should be it

external/duckdb/test/api/capi/test_capi_pending.cpp

@@ -0,0 +1,30 @@
+#include "capi_tester.hpp"
+#include "duckdb.h"
+
+using namespace duckdb;
+using namespace std;
+
+TEST_CASE("Test pending statements in C API", "[capi]") {
+	CAPITester tester;
+	CAPIPrepared prepared;
+	CAPIPending pending;
+	duckdb::unique_ptr<CAPIResult> result;
+
+	// open the database in in-memory mode
+	REQUIRE(tester.OpenDatabase(nullptr));
+	REQUIRE(prepared.Prepare(tester, "SELECT SUM(i) FROM range(1000000) tbl(i)"));
+	REQUIRE(pending.Pending(prepared));
+
+	while (true) {
+		auto state = pending.ExecuteTask();
+		REQUIRE(state != DUCKDB_PENDING_ERROR);
+		if (duckdb_pending_execution_is_finished(state)) {
+			break;
+		}
+	}
+
+	result = pending.Execute();
+	REQUIRE(result);
+	REQUIRE(!result->HasError());
+	REQUIRE(result->Fetch<int64_t>(0, 0) == 499999500000LL);
+}