[sf] Add TransactionTraceWriterTests

Bug: 238781169, 294989213
Test: presubmit
Change-Id: Ibeb4118b08424ae28f4f48442f977a433aeff18e
diff --git a/services/surfaceflinger/tests/unittests/TransactionTraceWriterTest.cpp b/services/surfaceflinger/tests/unittests/TransactionTraceWriterTest.cpp
new file mode 100644
index 0000000..379135e
--- /dev/null
+++ b/services/surfaceflinger/tests/unittests/TransactionTraceWriterTest.cpp
@@ -0,0 +1,79 @@
+/*
+ * Copyright 2023 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.
+ */
+
+#undef LOG_TAG
+#define LOG_TAG "TransactionTraceWriterTest"
+
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+#include <log/log.h>
+#include <filesystem>
+
+#include "TestableSurfaceFlinger.h"
+
+namespace android {
+
+class TransactionTraceWriterTest : public testing::Test {
+protected:
+    std::string mFilename = "/data/local/tmp/testfile_transaction_trace.winscope";
+
+    void SetUp() { mFlinger.initTransactionTraceWriter(); }
+    void TearDown() { std::filesystem::remove(mFilename); }
+
+    void verifyTraceFile() {
+        std::fstream file(mFilename, std::ios::in);
+        ASSERT_TRUE(file.is_open());
+        std::string line;
+        char magicNumber[8];
+        file.read(magicNumber, 8);
+        EXPECT_EQ("\tTNXTRAC", std::string(magicNumber, magicNumber + 8));
+    }
+
+    TestableSurfaceFlinger mFlinger;
+};
+
+TEST_F(TransactionTraceWriterTest, canWriteToFile) {
+    TransactionTraceWriter::getInstance().invokeForTest(mFilename, /* overwrite */ true);
+    EXPECT_EQ(access(mFilename.c_str(), F_OK), 0);
+    verifyTraceFile();
+}
+
+TEST_F(TransactionTraceWriterTest, canOverwriteFile) {
+    std::string testLine = "test";
+    {
+        std::ofstream file(mFilename, std::ios::out);
+        file << testLine;
+    }
+    TransactionTraceWriter::getInstance().invokeForTest(mFilename, /* overwrite */ true);
+    verifyTraceFile();
+}
+
+TEST_F(TransactionTraceWriterTest, doNotOverwriteFile) {
+    std::string testLine = "test";
+    {
+        std::ofstream file(mFilename, std::ios::out);
+        file << testLine;
+    }
+    TransactionTraceWriter::getInstance().invokeForTest(mFilename, /* overwrite */ false);
+    {
+        std::fstream file(mFilename, std::ios::in);
+        ASSERT_TRUE(file.is_open());
+        std::string line;
+        std::getline(file, line);
+        EXPECT_EQ(line, testLine);
+    }
+}
+} // namespace android
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