Add a small testcase for snapshot writer.

update_engine heavily relies on snapshot writer, add a testcase

Test: th
Change-Id: I4e3cf47dcff92ec4df0b3b84a0a8c7cb7866e7e7

fs_mgr/libsnapshot/Android.bp

@@ -251,6 +251,7 @@ cc_defaults {
         "snapshot_metadata_updater_test.cpp",
         "snapshot_reader_test.cpp",
         "snapshot_test.cpp",
+        "snapshot_writer_test.cpp",
     ],
     shared_libs: [
         "libbinder",

fs_mgr/libsnapshot/snapshot_writer.cpp

@@ -90,7 +90,9 @@ std::unique_ptr<FileDescriptor> CompressedSnapshotWriter::OpenReader() {
     }
 
     const auto& cow_options = options();
-    reader->SetBlockDeviceSize(*cow_options.max_blocks * cow_options.block_size);
+    if (cow_options.max_blocks) {
+        reader->SetBlockDeviceSize(*cow_options.max_blocks * cow_options.block_size);
+    }
 
     return reader;
 }

fs_mgr/libsnapshot/snapshot_writer_test.cpp

@@ -0,0 +1,62 @@
+//
+// Copyright (C) 2021 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.
+//
+
+#include <libsnapshot/snapshot.h>
+
+#include <unordered_set>
+
+#include <android-base/file.h>
+#include <gtest/gtest.h>
+#include <libsnapshot/snapshot_writer.h>
+#include <payload_consumer/file_descriptor.h>
+
+namespace android::snapshot {
+class CompressedSnapshotWriterTest : public ::testing::Test {
+  public:
+    static constexpr size_t BLOCK_SIZE = 4096;
+};
+
+TEST_F(CompressedSnapshotWriterTest, ReadAfterWrite) {
+    TemporaryFile cow_device_file{};
+    android::snapshot::CowOptions options{.block_size = BLOCK_SIZE};
+    android::snapshot::CompressedSnapshotWriter snapshot_writer{options};
+    snapshot_writer.SetCowDevice(android::base::unique_fd{cow_device_file.fd});
+    snapshot_writer.Initialize();
+    std::vector<unsigned char> buffer;
+    buffer.resize(BLOCK_SIZE);
+    std::fill(buffer.begin(), buffer.end(), 123);
+
+    ASSERT_TRUE(snapshot_writer.AddRawBlocks(0, buffer.data(), buffer.size()));
+    ASSERT_TRUE(snapshot_writer.Finalize());
+    auto cow_reader = snapshot_writer.OpenReader();
+    ASSERT_NE(cow_reader, nullptr);
+    ASSERT_TRUE(snapshot_writer.AddRawBlocks(1, buffer.data(), buffer.size()));
+    ASSERT_TRUE(snapshot_writer.AddRawBlocks(2, buffer.data(), buffer.size()));
+    ASSERT_TRUE(snapshot_writer.Finalize());
+    // After wrigin some data, if we call OpenReader() again, writes should
+    // be visible to the newly opened reader. update_engine relies on this
+    // behavior for verity writes.
+    cow_reader = snapshot_writer.OpenReader();
+    ASSERT_NE(cow_reader, nullptr);
+    std::vector<unsigned char> read_back;
+    read_back.resize(buffer.size());
+    cow_reader->Seek(BLOCK_SIZE, SEEK_SET);
+    const auto bytes_read = cow_reader->Read(read_back.data(), read_back.size());
+    ASSERT_EQ((size_t)(bytes_read), BLOCK_SIZE);
+    ASSERT_EQ(read_back, buffer);
+}
+
+}  // namespace android::snapshot