Check _next on SQL views: 400 for a non-integer, clamp a negative to zero

A view pages by row offset, so table_view_data converted _next with a
bare int() and any other value raised an unhandled ValueError (HTTP 500).
Raise BadRequest instead, as the same function does for a bad _size.

Clamp a negative offset to zero before it is used for both the query and
the next link, so a view handles a negative _next as a table does: the
first page, with a next link that continues from it. Previously the next
link was computed from the negative value and came out missing, negative
or overlapping the first page.

Fixes #2982

datasette/views/table.py

@@ -2146,8 +2146,15 @@ async def table_view_data(
     if _next:
         sort_value = None
         if is_view:
-            # _next is an offset
-            offset = f" offset {int(_next)}"
+            # _next is an offset. A negative one starts from the first row, as a
+            # negative keyset token does for a table, and pagination continues
+            # from there.
+            try:
+                offset_value = max(int(_next), 0)
+            except ValueError:
+                raise BadRequest("_next must be an integer")
+            _next = str(offset_value)
+            offset = f" offset {offset_value}"
         else:
             components = urlsafe_components(_next)
             # If a sort order is applied and there are multiple components,

tests/test_table_api.py

@@ -355,6 +355,37 @@ async def test_validate_page_size(ds_client, path, expected_error):
     assert response.status_code == 400
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "next_value",
+    (
+        "dog",
+        "1e3",
+        # A facet link whose "&sect" a client decoded as an HTML entity
+        "25%C2%A7or_name%3DOil",
+    ),
+)
+async def test_validate_view_next(ds_client, next_value):
+    # Views paginate by offset, so _next has to be an integer
+    response = await ds_client.get(f"/fixtures/paginated_view.json?_next={next_value}")
+    assert response.status_code == 400
+    assert response.json()["error"] == "_next must be an integer"
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("next_value", ("-2", "-5", "-10"))
+async def test_view_negative_next_starts_from_first_row(ds_client, next_value):
+    # As for a table, a negative _next starts from the first row and the
+    # next link continues the pagination from there
+    negative = await ds_client.get(
+        f"/fixtures/paginated_view.json?_size=5&_next={next_value}"
+    )
+    first = await ds_client.get("/fixtures/paginated_view.json?_size=5")
+    assert negative.status_code == 200
+    assert negative.json()["rows"] == first.json()["rows"]
+    assert negative.json()["next"] == first.json()["next"] == "5"
+
+
 @pytest.mark.asyncio
 async def test_page_size_zero(ds_client):
     """For _size=0 we return the counts, empty rows and no continuation token"""