MDEV-32050: Boost innodb_purge_batch_size on slow shutdown

A slow shutdown using the previous default innodb_purge_batch_size=300
could be extremely slow, employing at most a few CPU cores on the average.
Let us use the maximum batch size in order to increase throughput.

Reviewed by: Vladislav Lesin

storage/innobase/handler/ha_innodb.cc

@@ -18854,7 +18854,7 @@ static MYSQL_SYSVAR_ULONG(purge_batch_size, srv_purge_batch_size,
   NULL, NULL,
   1000,			/* Default setting */
   1,			/* Minimum value */
-  5000, 0);		/* Maximum value */
+  innodb_purge_batch_size_MAX, 0);
 
 static MYSQL_SYSVAR_UINT(purge_threads, srv_n_purge_threads,
   PLUGIN_VAR_OPCMDARG | PLUGIN_VAR_READONLY,

storage/innobase/include/trx0types.h

@@ -47,6 +47,7 @@ static const ulint TRX_SYS_SPACE = 0;
 static const ulint TRX_MAGIC_N = 91118598;
 
 constexpr uint innodb_purge_threads_MAX= 32;
+constexpr uint innodb_purge_batch_size_MAX= 5000;
 
 /** Transaction states (trx_t::state) */
 enum trx_state_t {

storage/innobase/srv/srv0srv.cc

@@ -1779,7 +1779,10 @@ void srv_purge_shutdown()
   if (purge_sys.enabled())
   {
     if (!srv_fast_shutdown && !opt_bootstrap)
+    {
+      srv_purge_batch_size= innodb_purge_batch_size_MAX;
       srv_update_purge_thread_count(innodb_purge_threads_MAX);
+    }
     size_t history_size= trx_sys.history_size();
     while (!srv_purge_should_exit(history_size))
     {