file utilities : added write_binary_atomic() method.

openvpn/common/fileatomic.hpp

@@ -0,0 +1,64 @@
+//    OpenVPN -- An application to securely tunnel IP networks
+//               over a single port, with support for SSL/TLS-based
+//               session authentication and key exchange,
+//               packet encryption, packet authentication, and
+//               packet compression.
+//
+//    Copyright (C) 2012-2015 OpenVPN Technologies, Inc.
+//
+//    This program is free software: you can redistribute it and/or modify
+//    it under the terms of the GNU Affero General Public License Version 3
+//    as published by the Free Software Foundation.
+//
+//    This program is distributed in the hope that it will be useful,
+//    but WITHOUT ANY WARRANTY; without even the implied warranty of
+//    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+//    GNU Affero General Public License for more details.
+//
+//    You should have received a copy of the GNU Affero General Public License
+//    along with this program in the COPYING file.
+//    If not, see <http://www.gnu.org/licenses/>.
+
+// Atomic file-handling methods.
+
+#ifndef OPENVPN_COMMON_FILEATOMIC_H
+#define OPENVPN_COMMON_FILEATOMIC_H
+
+#include <stdio.h> // for rename()
+#include <errno.h>
+#include <cstring>
+
+#include <openvpn/common/platform.hpp>
+#include <openvpn/common/file.hpp>
+#include <openvpn/common/hexstr.hpp>
+#include <openvpn/random/randapi.hpp>
+
+#if defined(OPENVPN_PLATFORM_WIN)
+#error atomic file methods not supported on Windows
+#endif
+
+namespace openvpn {
+  // Atomically write binary buffer to file (relies on
+  // the atomicity of rename())
+  inline void write_binary_atomic(const std::string& filename,
+				  const Buffer& buf,
+				  RandomAPI& rng)
+  {
+    // generate temporary filename
+    unsigned char data[16];
+    rng.rand_fill(data);
+    const std::string tfn = filename + '.' + render_hex(data, sizeof(data));
+
+    // write to temporary file
+    write_binary(tfn, buf);
+
+    // then move into position
+    if (::rename(tfn.c_str(), filename.c_str()) == -1)
+      {
+	const int eno = errno;
+	OPENVPN_THROW(open_file_error, "error moving '" << tfn << "' -> '" << filename << "' : " << std::strerror(eno));
+      }
+  }
+}
+
+#endif