BUG#13900 DATETIME data changes after inserting a new row in a InnoDB table

Applying patch from Marko.
All tests pass in pentium-debug-max build on Linux.

innobase/include/rem0rec.h

@@ -312,6 +312,15 @@ rec_offs_nth_extern(
 	const ulint*	offsets,/* in: array returned by rec_get_offsets() */
 	ulint		n);	/* in: nth field */
 /**********************************************************
+Returns nonzero if the SQL NULL bit is set in nth field of rec. */
+UNIV_INLINE
+ulint
+rec_offs_nth_sql_null(
+/*==================*/
+				/* out: nonzero if SQL NULL */
+	const ulint*	offsets,/* in: array returned by rec_get_offsets() */
+	ulint		n);	/* in: nth field */
+/**********************************************************
 Gets the physical size of a field. */
 UNIV_INLINE
 ulint

innobase/include/rem0rec.ic

@@ -954,6 +954,22 @@ rec_offs_nth_extern(
 				& REC_OFFS_EXTERNAL));
 }
 
+/**********************************************************
+Returns nonzero if the SQL NULL bit is set in nth field of rec. */
+UNIV_INLINE
+ulint
+rec_offs_nth_sql_null(
+/*==================*/
+				/* out: nonzero if SQL NULL */
+	const ulint*	offsets,/* in: array returned by rec_get_offsets() */
+	ulint		n)	/* in: nth field */
+{
+	ut_ad(rec_offs_validate(NULL, NULL, offsets));
+	ut_ad(n < rec_offs_n_fields(offsets));
+	return(UNIV_UNLIKELY(rec_offs_base(offsets)[1 + n]
+				& REC_OFFS_SQL_NULL));
+}
+
 /**********************************************************
 Gets the physical size of a field. */
 UNIV_INLINE

innobase/row/row0upd.c

@@ -395,6 +395,18 @@ row_upd_changes_field_size_or_external(
 
 		old_len = rec_offs_nth_size(offsets, upd_field->field_no);
 
+		if (rec_offs_comp(offsets)
+		    && rec_offs_nth_sql_null(offsets, upd_field->field_no)) {
+			/* Note that in the compact table format, for a
+			variable length field, an SQL NULL will use zero
+			bytes in the offset array at the start of the physical
+			record, but a zero-length value (empty string) will
+			use one byte! Thus, we cannot use update-in-place
+			if we update an SQL NULL varchar to an empty string! */
+
+			old_len = UNIV_SQL_NULL;
+		}
+
 		if (old_len != new_len) {
 
 			return(TRUE);