MDEV-16092 Crash in encryption.create_or_replace

If the tablespace is dropped or truncated after the
space->is_stopping() check in fil_crypt_get_page_throttle_func(),
we would proceed to request the page, and eventually report a fatal
error.

buf_page_get_gen(): Do not retry reading if mode==BUF_GET_POSSIBLY_FREED.

lock_rec_block_validate(): Be prepared for a NULL return value when
invoking buf_page_get_gen() with mode=BUF_GET_POSSIBLY_FREED.

storage/innobase/buf/buf0buf.cc

@@ -2,7 +2,7 @@
 
 Copyright (c) 1995, 2016, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2008, Google Inc.
-Copyright (c) 2013, 2017, MariaDB Corporation.
+Copyright (c) 2013, 2018, MariaDB Corporation.
 
 Portions of this file contain modifications contributed and copyrighted by
 Google, Inc. Those modifications are gratefully acknowledged and are described
@@ -3189,6 +3189,11 @@ buf_page_get_gen(
 					      ibuf_inside(mtr));
 
 			retries = 0;
+		} else if (mode == BUF_GET_POSSIBLY_FREED) {
+			if (err) {
+				*err = local_err;
+			}
+			return NULL;
 		} else if (retries < BUF_PAGE_READ_MAX_RETRIES) {
 			++retries;
 

storage/innobase/lock/lock0lock.cc

@@ -6895,10 +6895,10 @@ lock_rec_block_validate(
 			page_no, RW_X_LATCH, NULL,
 			BUF_GET_POSSIBLY_FREED,
 			__FILE__, __LINE__, &mtr);
-
-		buf_block_dbg_add_level(block, SYNC_NO_ORDER_CHECK);
-
-		ut_ad(lock_rec_validate_page(block));
+		if (block) {
+			buf_block_dbg_add_level(block, SYNC_NO_ORDER_CHECK);
+			ut_ad(lock_rec_validate_page(block));
+		}
 		mtr_commit(&mtr);
 
 		fil_space_release(space);

storage/xtradb/buf/buf0buf.cc

@@ -2,7 +2,7 @@
 
 Copyright (c) 1995, 2016, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2008, Google Inc.
-Copyright (c) 2013, 2017, MariaDB Corporation.
+Copyright (c) 2013, 2018, MariaDB Corporation.
 
 Portions of this file contain modifications contributed and copyrighted by
 Google, Inc. Those modifications are gratefully acknowledged and are described
@@ -3088,6 +3088,11 @@ buf_page_get_gen(
 					      ibuf_inside(mtr), trx);
 
 			retries = 0;
+		} else if (mode == BUF_GET_POSSIBLY_FREED) {
+			if (err) {
+				*err = local_err;
+			}
+			return NULL;
 		} else if (retries < BUF_PAGE_READ_MAX_RETRIES) {
 			++retries;
 

storage/xtradb/lock/lock0lock.cc

@@ -6955,10 +6955,10 @@ lock_rec_block_validate(
 			page_no, RW_X_LATCH, NULL,
 			BUF_GET_POSSIBLY_FREED,
 			__FILE__, __LINE__, &mtr);
-
-		buf_block_dbg_add_level(block, SYNC_NO_ORDER_CHECK);
-
-		ut_ad(lock_rec_validate_page(block));
+		if (block) {
+			buf_block_dbg_add_level(block, SYNC_NO_ORDER_CHECK);
+			ut_ad(lock_rec_validate_page(block));
+		}
 		mtr_commit(&mtr);
 
 		fil_space_release(space);