feat(bigtable): implement bigtable::ResultSourceInterface (#15575)

* feat(bigtable): implement bigtable::ResultSourceInterface

Author: mpeddada1

google/cloud/bigtable/CMakeLists.txt

@@ -234,6 +234,7 @@ add_library(
     read_modify_write_rule.h
     resource_names.cc
     resource_names.h
+    results.h
     retry_policy.h
     row.h
     row_key.h

google/cloud/bigtable/google_cloud_cpp_bigtable.bzl

@@ -120,6 +120,7 @@ google_cloud_cpp_bigtable_hdrs = [
     "query_row.h",
     "read_modify_write_rule.h",
     "resource_names.h",
+    "results.h",
     "retry_policy.h",
     "row.h",
     "row_key.h",

google/cloud/bigtable/results.h

@@ -0,0 +1,62 @@
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef GOOGLE_CLOUD_CPP_GOOGLE_CLOUD_BIGTABLE_RESULTS_H
+#define GOOGLE_CLOUD_CPP_GOOGLE_CLOUD_BIGTABLE_RESULTS_H
+
+#include "google/cloud/bigtable/query_row.h"
+#include "google/cloud/bigtable/version.h"
+#include "google/cloud/status_or.h"
+
+namespace google {
+namespace cloud {
+namespace bigtable {
+GOOGLE_CLOUD_CPP_INLINE_NAMESPACE_BEGIN
+
+/**
+ * Defines the interface for `RowStream` implementations.
+ *
+ * The `RowStream` class represents a stream of `Rows` returned from
+ * `bigtable::Table::ReadRows()`. There are
+ * different implementations depending the the RPC. Applications can also
+ * mock this class when testing their code and mocking the `bigtable::Table`
+ * behavior.
+ */
+class ResultSourceInterface {
+ public:
+  virtual ~ResultSourceInterface() = default;
+
+  /**
+   * Returns the next row in the stream.
+   *
+   * @return if the stream is interrupted due to a failure the
+   *   `StatusOr<bigtable::QueryRow>` contains the error.  The function returns
+   * a successful `StatusOr<>` with a `bigtable::QueryRow` with an empty
+   * row_key() to indicate end-of-stream.
+   */
+  virtual StatusOr<bigtable::QueryRow> NextRow() = 0;
+
+  /**
+   * Returns metadata about the result set.
+   */
+  virtual absl::optional<google::bigtable::v2::ResultSetMetadata>
+  Metadata() = 0;
+};
+
+GOOGLE_CLOUD_CPP_INLINE_NAMESPACE_END
+}  // namespace bigtable
+}  // namespace cloud
+}  // namespace google
+
+#endif  // GOOGLE_CLOUD_CPP_GOOGLE_CLOUD_BIGTABLE_RESULTS_H