MDEV-9895: Assertion `n_rows > 0' failed in Frame_cursor* get_frame_cursor

n=0 in "ROWS 0 PRECEDING" is valid, add handling for it:
- Adjust the assert
- Bottom bound of 'ROW 0 PRECEDING' is actually looking at the current
  row, that is, it needs to process partition's first row directly in
  Frame_n_rows_preceding::next_partition().
- Added testcases

mysql-test/r/win.result

@@ -1841,3 +1841,64 @@ insert into t1 values (1),(2);
 SELECT MAX(i) OVER (PARTITION BY (i)) FROM t1;
 ERROR 42000: This version of MariaDB doesn't yet support 'This aggregate as window function'
 drop table t1;
+#
+# Check the 0 in ROWS 0 PRECEDING
+#
+create table t1 (
+part_id int,
+pk int,
+a int
+);
+insert into t1 values (1, 1, 1);
+insert into t1 values (1, 2, 2);
+insert into t1 values (1, 3, 4);
+insert into t1 values (1, 4, 8);
+select 
+pk, a,
+sum(a) over (order by pk rows between 0 preceding and current row)
+from t1;
+pk	a	sum(a) over (order by pk rows between 0 preceding and current row)
+1	1	1
+2	2	2
+3	4	4
+4	8	8
+select 
+pk, a, 
+sum(a) over (order by pk rows between 1 preceding and 0 preceding)
+from t1;
+pk	a	sum(a) over (order by pk rows between 1 preceding and 0 preceding)
+1	1	1
+2	2	3
+3	4	6
+4	8	12
+insert into t1 values (200, 1, 1);
+insert into t1 values (200, 2, 2);
+insert into t1 values (200, 3, 4);
+insert into t1 values (200, 4, 8);
+select 
+part_id, pk, a,
+sum(a) over (partition by part_id order by pk rows between 0 preceding and current row)
+from t1;
+part_id	pk	a	sum(a) over (partition by part_id order by pk rows between 0 preceding and current row)
+1	1	1	1
+1	2	2	2
+1	3	4	4
+1	4	8	8
+200	1	1	1
+200	2	2	2
+200	3	4	4
+200	4	8	8
+select 
+part_id, pk, a, 
+sum(a) over (partition by part_id order by pk rows between 1 preceding and 0 preceding)
+from t1;
+part_id	pk	a	sum(a) over (partition by part_id order by pk rows between 1 preceding and 0 preceding)
+1	1	1	1
+1	2	2	3
+1	3	4	6
+1	4	8	12
+200	1	1	1
+200	2	2	3
+200	3	4	6
+200	4	8	12
+drop table t1;

mysql-test/t/win.test

@@ -1124,3 +1124,44 @@ insert into t1 values (1),(2);
 SELECT MAX(i) OVER (PARTITION BY (i)) FROM t1;
 drop table t1;
 
+--echo #
+--echo # Check the 0 in ROWS 0 PRECEDING
+--echo #
+
+create table t1 (
+  part_id int,
+  pk int,
+  a int
+);
+
+insert into t1 values (1, 1, 1);
+insert into t1 values (1, 2, 2);
+insert into t1 values (1, 3, 4);
+insert into t1 values (1, 4, 8);
+
+select 
+  pk, a,
+  sum(a) over (order by pk rows between 0 preceding and current row)
+from t1;
+
+select 
+  pk, a, 
+  sum(a) over (order by pk rows between 1 preceding and 0 preceding)
+from t1;
+
+insert into t1 values (200, 1, 1);
+insert into t1 values (200, 2, 2);
+insert into t1 values (200, 3, 4);
+insert into t1 values (200, 4, 8);
+select 
+  part_id, pk, a,
+  sum(a) over (partition by part_id order by pk rows between 0 preceding and current row)
+from t1;
+
+select 
+  part_id, pk, a, 
+  sum(a) over (partition by part_id order by pk rows between 1 preceding and 0 preceding)
+from t1;
+
+drop table t1;
+

sql/sql_window.cc

@@ -1279,6 +1279,14 @@ class Frame_n_rows_preceding : public Frame_cursor
         - top bound should remove row (#n-3) from the window function.
     */
     n_rows_to_skip= n_rows + (is_top_bound? 1:0) - 1;
+
+    /* Bottom bound "ROWS 0 PRECEDING" is a special case: */
+    if (n_rows_to_skip == -1)
+    {
+      cursor.get_next();
+      item->add();
+      n_rows_to_skip= 0;
+    }
   }
 
   void next_row(Item_sum* item)
@@ -1496,7 +1504,7 @@ Frame_cursor *get_frame_cursor(Window_frame *frame, bool is_top_bound)
       longlong n_rows= bound->offset->val_int();
       /* These should be handled in the parser */
       DBUG_ASSERT(!bound->offset->null_value);
-      DBUG_ASSERT(n_rows > 0);
+      DBUG_ASSERT(n_rows >= 0);
       if (is_preceding)
         return new Frame_n_rows_preceding(is_top_bound, n_rows);
       else