in reply to Avoiding race conditions

I have a calendaring system which I tinker with from time to time and I make PDF reports of schedules and such. What I did was make the text into a POD format, then use POD::Pdf (well, a hacked up version). But, that may not fit your needs :) I have used some other PDF tooks in the past and suggest trying a few to see what is best for your app.

As for the filenames. Is the 'user' actually logged in (via some way you have an equiv of REMOTE_USER)? I have used things like $ENV{REMOTE_USER}.$$.file.ext for simple anti-race names. I also use the following to generate a random name: 


my @set    = ('a' .. 'z', 'A' .. 'Z');
my $length = 8; # Change length if desired       
my $random = join "" => map {$set [rand @set]} 1 .. $length;
my $tmp_file = $random . "." . $$ . ".ext";

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Many people have their own way to get a unique name. I tend to like adding a login name (if available), the PID (assuming I will unlink() the file when done), and some random string. If you are extra paranoid (or extra careful :) you could also add a sempahore file with the name of your temp file, and lock it. If you can't get a lock on it, then the name must be in use and you can generate another.

Cheers,
KM

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Re: Re: Avoiding race conditions
by swiftone (Curate) on Apr 24, 2001 at 21:03 UTC
    POD::Pdf:
    My data is in two column table format, so I don't think POD will pull it off, but I'll give it a glance anyway. Does POD support pagebreaks?

    Unique features:
    No, sadly the user is not logged in. This isn't mod_perl, just straight CGI, so I could use the PID I suppose. hmm. Other than a cronjob, I have no idea how to automatically unlink the file, since the CGI creating the PDF will end presumably long before the PDF is completely downloaded.
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      My data is in two column table format, so I don't think POD will pull it off, but I'll give it a glance anyway. Does POD support pagebreaks?

      Yeah, I would experiment with other POD::*, or a *2pdf utility. POD doesn't do pagebreaks (as far as I have ever read), but the PDF module/utility will give you a way to define the page dimensions.. so it should add pagebreaks.

      No, sadly the user is not logged in. This isn't mod_perl, just straight CGI, so I could use the PID I suppose. hmm. Other than a cronjob, I have no idea how to automatically unlink the file, since the CGI creating the PDF will end presumably long before the PDF is completely downloaded.

      I do this: 

      my $out = pod2pdf(@args);
print "Content-Type: application/pdf\n";
print "Content-Disposition: attachment; filename=$username.tasks.pdf\n
+\n";
unlink "./pod/$username.tasks.$$.pod";
binmode(STDOUT);
return print $out;

      [download]

      Of course, YMMV depending on the module you use. I think I had hacked POD::Pdf to return the Pdf data instead of just writing it to the file. I don't recall. But, otherwise you could just read the file in and print it:

      my $out;
{ 
    local $/ = undef;
    open(PDF, "$file") or die "foo $!";
    $out = <PDF>;
    close PDF;
}

      [download]

      Or whatever fits your fancy.

      Cheers,
      KM

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[d/l]
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        oooh! Now we're getting to the good stuff: 
        print "Content-Type: application/pdf\n";
print "Content-Disposition: attachment; filename=$username.tasks.pdf\n\n";
        I didn't know about that "Content-Disposition" header. That fixes the unlinking problem.

        pause to test

        Neat! This actually solves the entire temporary filename aspect, since I now no longer need a file! ( I can just deliver the content directly). The previous problem was convincing the browser to not try to save pdfprogram.cgi?PARAMETERS=BLAH as the filename.

        Thanks a ton!

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