM t;

storage/innobase/handler/ha_innodb.cc

@@ -238,6 +238,8 @@ static uint innodb_max_purge_lag_wait;
 static void innodb_max_purge_lag_wait_update(THD *thd, st_mysql_sys_var *,
                                              void *, const void *limit)
 {
+  if (high_level_read_only)
+    return;
   const uint l= *static_cast<const uint*>(limit);
   if (trx_sys.rseg_history_len <= l)
     return;

storage/innobase/srv/srv0srv.cc

@@ -1902,8 +1902,7 @@ srv_active_wake_master_thread_low()
 void
 srv_wake_purge_thread_if_not_active()
 {
-	ut_ad(!srv_read_only_mode);
-	ut_ad(!mutex_own(&srv_sys.mutex));
+	ut_ad(srv_read_only_mode || !mutex_own(&srv_sys.mutex));
 
 	if (purge_sys.enabled() && !purge_sys.paused()
 	    && !srv_sys.n_threads_active[SRV_PURGE]

storage/innobase/trx/trx0trx.cc

@@ -1477,8 +1477,7 @@ inline void trx_t::commit_in_memory(const mtr_t *mtr)
   trx_mutex_exit(this);
 
   ut_a(error_state == DB_SUCCESS);
-  if (!srv_read_only_mode)
-    srv_wake_purge_thread_if_not_active();
+  srv_wake_purge_thread_if_not_active();
 }
 
 /** Commit the transaction in a mini-transaction.