MDEV-37299: Fix crash when server read-only and encrption ON

Modified srv_start to call fil_crypt_threads_init() only
when srv_read_only_mode is not set.

Modified encryption.innodb-read-only to capture number of
encryption threads created for both scenarios when
server is not read only as well as when server is read only.

Author: mariadb-satishkumar

mysql-test/suite/encryption/r/innodb-read-only.result

@@ -1,4 +1,16 @@
 # Wait max 10 min for key encryption threads to encrypt all spaces
 # Success!
+SET GLOBAL innodb_encryption_threads=4;
+SELECT COUNT(*) AS encrypt_threads_running
+FROM performance_schema.threads
+WHERE NAME LIKE '%encrypt%';
+encrypt_threads_running
+4
 # restart: --innodb-read-only=1 --innodb-encrypt-tables=1
+SET GLOBAL innodb_encryption_threads=4;
+SELECT COUNT(*) AS encrypt_threads_running
+FROM performance_schema.threads
+WHERE NAME LIKE '%encrypt%';
+encrypt_threads_running
+0
 # All done

mysql-test/suite/encryption/t/innodb-read-only.test

@@ -25,10 +25,22 @@ if (!$success)
 }
 --echo # Success!
 
+# Server in normal mode
+SET GLOBAL innodb_encryption_threads=4;
+SELECT COUNT(*) AS encrypt_threads_running
+FROM performance_schema.threads
+WHERE NAME LIKE '%encrypt%';
+
 #
 # MDEV-11835: InnoDB: Failing assertion: free_slot != NULL on
 # restarting server with encryption and read-only
 #
 --let $restart_parameters= --innodb-read-only=1 --innodb-encrypt-tables=1
 --source include/restart_mysqld.inc
+
+# Server read-only mode
+SET GLOBAL innodb_encryption_threads=4;
+SELECT COUNT(*) AS encrypt_threads_running
+FROM performance_schema.threads
+WHERE NAME LIKE '%encrypt%';
 --echo # All done

storage/innobase/fil/fil0crypt.cc

@@ -2108,6 +2108,9 @@ Adjust thread count for key rotation
 @param[in]	enw_cnt		Number of threads to be used */
 void fil_crypt_set_thread_cnt(const uint new_cnt)
 {
+	if (srv_read_only_mode)
+		return;
+
 	if (!fil_crypt_threads_inited) {
 		if (srv_shutdown_state != SRV_SHUTDOWN_NONE)
 			return;
@@ -2261,6 +2264,8 @@ void fil_crypt_set_encrypt_tables(ulong val)
 Init threads for key rotation */
 void fil_crypt_threads_init()
 {
+	ut_ad(!srv_read_only_mode);
+
 	if (!fil_crypt_threads_inited) {
 		pthread_cond_init(&fil_crypt_cond, nullptr);
 		pthread_cond_init(&fil_crypt_threads_cond, nullptr);