Fix assertion failure in cursor_fetcher_rewind

The code in cursor_fetcher_rewind asserted that there always
is an associated request which is not true if EOF was reached
already. Found by sqlsmith.

Fixes #3786

Author: svenklemm

tsl/src/remote/cursor_fetcher.c

@@ -421,7 +421,7 @@ cursor_fetcher_rewind(DataFetcher *df)
 	{
 		char sql[64];
 
-		Assert(cursor->state.data_req != NULL);
+		Assert(cursor->state.eof || cursor->state.data_req != NULL);
 
 		if (!cursor->state.eof)
 			async_request_discard_response(cursor->state.data_req);

tsl/test/shared/expected/dist_fetcher_type.out

@@ -91,3 +91,31 @@ limit 1;
 (1 row)
 
 reset timescaledb.remote_data_fetcher;
+-- #3786 test for assertion failure in cursor_fetcher_rewind
+SET jit TO off;
+SELECT *
+FROM devices AS d
+WHERE
+  EXISTS(
+    SELECT 1
+    FROM metrics_dist AS m,
+      LATERAL(
+        SELECT 1
+        FROM insert_test it
+        WHERE
+          EXISTS(
+            SELECT 1
+            FROM dist_chunk_copy AS ref_2
+            WHERE
+              it.id IS NOT NULL AND
+              EXISTS(SELECT d.name AS c0 FROM metrics_int WHERE NULL::TIMESTAMP <= m.time)
+          )
+      ) AS l
+    WHERE d.name ~~ d.name
+  )
+ORDER BY 1,2;
+ device_id | name 
+-----------+------
+(0 rows)
+
+RESET jit;

tsl/test/shared/sql/dist_fetcher_type.sql

@@ -37,3 +37,28 @@ where t1.id = t2.id + 1
 limit 1;
 
 reset timescaledb.remote_data_fetcher;
+
+-- #3786 test for assertion failure in cursor_fetcher_rewind
+SET jit TO off;
+SELECT *
+FROM devices AS d
+WHERE
+  EXISTS(
+    SELECT 1
+    FROM metrics_dist AS m,
+      LATERAL(
+        SELECT 1
+        FROM insert_test it
+        WHERE
+          EXISTS(
+            SELECT 1
+            FROM dist_chunk_copy AS ref_2
+            WHERE
+              it.id IS NOT NULL AND
+              EXISTS(SELECT d.name AS c0 FROM metrics_int WHERE NULL::TIMESTAMP <= m.time)
+          )
+      ) AS l
+    WHERE d.name ~~ d.name
+  )
+ORDER BY 1,2;
+RESET jit;