MDEV-28840 innodb_undo_log_truncate is not crash-safe

trx_purge_free_segment(): Do mark that the block will be modified.

It seems possible that this regression was introduced by the
changes to the page-freeing logic
in commit 4179f93 (MDEV-18976).

Tested by: Matthias Leich

storage/innobase/include/mtr0mtr.h

@@ -290,9 +290,9 @@ struct mtr_t {
   @param[in]	type	object type: MTR_MEMO_PAGE_X_FIX, ... */
   void release_page(const void *ptr, mtr_memo_type_t type);
 
-private:
   /** Note that the mini-transaction will modify data. */
   void flag_modified() { m_modifications = true; }
+private:
   /** Mark the given latched page as modified.
   @param block   page that will be modified */
   void modify(const buf_block_t& block);

storage/innobase/include/mtr0mtr.inl

@@ -46,7 +46,7 @@ mtr_t::memo_push(void* object, mtr_memo_type_t type)
 	ut_ad(object != NULL);
 	ut_ad(type >= MTR_MEMO_PAGE_S_FIX);
 	ut_ad(type <= MTR_MEMO_SPACE_S_LOCK);
-	ut_ad(ut_is_2pow(type));
+	ut_ad(type == MTR_MEMO_PAGE_X_MODIFY || ut_is_2pow(type));
 
 	/* If this mtr has x-fixed a clean page then we set
 	the made_dirty flag. This tells us if we need to

storage/innobase/trx/trx0purge.cc

@@ -402,8 +402,9 @@ static dberr_t trx_purge_free_segment(trx_rseg_t *rseg, fil_addr_t hdr_addr)
       block->fix();
       mtr.commit();
       mtr.start();
+      mtr.flag_modified();
       mtr.memo_push(rseg_hdr, MTR_MEMO_PAGE_X_FIX);
-      mtr.memo_push(block, MTR_MEMO_PAGE_X_FIX);
+      mtr.memo_push(block, MTR_MEMO_PAGE_X_MODIFY);
       rseg->latch.wr_lock(SRW_LOCK_CALL);
       rseg_hdr->page.lock.x_lock();
       block->page.lock.x_lock();