Try to fix safeWriteToFile producing empty files on Windows (#14085)

Use win32 APIs to write the temporary file before copying to the final
destination. Because we've observed the final file being empty, we
suspect that std::ostream::flush is not flushing.

Also add a test for it.

src/filesys.cpp

@@ -758,14 +758,32 @@ bool safeWriteToFile(const std::string &path, const std::string &content)
 	std::string tmp_file = path + ".~mt";
 
 	// Write to a tmp file
+	bool tmp_success = false;
+
+#ifdef _WIN32
+	// We've observed behavior suggesting that the MSVC implementation of std::ofstream::flush doesn't
+	// actually flush, so we use win32 APIs.
+	HANDLE tmp_handle = CreateFile(
+		tmp_file.c_str(), GENERIC_WRITE, 0, nullptr, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, nullptr);
+	if (tmp_handle == INVALID_HANDLE_VALUE) {
+		return false;
+	}
+	DWORD bytes_written;
+	tmp_success = (WriteFile(tmp_handle, content.c_str(), content.size(), &bytes_written, nullptr) &&
+					FlushFileBuffers(tmp_handle));
+	CloseHandle(tmp_handle);
+#else
 	std::ofstream os(tmp_file.c_str(), std::ios::binary);
-	if (!os.good())
+	if (!os.good()) {
 		return false;
+	}
 	os << content;
 	os.flush();
 	os.close();
-	if (os.fail()) {
-		// Remove the temporary file because writing it failed and it's useless.
+	tmp_success = !os.fail();
+#endif
+
+	if (!tmp_success) {
 		remove(tmp_file.c_str());
 		return false;
 	}
@@ -777,14 +795,12 @@ bool safeWriteToFile(const std::string &path, const std::string &content)
 	// When creating the file, it can cause Windows Search indexer, virus scanners and other apps
 	// to query the file. This can make the move file call below fail.
 	// We retry up to 5 times, with a 1ms sleep between, before we consider the whole operation failed
-	int number_attempts = 0;
-	while (number_attempts < 5) {
+	for (int attempt = 0; attempt < 5; attempt++) {
 		rename_success = MoveFileEx(tmp_file.c_str(), path.c_str(),
 				MOVEFILE_REPLACE_EXISTING | MOVEFILE_WRITE_THROUGH);
 		if (rename_success)
 			break;
 		sleep_ms(1);
-		++number_attempts;
 	}
 #else
 	// On POSIX compliant systems rename() is specified to be able to swap the

src/unittest/CMakeLists.txt

@@ -9,7 +9,7 @@ set (UNITTEST_SRCS
 	${CMAKE_CURRENT_SOURCE_DIR}/test_compression.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/test_connection.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/test_craft.cpp
-	${CMAKE_CURRENT_SOURCE_DIR}/test_filepath.cpp
+	${CMAKE_CURRENT_SOURCE_DIR}/test_filesys.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/test_inventory.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/test_irrptr.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/test_lua.cpp

src/unittest/test_filepath.cpp → src/unittest/test_filesys.cpp

@@ -26,10 +26,11 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "nodedef.h"
 #include "noise.h"
 
-class TestFilePath : public TestBase {
+class TestFileSys : public TestBase
+{
 public:
-	TestFilePath() { TestManager::registerTestModule(this); }
-	const char *getName() { return "TestFilePath"; }
+	TestFileSys() {	TestManager::registerTestModule(this); }
+	const char *getName() {	return "TestFileSys"; }
 
 	void runTests(IGameDef *gamedef);
 
@@ -38,17 +39,19 @@ class TestFilePath : public TestBase {
 	void testRemoveLastPathComponent();
 	void testRemoveLastPathComponentWithTrailingDelimiter();
 	void testRemoveRelativePathComponent();
+	void testSafeWriteToFile();
 };
 
-static TestFilePath g_test_instance;
+static TestFileSys g_test_instance;
 
-void TestFilePath::runTests(IGameDef *gamedef)
+void TestFileSys::runTests(IGameDef *gamedef)
 {
 	TEST(testIsDirDelimiter);
 	TEST(testPathStartsWith);
 	TEST(testRemoveLastPathComponent);
 	TEST(testRemoveLastPathComponentWithTrailingDelimiter);
 	TEST(testRemoveRelativePathComponent);
+	TEST(testSafeWriteToFile);
 }
 
 ////////////////////////////////////////////////////////////////////////////////
@@ -74,7 +77,7 @@ std::string p(std::string path)
 }
 
 
-void TestFilePath::testIsDirDelimiter()
+void TestFileSys::testIsDirDelimiter()
 {
 	UASSERT(fs::IsDirDelimiter('/') == true);
 	UASSERT(fs::IsDirDelimiter('A') == false);
@@ -87,7 +90,7 @@ void TestFilePath::testIsDirDelimiter()
 }
 
 
-void TestFilePath::testPathStartsWith()
+void TestFileSys::testPathStartsWith()
 {
 	const int numpaths = 12;
 	std::string paths[numpaths] = {
@@ -163,7 +166,7 @@ void TestFilePath::testPathStartsWith()
 }
 
 
-void TestFilePath::testRemoveLastPathComponent()
+void TestFileSys::testRemoveLastPathComponent()
 {
 	std::string path, result, removed;
 
@@ -200,7 +203,7 @@ void TestFilePath::testRemoveLastPathComponent()
 }
 
 
-void TestFilePath::testRemoveLastPathComponentWithTrailingDelimiter()
+void TestFileSys::testRemoveLastPathComponentWithTrailingDelimiter()
 {
 	std::string path, result, removed;
 
@@ -236,7 +239,7 @@ void TestFilePath::testRemoveLastPathComponentWithTrailingDelimiter()
 }
 
 
-void TestFilePath::testRemoveRelativePathComponent()
+void TestFileSys::testRemoveRelativePathComponent()
 {
 	std::string path, result;
 
@@ -262,3 +265,15 @@ void TestFilePath::testRemoveRelativePathComponent()
 	result = fs::RemoveRelativePathComponents(path);
 	UASSERT(result == p("/a/e"));
 }
+
+
+void TestFileSys::testSafeWriteToFile()
+{
+	const std::string dest_path = fs::TempPath() + DIR_DELIM + "testSafeWriteToFile.txt";
+	const std::string test_data("hello\0world", 11);
+	fs::safeWriteToFile(dest_path, test_data);
+	UASSERT(fs::PathExists(dest_path));
+	std::string contents_actual;
+	UASSERT(fs::ReadFile(dest_path, contents_actual));
+	UASSERT(contents_actual == test_data);
+}