Lightweight Databases

Copy With Changes

This version (which we saw earlier) is safer:

        sub delete_user {
          my ($file, $target_user) = @_;
          open my $rfh, "<", $file or die ...;
          open my $wfh, ">", "$file.tmp" or die ...;
          while (<$rfh>) {
            my ($user) = split /:/;
            print $wfh unless $user eq $target_user;
          }
          close $rfh or die ...; 
          close $wfh or die ...;
          rename "$file.tmp", $file or die ...;
        }

rename is guaranteed to be atomic:
- At every instant, exactly one version of the file exists
- If the function fails, or Perl crashes, the old file is untouched
- At the moment the rename succeeds, the entire new file is in place
- (Warning: file.tmp and file must be on the same filesystem)
Why doesn't -i do it this way?
- No good reason; coming in 5.10.