ignore empty input chunks generated by joins

src/Processors/Transforms/WindowTransform.cpp

@@ -975,7 +975,14 @@ void WindowTransform::appendChunk(Chunk & chunk)
     // have it if it's end of data, though.
     if (!input_is_finished)
     {
-        assert(chunk.hasRows());
+        if (!chunk.hasRows())
+        {
+            // Joins may generate empty input chunks when it's not yet end of
+            // input. Just ignore them. They probably shouldn't be sending empty
+            // chunks up the pipeline, but oh well.
+            return;
+        }
+
         blocks.push_back({});
         auto & block = blocks.back();
         // Use the number of rows from the Chunk, because it is correct even in

tests/queries/0_stateless/01591_window_functions.reference

@@ -1108,3 +1108,8 @@ from (
 -- -INT_MIN row offset that can lead to problems with negation, found when fuzzing
 -- under UBSan. Should be limited to at most INT_MAX.
 select count() over (rows between 2147483648 preceding and 2147493648 following) from numbers(2); -- { serverError 36 }
+-- Somehow in this case WindowTransform gets empty input chunks not marked as
+-- input end, and then two (!) empty input chunks marked as input end. Whatever.
+select count() over () from (select 1 a) l inner join (select 2 a) r using a;
+-- This case works as expected, one empty input chunk marked as input end.
+select count() over () where null;

tests/queries/0_stateless/01591_window_functions.sql

@@ -414,3 +414,9 @@ from (
 -- -INT_MIN row offset that can lead to problems with negation, found when fuzzing
 -- under UBSan. Should be limited to at most INT_MAX.
 select count() over (rows between 2147483648 preceding and 2147493648 following) from numbers(2); -- { serverError 36 }
+
+-- Somehow in this case WindowTransform gets empty input chunks not marked as
+-- input end, and then two (!) empty input chunks marked as input end. Whatever.
+select count() over () from (select 1 a) l inner join (select 2 a) r using a;
+-- This case works as expected, one empty input chunk marked as input end.
+select count() over () where null;