Add unittest for set next op index

In order to add this unittest, we need to ensure that a checkpoint is
saved after each InstallOp. However, current logic throttles checkpoint
by time, it also skips checkpoint if buffer offset is the same. To
address these issues,
        1. Add a ShouldCheckpoint() to control whether to throttle
        checkpoint based on time, unittest overrides this and just
        return true
        2. Modify logic to only skip a checkpoint if operation_index
        hasn't changed. For SOURCE_COPY operations, buffer offset won't
        be advanced but operation index will.

Test: treehugger
Change-Id: Ib81bfe0c4ecb7200096b4b22390fb1f2fcff1581
diff --git a/payload_consumer/mock_partition_writer.h b/payload_consumer/mock_partition_writer.h
new file mode 100644
index 0000000..b056010
--- /dev/null
+++ b/payload_consumer/mock_partition_writer.h
@@ -0,0 +1,68 @@
+//
+// Copyright (C) 2020 The Android Open Source Project
+//
+// 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
+//
+//      http://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 UPDATE_ENGINE_MOCK_PARTITION_WRITER_H_
+#define UPDATE_ENGINE_MOCK_PARTITION_WRITER_H_
+
+#include <gmock/gmock.h>
+
+#include "common/error_code.h"
+#include "payload_generator/delta_diff_generator.h"
+#include "update_engine/payload_consumer/partition_writer.h"
+
+namespace chromeos_update_engine {
+class MockPartitionWriter : public PartitionWriter {
+ public:
+  MockPartitionWriter() : PartitionWriter({}, {}, nullptr, kBlockSize, false) {}
+  virtual ~MockPartitionWriter() = default;
+
+  // Perform necessary initialization work before InstallOperation can be
+  // applied to this partition
+  MOCK_METHOD(bool, Init, (const InstallPlan*, bool, size_t), (override));
+
+  // |CheckpointUpdateProgress| will be called after SetNextOpIndex(), but it's
+  // optional. DeltaPerformer may or may not call this everytime an operation is
+  // applied.
+  MOCK_METHOD(void, CheckpointUpdateProgress, (size_t), (override));
+
+  // These perform a specific type of operation and return true on success.
+  // |error| will be set if source hash mismatch, otherwise |error| might not be
+  // set even if it fails.
+  MOCK_METHOD(bool,
+              PerformReplaceOperation,
+              (const InstallOperation&, const void*, size_t),
+              (override));
+  MOCK_METHOD(bool,
+              PerformZeroOrDiscardOperation,
+              (const InstallOperation&),
+              (override));
+  MOCK_METHOD(bool,
+              PerformSourceCopyOperation,
+              (const InstallOperation&, ErrorCode*),
+              (override));
+  MOCK_METHOD(bool,
+              PerformSourceBsdiffOperation,
+              (const InstallOperation&, ErrorCode*, const void*, size_t),
+              (override));
+  MOCK_METHOD(bool,
+              PerformPuffDiffOperation,
+              (const InstallOperation&, ErrorCode*, const void*, size_t),
+              (override));
+};
+
+}  // namespace chromeos_update_engine
+
+#endif