MDEV-30479 optimization: Invoke recv_sys_t::trim() earlier

recv_sys_t::parse(): Discard old page-level redo log when parsing
a TRIM_PAGES record.

recv_sys_t::apply(): trim() was invoked in parse() already.

recv_sys_t::truncated_undo_spaces[]: Only store the size, no LSN.

storage/innobase/include/log0recv.h

@@ -266,15 +266,9 @@ struct recv_sys_t
   @param lsn     log sequence number of the shrink operation */
   inline void trim(const page_id_t page_id, lsn_t lsn);
 
-  /** Undo tablespaces for which truncate has been logged
-  (indexed by page_id_t::space() - srv_undo_space_id_start) */
-  struct trunc
-  {
-    /** log sequence number of FILE_CREATE, or 0 if none */
-    lsn_t lsn;
-    /** truncated size of the tablespace, or 0 if not truncated */
-    unsigned pages;
-  } truncated_undo_spaces[127];
+  /** Truncated undo tablespace size for which truncate has been logged
+  (indexed by page_id_t::space() - srv_undo_space_id_start), or 0 */
+  unsigned truncated_undo_spaces[127];
 
 public:
   /** The contents of the doublewrite buffer */

storage/innobase/log/log0recv.cc

@@ -1949,8 +1949,10 @@ bool recv_sys_t::parse(lsn_t checkpoint_lsn, store_t *store, bool apply)
             goto record_corrupted;
           static_assert(UT_ARR_SIZE(truncated_undo_spaces) ==
                         TRX_SYS_MAX_UNDO_SPACES, "compatibility");
-          truncated_undo_spaces[space_id - srv_undo_space_id_start]=
-            { recovered_lsn, page_no };
+          /* The entire undo tablespace will be reinitialized by
+          innodb_undo_log_truncate=ON. Discard old log for all pages. */
+          trim({space_id, 0}, recovered_lsn);
+          truncated_undo_spaces[space_id - srv_undo_space_id_start]= page_no;
           if (undo_space_trunc)
             undo_space_trunc(space_id);
 #endif
@@ -2675,17 +2677,15 @@ void recv_sys_t::apply(bool last_batch)
 
     for (auto id= srv_undo_tablespaces_open; id--;)
     {
-      const trunc& t= truncated_undo_spaces[id];
-      if (t.lsn)
+      if (unsigned pages= truncated_undo_spaces[id])
       {
-        trim(page_id_t(id + srv_undo_space_id_start, 0), t.lsn);
-        if (fil_space_t *space = fil_space_get(id + srv_undo_space_id_start))
+        if (fil_space_t *space= fil_space_get(id + srv_undo_space_id_start))
         {
           ut_ad(UT_LIST_GET_LEN(space->chain) == 1);
           fil_node_t *file= UT_LIST_GET_FIRST(space->chain);
           ut_ad(file->is_open());
           os_file_truncate(file->name, file->handle,
-                           os_offset_t{t.pages} << srv_page_size_shift, true);
+                           os_offset_t{pages} << srv_page_size_shift, true);
         }
       }
     }