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... 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.


In reply to Re^2: Perl Best Practices book: is this one a best practice or a dodgy practice? by eyepopslikeamosquito
in thread Perl Best Practices book: is this one a best practice or a dodgy practice? by eyepopslikeamosquito

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