view: Fix panic if offset is longer than result set

Specifically in the case that a query

- Has ORDER BY, LIMIT, and OFFSET
- Has aggregates with a group by
- *doesn't* filter by a primary or unique column

If the OFFSET is longer than the length of the result set, we'd panic
due to a bounds checking bug in `Vec::drain` inside the ResultIterator.
This fixes that by just clearing the result set entirely if the offset
is >= the length of the result set.

Fixes: ENG-2970
Change-Id: Iadadbe4ee986f0a991745814a927813a946dd9ac
Reviewed-on: https://gerrit.readyset.name/c/readyset/+/4805
Tested-by: Buildkite CI
Reviewed-by: Fran Noriega <fran@readyset.io>

logictests/pagination.test

@@ -4,6 +4,7 @@ create table t1 (grp int, val int);
 statement ok
 insert into t1 (grp, val) values (1, 1), (1, 2), (1, 3), (1, 4), (1, 5), (1, 6);
 
+onlyif readyset
 statement ok
 create cache from
 select val from t1 where grp = ? order by val asc limit 3 offset ?;
@@ -43,6 +44,7 @@ select val from t1 where grp = ? order by val asc limit ? offset ?;
 statement ok
 create table t2 (grp2 int, val int);
 
+onlyif readyset
 statement ok
 create cache from
 select t1.val
@@ -89,3 +91,14 @@ offset ?
 1
 2
 3
+
+query II nosort
+select max(val), grp
+from t1
+where grp >= ?
+group by grp
+order by grp asc
+limit 10
+offset 1234
+? = 1
+----

readyset-client/src/view/results.rs

@@ -262,7 +262,11 @@ impl ResultIterator {
                 });
 
                 if let Some(offset) = offset {
-                    results.drain(offset..);
+                    if offset >= results.len() {
+                        results.clear();
+                    } else {
+                        results.drain(offset..);
+                    }
                 }
 
                 return ResultIterator::owned(vec![Results {