CDRIVER-5515 fix assert when legacy exhaust cursor receives no documents (#1562)

* add regression test

Create an exhaust cursor with empty results. Fails an assertion when using legacy op codes.

* CDRIVER-5515 handle NULL for OP_REPLY documents

Avoids an assertion failure in bson_reader_new_from_data if a legacy
exhaust cursor receives an empty batch of documents.

Author: kevinAlbs

src/libmongoc/src/mongoc/mongoc-cursor-legacy.c

@@ -244,9 +244,14 @@ _mongoc_cursor_op_getmore (mongoc_cursor_t *cursor,
 
    cursor->cursor_id = mcd_rpc_op_reply_get_cursor_id (response->rpc);
 
-   response->reader = bson_reader_new_from_data (
-      mcd_rpc_op_reply_get_documents (response->rpc),
-      mcd_rpc_op_reply_get_documents_len (response->rpc));
+   const void *documents = mcd_rpc_op_reply_get_documents (response->rpc);
+   if (documents == NULL) {
+      // Use a non-NULL pointer to satisfy precondition of
+      // `bson_reader_new_from_data`:
+      documents = "";
+   }
+
+   response->reader = bson_reader_new_from_data (documents, mcd_rpc_op_reply_get_documents_len (response->rpc));
 
    _mongoc_cursor_monitor_succeeded (cursor,
                                      response,
@@ -634,9 +639,14 @@ _mongoc_cursor_op_query_find (mongoc_cursor_t *cursor,
 
    cursor->cursor_id = mcd_rpc_op_reply_get_cursor_id (response->rpc);
 
-   response->reader = bson_reader_new_from_data (
-      mcd_rpc_op_reply_get_documents (response->rpc),
-      mcd_rpc_op_reply_get_documents_len (response->rpc));
+   const void *documents = mcd_rpc_op_reply_get_documents (response->rpc);
+   if (documents == NULL) {
+      // Use a non-NULL pointer to satisfy precondition of
+      // `bson_reader_new_from_data`:
+      documents = "";
+   }
+
+   response->reader = bson_reader_new_from_data (documents, mcd_rpc_op_reply_get_documents_len (response->rpc));
 
    if (_mongoc_cursor_get_opt_bool (cursor, MONGOC_CURSOR_EXHAUST)) {
       cursor->in_exhaust = true;

src/libmongoc/tests/test-mongoc-exhaust.c

@@ -375,6 +375,45 @@ test_exhaust_cursor_works (void *context)
    mongoc_client_destroy (client);
 }
 
+// `test_exhaust_cursor_no_match` is a regression test for CDRIVER-5515
+static void
+test_exhaust_cursor_no_match (void *context)
+{
+   BSON_UNUSED (context);
+   bson_error_t error;
+   mongoc_client_t *client = test_framework_new_default_client ();
+   mongoc_collection_t *coll = mongoc_client_get_collection (client, "db", "coll");
+
+   // Drop collection to remove prior test data.
+   mongoc_collection_drop (coll, NULL /* ignore error */);
+
+   mongoc_cursor_t *cursor =
+      mongoc_collection_find_with_opts (coll, tmp_bson ("{}"), tmp_bson ("{'exhaust': true }"), NULL /* read_prefs */);
+   const bson_t *result;
+   size_t count = 0;
+   while (mongoc_cursor_next (cursor, &result)) {
+      count++;
+   }
+   // Expect an error if exhaust cursors are not supported.
+   const bool sharded = _mongoc_topology_get_type (cursor->client->topology) == MONGOC_TOPOLOGY_SHARDED;
+   int64_t wire_version;
+   test_framework_get_max_wire_version (&wire_version);
+   if (sharded && wire_version < WIRE_VERSION_MONGOS_EXHAUST) {
+      ASSERT (mongoc_cursor_error (cursor, &error));
+      ASSERT_ERROR_CONTAINS (error, MONGOC_ERROR_CURSOR, MONGOC_ERROR_CURSOR_INVALID_CURSOR, "exhaust cursors require");
+   } else {
+      // Expect no error.
+      ASSERT_OR_PRINT (!mongoc_cursor_error (cursor, &error), error);
+      // Expect no results.
+      ASSERT_CMPSIZE_T (count, ==, 0);
+   }
+
+   mongoc_cursor_destroy (cursor);
+   mongoc_collection_destroy (coll);
+   mongoc_client_destroy (client);
+}
+
+
 static void
 test_exhaust_cursor_single (void *context)
 {
@@ -795,6 +834,8 @@ test_exhaust_install (TestSuite *suite)
                       NULL,
                       NULL,
                       skip_if_no_exhaust);
+   TestSuite_AddFull (
+      suite, "/Client/exhaust_cursor/no_match", test_exhaust_cursor_no_match, NULL, NULL, skip_if_no_exhaust);
    TestSuite_AddFull (suite,
                       "/Client/exhaust_cursor/single",
                       test_exhaust_cursor_single,