Re^2: Perl Best Practices book: is this one a best practice or a dodgy practice?

[eyepopslikeamosquito]

... the second suggested solution--using the IO::Insitu module--does use a back-up strategy to ensure that data is not lost if the program abends.

True. But it is still not re-runnable. Which makes it dangerous in the hands of naive users who interrupt a program with CTRL-C, then re-run it. If they do that, they may suffer permanent data loss and without being aware of it.

It seems to me that you can get re-runnability with little extra effort: simply write the temporary file first and only overwrite the original (via atomic rename) after the temporary has been successfully written.

As a test, I pressed CTRL-C midway through running this test program:

use strict;
use warnings;

use IO::InSitu;

my $infile_name   = 'fred.tmp';
my $outfile_name  = $infile_name;

my ($in, $out) = open_rw($infile_name, $outfile_name);

for my $line (<$in>) {
    print {$out} transform($line);
}

# Try pressing CTRL-C while file is being updated.
sub transform {
    sleep 1;
    return "hello:" . $_[0];
}

This is what I saw:

total 20
drwxrwxr-x   2 andrew andrew 4096 Sep  3 14:44 ./
-rw-rw-r--   1 andrew andrew    0 Sep  3 14:42 fred.tmp
-rw-rw-r--   1 andrew andrew  191 Sep  3 14:42 fred.tmp.bak
drwxrwxr-x  11 andrew andrew 4096 Sep  3 14:42 ../
-rw-rw-r--   1 andrew andrew  288 Sep  3 14:41 tsitu1.pl

Now, of course, blindly re-running the test program resulted in permanent data loss (an empty fred.tmp file in this example).

Update: Just to clarify, this problem is broader than the naive user scenario given above and may bite you anytime a script is automatically rerun after an interruption -- a script that is run automatically at boot time, for example.

Further update: More detail on Win32 rename, related to tye's response below, can now be found at Re^7: Read in hostfile, modify, output.