MDEV-22495 Assertion ...status != buf_page_t::FREED in ibuf_read_merge_pages()

ibuf_read_merge_pages(): Request a possibly freed page.
The change buffer is discarded lazily for freed pages either
by this function or when buf_page_create() reuses a page.

buf_page_get_low(): Relax a debug assertion.
Do not attempt change buffer merge on freed pages.

ibuf_merge_or_delete_for_page(): Assert that the page state is NORMAL.
INIT_ON_FLUSH is not possible, because in that case buf_page_create()
should have removed any buffered changes for the page.

buf_page_get_gen(): Apply buffered changes also in the case when
we can avoid reading the page based on buffered redo log records.
This addresses a hard-to-reproduce scenario that was broken in
commit 6697135c6d.

storage/innobase/buf/buf0buf.cc

@@ -3298,6 +3298,7 @@ buf_page_get_low(
 	      || (rw_latch == RW_NO_LATCH));
 	ut_ad(!allow_ibuf_merge
 	      || mode == BUF_GET
+	      || mode == BUF_GET_POSSIBLY_FREED
 	      || mode == BUF_GET_IF_IN_POOL
 	      || mode == BUF_GET_IF_IN_POOL_OR_WATCH);
 
@@ -3907,7 +3908,8 @@ evict_from_pool:
 		return NULL;
 	}
 
-	if (allow_ibuf_merge
+	if (fix_block->page.status != buf_page_t::FREED
+	    && allow_ibuf_merge
 	    && fil_page_get_type(fix_block->frame) == FIL_PAGE_INDEX
 	    && page_is_leaf(fix_block->frame)) {
 		rw_lock_x_lock_inline(&fix_block->lock, 0, file, line);
@@ -3974,9 +3976,27 @@ buf_page_get_gen(
   {
     block->fix();
     ut_ad(rw_lock_s_lock_nowait(block->debug_latch, file, line));
-    block= buf_page_mtr_lock(block, rw_latch, mtr, file, line);
     if (err)
       *err= DB_SUCCESS;
+    const bool must_merge= allow_ibuf_merge &&
+      ibuf_page_exists(page_id, block->zip_size());
+    if (block->page.status == buf_page_t::FREED)
+      ut_ad(mode == BUF_GET_POSSIBLY_FREED || mode == BUF_PEEK_IF_IN_POOL);
+    else if (must_merge && fil_page_get_type(block->frame) == FIL_PAGE_INDEX &&
+	     page_is_leaf(block->frame))
+    {
+      rw_lock_x_lock_inline(&block->lock, 0, file, line);
+      block->page.ibuf_exist= false;
+      ibuf_merge_or_delete_for_page(block, page_id, block->zip_size(), true);
+
+      if (rw_latch == RW_X_LATCH)
+      {
+        mtr->memo_push(block, MTR_MEMO_PAGE_X_FIX);
+	return block;
+      }
+      rw_lock_x_unlock(&block->lock);
+    }
+    block= buf_page_mtr_lock(block, rw_latch, mtr, file, line);
     return block;
   }
 

storage/innobase/ibuf/ibuf0ibuf.cc

@@ -2325,8 +2325,8 @@ tablespace_deleted:
 			mtr.start();
 			dberr_t err;
 			buf_page_get_gen(page_id_t(space_id, page_nos[i]),
-					 zip_size,
-					 RW_X_LATCH, NULL, BUF_GET,
+					 zip_size, RW_X_LATCH, nullptr,
+					 BUF_GET_POSSIBLY_FREED,
 					 __FILE__, __LINE__, &mtr, &err, true);
 			mtr.commit();
 			if (err == DB_TABLESPACE_DELETED) {
@@ -4221,8 +4221,9 @@ ibuf_merge_or_delete_for_page(
 	ulint		mops[IBUF_OP_COUNT];
 	ulint		dops[IBUF_OP_COUNT];
 
-	ut_ad(block == NULL || page_id == block->page.id);
+	ut_ad(!block || page_id == block->page.id);
 	ut_ad(!block || buf_block_get_state(block) == BUF_BLOCK_FILE_PAGE);
+	ut_ad(!block || block->page.status == buf_page_t::NORMAL);
 
 	if (trx_sys_hdr_page(page_id)
 	    || fsp_is_system_temporary(page_id.space())) {