branches/5.1: Merge r4003 from branches/5.0:

rec_set_nth_field(): When the field already is SQL null,
do nothing when it is being changed to SQL null. (Bug #41571)

Normally, MySQL does not pass "do-nothing" updates to the storage engine.
When it does and a column of an InnoDB table that is in ROW_FORMAT=COMPACT
is being updated from NULL to NULL, the InnoDB buffer pool will be corrupted
without this fix.

rb://81 approved by Heikki Tuuri

include/rem0rec.h

@@ -365,8 +365,9 @@ rec_set_field_extern_bits(
 /***************************************************************
 This is used to modify the value of an already existing field in a record.
 The previous value must have exactly the same size as the new value. If len
-is UNIV_SQL_NULL then the field is treated as an SQL null for old-style
-records. For new-style records, len must not be UNIV_SQL_NULL. */
+is UNIV_SQL_NULL then the field is treated as an SQL null.
+For records in ROW_FORMAT=COMPACT (new-style records), len must not be
+UNIV_SQL_NULL unless the field already is SQL null. */
 UNIV_INLINE
 void
 rec_set_nth_field(
@@ -375,11 +376,7 @@ rec_set_nth_field(
 	const ulint*	offsets,/* in: array returned by rec_get_offsets() */
 	ulint		n,	/* in: index number of the field */
 	const void*	data,	/* in: pointer to the data if not SQL null */
-	ulint		len);	/* in: length of the data or UNIV_SQL_NULL.
-				If not SQL null, must have the same
-				length as the previous value.
-				If SQL null, previous value must be
-				SQL null. */
+	ulint		len);	/* in: length of the data or UNIV_SQL_NULL */
 /**************************************************************
 The following function returns the data size of an old-style physical
 record, that is the sum of field lengths. SQL null fields

include/rem0rec.ic

@@ -1219,8 +1219,9 @@ rec_get_nth_field_size(
 /***************************************************************
 This is used to modify the value of an already existing field in a record.
 The previous value must have exactly the same size as the new value. If len
-is UNIV_SQL_NULL then the field is treated as an SQL null for old-style
-records. For new-style records, len must not be UNIV_SQL_NULL. */
+is UNIV_SQL_NULL then the field is treated as an SQL null.
+For records in ROW_FORMAT=COMPACT (new-style records), len must not be
+UNIV_SQL_NULL unless the field already is SQL null. */
 UNIV_INLINE
 void
 rec_set_nth_field(
@@ -1230,11 +1231,7 @@ rec_set_nth_field(
 	ulint		n,	/* in: index number of the field */
 	const void*	data,	/* in: pointer to the data
 				if not SQL null */
-	ulint		len)	/* in: length of the data or UNIV_SQL_NULL.
-				If not SQL null, must have the same
-				length as the previous value.
-				If SQL null, previous value must be
-				SQL null. */
+	ulint		len)	/* in: length of the data or UNIV_SQL_NULL */
 {
 	byte*	data2;
 	ulint	len2;
@@ -1242,9 +1239,11 @@ rec_set_nth_field(
 	ut_ad(rec);
 	ut_ad(rec_offs_validate(rec, NULL, offsets));
 
-	if (len == UNIV_SQL_NULL) {
-		ut_ad(!rec_offs_comp(offsets));
-		rec_set_nth_field_sql_null(rec, n);
+	if (UNIV_UNLIKELY(len == UNIV_SQL_NULL)) {
+		if (!rec_offs_nth_sql_null(offsets, n)) {
+			ut_a(!rec_offs_comp(offsets));
+			rec_set_nth_field_sql_null(rec, n);
+		}
 
 		return;
 	}