MDEV-34519 innodb_log_checkpoint_now crashes when innodb_read_only is enabled

During read only mode, InnoDB doesn't allow checkpoint to happen.
So InnoDB should throw the warning when InnoDB tries to
force the checkpoint when innodb_read_only = 1 or
innodb_force_recovery = 6.

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

@@ -5,4 +5,17 @@ CREATE TABLE t1(f1 INT NOT NULL, f2 TEXT)ENGINE=InnoDB;
 INSERT INTO t1 SELECT seq, repeat('a', 4000) FROM seq_1_to_1800;
 # restart: --debug_dbug=+d,before_final_redo_apply --innodb_log_file_size=8M
 # restart: --innodb_log_file_size=10M
+#
+# MDEV-34519 innodb_log_checkpoint_now crashes when
+#              innodb_read_only is enabled
+#
+# restart: --innodb-force-recovery=6
+SET GLOBAL innodb_log_checkpoint_now=1;
+Warnings:
+Warning	138	InnoDB doesn't force checkpoint when innodb-force-recovery=6.
+# restart: --innodb-read-only=1
+SET GLOBAL innodb_log_checkpoint_now=1;
+Warnings:
+Warning	138	InnoDB doesn't force checkpoint when innodb-read-only=1.
+# restart
 DROP TABLE t1;

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

@@ -14,4 +14,18 @@ let $shutdown_timeout=0;
 let $restart_parameters=--innodb_log_file_size=10M;
 let $shutdown_timeout=;
 --source include/restart_mysqld.inc
+
+--echo #
+--echo # MDEV-34519 innodb_log_checkpoint_now crashes when
+--echo #              innodb_read_only is enabled
+--echo #
+--let $restart_parameters=--innodb-force-recovery=6
+--source include/restart_mysqld.inc
+SET GLOBAL innodb_log_checkpoint_now=1;
+--let $restart_parameters=--innodb-read-only=1
+--source include/restart_mysqld.inc
+SET GLOBAL innodb_log_checkpoint_now=1;
+let $restart_parameters=;
+--source include/restart_mysqld.inc
+
 DROP TABLE t1;

storage/innobase/handler/ha_innodb.cc

@@ -18378,9 +18378,24 @@ wait_background_drop_list_empty(THD*, st_mysql_sys_var*, void*, const void*)
 Force innodb to checkpoint. */
 static
 void
-checkpoint_now_set(THD*, st_mysql_sys_var*, void*, const void* save)
+checkpoint_now_set(THD* thd, st_mysql_sys_var*, void*, const void* save)
 {
-	if (*(my_bool*) save) {
+	if (!*(my_bool*) save) {
+		return;
+	}
+
+	if (srv_read_only_mode) {
+		push_warning_printf(thd, Sql_condition::WARN_LEVEL_WARN,
+				    HA_ERR_UNSUPPORTED,
+				    "InnoDB doesn't force checkpoint "
+				    "when %s",
+				    (srv_force_recovery
+				     == SRV_FORCE_NO_LOG_REDO)
+				    ? "innodb-force-recovery=6."
+				    : "innodb-read-only=1.");
+		return;
+	}
+
 	mysql_mutex_unlock(&LOCK_global_system_variables);
 
 	lsn_t lsn;
@@ -18399,7 +18414,6 @@ checkpoint_now_set(THD*, st_mysql_sys_var*, void*, const void* save)
 	}
 
 	mysql_mutex_lock(&LOCK_global_system_variables);
-	}
 }
 
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