Introduce AtomicFile

CMakeLists.txt

@@ -163,7 +163,7 @@ else()
   set(LOGROTATE_CREATE "\n\tcreate 644 ${ICINGA2_USER} ${ICINGA2_GROUP}")
 endif()
 
-find_package(Boost ${BOOST_MIN_VERSION} COMPONENTS coroutine context date_time filesystem thread system program_options regex REQUIRED)
+find_package(Boost ${BOOST_MIN_VERSION} COMPONENTS coroutine context date_time filesystem iostreams thread system program_options regex REQUIRED)
 
 # Boost.Coroutine2 (the successor of Boost.Coroutine)
 # (1) doesn't even exist in old Boost versions and

lib/base/CMakeLists.txt

@@ -16,6 +16,7 @@ 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 +1,116 @@
+/* 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 <io.h>
+#	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))
+{
+	m_TempFilename = m_Path + ".tmp.XXXXXX";
+
+#ifdef _WIN32
+	m_Fd = Utility::MksTemp(&m_TempFilename[0]);
+#else /* _WIN32 */
+	m_Fd = mkstemp(&m_TempFilename[0]);
+#endif /* _WIN32 */
+
+	if (m_Fd < 0) {
+		auto error (errno);
+
+		BOOST_THROW_EXCEPTION(posix_error()
+			<< boost::errinfo_api_function("mkstemp")
+			<< boost::errinfo_errno(error)
+			<< boost::errinfo_file_name(m_TempFilename));
+	}
+
+	try {
+		exceptions(failbit | badbit);
+
+		open(boost::iostreams::file_descriptor(
+			m_Fd,
+			// Rationale: https://github.com/boostorg/iostreams/issues/152
+			boost::iostreams::never_close_handle
+		));
+
+		if (chmod(m_TempFilename.CStr(), mode) < 0) {
+			auto error (errno);
+
+			BOOST_THROW_EXCEPTION(posix_error()
+				<< boost::errinfo_api_function("chmod")
+				<< boost::errinfo_errno(error)
+				<< boost::errinfo_file_name(m_TempFilename));
+		}
+	} catch (...) {
+		if (is_open()) {
+			close();
+		}
+
+		(void)::close(m_Fd);
+		(void)unlink(m_TempFilename.CStr());
+		throw;
+	}
+}
+
+AtomicFile::~AtomicFile()
+{
+	if (is_open()) {
+		try {
+			close();
+		} catch (...) {
+			// Destructor must not throw
+		}
+	}
+
+	if (m_Fd >= 0) {
+		(void)::close(m_Fd);
+	}
+
+	if (!m_TempFilename.IsEmpty()) {
+		(void)unlink(m_TempFilename.CStr());
+	}
+}
+
+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();
+	(void)::close(m_Fd);
+	m_Fd = -1;
+
+	Utility::RenameFile(m_TempFilename, m_Path);
+	m_TempFilename = "";
+}

lib/base/atomic-file.hpp

@@ -0,0 +1,34 @@
+/* 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;
+	int m_Fd;
+};
+
+}
+
+#endif /* ATOMIC_FILE_H */

lib/base/utility.hpp

@@ -134,6 +134,10 @@ public:
 
 	static String CreateTempFile(const String& path, int mode, std::fstream& fp);
 
+#ifdef _WIN32
+	static int MksTemp(char *tmpl);
+#endif /* _WIN32 */
+
 #ifdef _WIN32
 	static String GetIcingaInstallPath();
 	static String GetIcingaDataPath();
@@ -185,10 +189,6 @@ public:
 private:
 	Utility();
 
-#ifdef _WIN32
-	static int MksTemp (char *tmpl);
-#endif /* _WIN32 */
-
 #ifdef I2_DEBUG
 	static double m_DebugTime;
 #endif /* I2_DEBUG */