MDEV-19783 Random crashes and corrupt data in INSTANT-added columns

The bug affects MariaDB Server 10.3 or later, but it makes sense
to improve CHECK TABLE in earlier versions already.

page_validate(): Check REC_INFO_MIN_REC_FLAG in the records.
This allows CHECK TABLE to catch more bugs.

storage/innobase/page/page0page.cc

@@ -2392,6 +2392,7 @@ page_validate(
 	ulint			data_size;
 	const rec_t*		rec;
 	const rec_t*		old_rec		= NULL;
+	const rec_t*		first_rec	= NULL;
 	ulint			offs;
 	ulint			n_slots;
 	ibool			ret		= FALSE;
@@ -2488,6 +2489,21 @@ page_validate(
 			goto func_exit;
 		}
 
+		if (rec == first_rec) {
+			if ((rec_get_info_bits(rec, page_is_comp(page))
+			     & REC_INFO_MIN_REC_FLAG)
+			    && page_is_leaf(page)) {
+				ib::error() << "REC_INFO_MIN_REC_FLAG "
+					"is set in a leaf-page record";
+				ret = false;
+			}
+		} else if (rec_get_info_bits(rec, page_is_comp(page))
+			   & REC_INFO_MIN_REC_FLAG) {
+			ib::error() << "REC_INFO_MIN_REC_FLAG record is not "
+				       "first in page";
+			ret = false;
+		}
+
 		/* Check that the records are in the ascending order */
 		if (count >= PAGE_HEAP_NO_USER_LOW
 		    && !page_rec_is_supremum(rec)) {
@@ -2599,6 +2615,11 @@ page_validate(
 		old_rec = rec;
 		rec = page_rec_get_next_const(rec);
 
+		if (page_rec_is_infimum(old_rec)
+		    && page_rec_is_user_rec(rec)) {
+			first_rec = rec;
+		}
+
 		/* set old_offsets to offsets; recycle offsets */
 		{
 			ulint* offs = old_offsets;