Introduce AtomicFile

lib/base/CMakeLists.txt

@@ -17,6 +17,7 @@ set(base_SOURCES
   application.cpp application.hpp application-ti.hpp application-version.cpp application-environment.cpp
   array.cpp array.hpp array-script.cpp
   atomic.hpp
+  atomic-file.cpp atomic-file.hpp
   base64.cpp base64.hpp
   boolean.cpp boolean.hpp boolean-script.cpp
   bulker.hpp

lib/base/atomic-file.cpp

@@ -0,0 +1,64 @@
+/* Icinga 2 | (c) 2022 Icinga GmbH | GPLv2+ */
+
+#include "base/atomic-file.hpp"
+#include "base/exception.hpp"
+#include "base/utility.hpp"
+#include <utility>
+
+#ifdef _WIN32
+#	include <windows.h>
+#else /* _WIN32 */
+#	include <errno.h>
+#	include <unistd.h>
+#endif /* _WIN32 */
+
+using namespace icinga;
+
+AtomicFile::AtomicFile(String path, int mode) : m_Path(std::move(path))
+{
+	exceptions(failbit | badbit);
+	m_TempFilename = Utility::CreateTempFile(m_Path + ".tmp.XXXXXX", mode, *this);
+}
+
+AtomicFile::~AtomicFile()
+{
+	if (!m_TempFilename.IsEmpty()) {
+		try {
+			Utility::Remove(m_TempFilename);
+		} catch (...) {
+			// A destructor must not throw
+		}
+	}
+}
+
+void AtomicFile::Commit()
+{
+	flush();
+
+	auto h ((*this)->handle());
+
+#ifdef _WIN32
+	if (!FlushFileBuffers(h)) {
+		auto err (GetLastError());
+
+		BOOST_THROW_EXCEPTION(win32_error()
+			<< boost::errinfo_api_function("FlushFileBuffers")
+			<< errinfo_win32_error(err)
+			<< boost::errinfo_file_name(m_TempFilename));
+	}
+#else /* _WIN32 */
+	if (fsync(h)) {
+		auto err (errno);
+
+		BOOST_THROW_EXCEPTION(posix_error()
+			<< boost::errinfo_api_function("fsync")
+			<< boost::errinfo_errno(err)
+			<< boost::errinfo_file_name(m_TempFilename));
+	}
+#endif /* _WIN32 */
+
+	close();
+
+	Utility::RenameFile(m_TempFilename, m_Path);
+	m_TempFilename = "";
+}

lib/base/atomic-file.hpp

@@ -0,0 +1,33 @@
+/* Icinga 2 | (c) 2022 Icinga GmbH | GPLv2+ */
+
+#ifndef ATOMIC_FILE_H
+#define ATOMIC_FILE_H
+
+#include "base/string.hpp"
+#include <boost/iostreams/device/file_descriptor.hpp>
+#include <boost/iostreams/stream.hpp>
+
+namespace icinga
+{
+
+/**
+ * Atomically replaces a file's content.
+ *
+ * @ingroup base
+ */
+class AtomicFile : public boost::iostreams::stream<boost::iostreams::file_descriptor>
+{
+public:
+	AtomicFile(String path, int mode);
+	~AtomicFile();
+
+	void Commit();
+
+private:
+	String m_Path;
+	String m_TempFilename;
+};
+
+}
+
+#endif /* ATOMIC_FILE_H */