+int writeFileAtomicNoThrow(
+ StringPiece filename,
+ iovec* iov,
+ int count,
+ mode_t permissions) {
+ // We write the data to a temporary file name first, then atomically rename
+ // it into place. This ensures that the file contents will always be valid,
+ // even if we crash or are killed partway through writing out data.
+ //
+ // Create a buffer that will contain two things:
+ // - A nul-terminated version of the filename
+ // - The temporary file name
+ std::vector<char> pathBuffer;
+ // Note that we have to explicitly pass in the size here to make
+ // sure the nul byte gets included in the data.
+ constexpr folly::StringPiece suffix(".XXXXXX\0", 8);
+ pathBuffer.resize((2 * filename.size()) + 1 + suffix.size());
+ // Copy in the filename and then a nul terminator
+ memcpy(pathBuffer.data(), filename.data(), filename.size());
+ pathBuffer[filename.size()] = '\0';
+ const char* const filenameCStr = pathBuffer.data();
+ // Now prepare the temporary path template
+ char* const tempPath = pathBuffer.data() + filename.size() + 1;
+ memcpy(tempPath, filename.data(), filename.size());
+ memcpy(tempPath + filename.size(), suffix.data(), suffix.size());
+
+ auto tmpFD = mkstemp(tempPath);
+ if (tmpFD == -1) {
+ return errno;
+ }
+ bool success = false;
+ SCOPE_EXIT {
+ if (tmpFD != -1) {
+ close(tmpFD);
+ }
+ if (!success) {
+ unlink(tempPath);
+ }
+ };
+
+ auto rc = writevFull(tmpFD, iov, count);
+ if (rc == -1) {
+ return errno;
+ }
+
+ rc = fchmod(tmpFD, permissions);
+ if (rc == -1) {
+ return errno;
+ }
+
+ // Close the file before renaming to make sure all data has
+ // been successfully written.
+ rc = close(tmpFD);
+ tmpFD = -1;
+ if (rc == -1) {
+ return errno;
+ }
+
+ rc = rename(tempPath, filenameCStr);
+ if (rc == -1) {
+ return errno;
+ }
+ success = true;
+ return 0;
+}
+
+void writeFileAtomic(
+ StringPiece filename,
+ iovec* iov,
+ int count,
+ mode_t permissions) {
+ auto rc = writeFileAtomicNoThrow(filename, iov, count, permissions);
+ checkPosixError(rc, "writeFileAtomic() failed to update ", filename);
+}
+
+void writeFileAtomic(StringPiece filename, ByteRange data, mode_t permissions) {
+ iovec iov;
+ iov.iov_base = const_cast<unsigned char*>(data.data());
+ iov.iov_len = data.size();
+ auto rc = writeFileAtomicNoThrow(filename, &iov, 1, permissions);
+ checkPosixError(rc, "writeFileAtomic() failed to update ", filename);
+}
+
+void writeFileAtomic(
+ StringPiece filename,
+ StringPiece data,
+ mode_t permissions) {
+ writeFileAtomic(filename, ByteRange(data), permissions);
+}