Atomic file copying

gistfile1.m

- (BOOL)atomicCopyItemAtURL:(NSURL *)sourceURL
                      toURL:(NSURL *)destinationURL
                      error:(NSError **)outError
{
  NSFileManager *manager = [NSFileManager defaultManager];
  
  // First copy into a temporary location where failure doesn't matter
  NSURL *tempDir = [manager URLForDirectory:NSItemReplacementDirectory
                                   inDomain:NSUserDomainMask
                          appropriateForURL:destinationURL
                                     create:YES
                                      error:outError];
  
  if (!tempDir) return NO;
  
  NSURL *tempURL = [tempDir URLByAppendingPathComponent:[destinationURL lastPathComponent]];
  
  BOOL result = [manager copyItemAtURL:sourceURL toURL:tempURL error:outError];
  if (result)
  {
    // Move the complete item into place, replacing any existing item in the process
    NSURL *resultingURL;
    result = [manager replaceItemAtURL:destinationURL
                         withItemAtURL:tempURL
                        backupItemName:nil
                               options:NSFileManagerItemReplacementUsingNewMetadataOnly
                      resultingItemURL:&resultingURL
                                 error:outError];
    
    if (result)
    {
      NSAssert([resultingURL.absoluteString isEqualToString:destinationURL.absoluteString],
               @"URL unexpectedly changed during replacement from:\n%@\nto:\n%@",
               destinationURL,
               resultingURL);
    }
  }
  
  // Clean up
  NSError *error;
  if (![manager removeItemAtURL:tempDir error:&error])
  {
    NSLog(@"Failed to remove temp directory after atomic copy: %@", error);
  }
  
  return result;
}