Add AtomicFile class for updating a file with rename-in-place

1 files changed, 80 insertions, 0 deletions

diff --git a/src/AtomicFile.cpp b/src/AtomicFile.cpp
new file mode 100644
index 00000000..994b8c1b
--- /dev/null
+++ b/src/AtomicFile.cpp
@@ -0,0 +1,80 @@
+// Copyright (C) 2019 Joel Rosdahl and other contributors
+//
+// See doc/AUTHORS.adoc for a complete list of contributors.
+//
+// This program is free software; you can redistribute it and/or modify it
+// under the terms of the GNU General Public License as published by the Free
+// Software Foundation; either version 3 of the License, or (at your option)
+// any later version.
+//
+// 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 General Public License for
+// more details.
+//
+// You should have received a copy of the GNU General Public License along with
+// this program; if not, write to the Free Software Foundation, Inc., 51
+// Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+
+#include "AtomicFile.hpp"
+
+#include "Error.hpp"
+#include "ccache.hpp"
+
+#include <cerrno>
+#include <fmt/core.h>
+#include <unistd.h>
+
+AtomicFile::AtomicFile(const std::string& path, Mode mode) : m_path(path)
+{
+  char* tmp_path = x_strdup(path.c_str());
+  int fd = create_tmp_fd(&tmp_path);
+  m_tmp_path = tmp_path;
+  m_stream.open(tmp_path,
+                mode == Mode::Binary ? std::ios::out | std::ios::binary
+                                     : std::ios::out);
+  free(tmp_path);
+  ::close(fd);
+
+  if (!m_stream) {
+    throw Error(fmt::format("failed to create {}: {}", path, strerror(errno)));
+  }
+}
+
+AtomicFile::~AtomicFile()
+{
+  if (m_stream.is_open()) {
+    // close() was not called so remove the lingering temporary file.
+    m_stream.close();
+    tmp_unlink(m_tmp_path.c_str());
+  }
+}
+
+void
+AtomicFile::write(const std::string& data)
+{
+  m_stream.write(data.data(), data.size());
+  if (!m_stream) {
+    throw Error(
+      fmt::format("failed to write data to {}: {}", m_path, strerror(errno)));
+  }
+}
+
+void
+AtomicFile::write(const std::vector<uint8_t>& data)
+{
+  m_stream.write(reinterpret_cast<const char*>(data.data()), data.size());
+  if (!m_stream) {
+    throw Error(
+      fmt::format("failed to write data to {}: {}", m_path, strerror(errno)));
+  }
+}
+
+void
+AtomicFile::close()
+{
+  m_stream.close();
+  if (x_rename(m_tmp_path.c_str(), m_path.c_str()) != 0) {
+    throw Error(fmt::format("failed to rename {} to {}", m_tmp_path, m_path));
+  }
+}