FindBin, __FILE__, $0 and PSGI woes

tl;dr: Do not use if (__FILE__ eq $0) in your .psgi code, that's not supposed to work since Plack 0.9971.

There's been a known issue in Starman and Plack based Perl web servers in general, where FindBin module used in your application doesn't return an expected value.

To make things even worse with Starman, when it hits the --max-requests and tries to restart the worker, it eventually crashes the process because the FindBin module can't even find the value of $0 and gets confused.

Today, after a quick discussion around this too frequently asked issues on the #plack IRC channel, I added a very simple commit that localizes the value of $0 to the path of .psgi file. It seems to solve those issues nicely: the use of FindBin correctly returns the expected path of .psgi file, and Starman doesn't crash when restarting.

However it turns out (thanks to clkao for the heads up), there're lots of .psgi files in the wild that has the following idiom, including my own scripts:

my $app = sub { ... };

if (__FILE__ eq $0) { 
    # called from the command line
    run_server_with($app);
} else {
    # called from PSGI web servers
    return $app;
}

It basically checks if __FILE__ is the same value as $0 and if it is so, consider that the .psgi (or .pl) script is being run from the command line, and try to launch the application with a server using modules like Plack::Runner. And the commit in question (already shipped as 0.9971 on CPAN) breaks this assumption.

The quick workaround we can think of are the use of other conditions, such as:

# PLACK_ENV is set by plackup etc.
unless ($ENV{PLACK_ENV}) { ... }

# caller is set if .psgi is evaluated
unless (caller) { ... }

# when Plack::Util::load_psgi() is called, package is automatically mangled
# similar to the Python equivalent idiom
if (__PACKAGE__ eq 'main') { ... }

I'm not 100% sure yet, whether it's better to revert the change, or to recommend everyone to stop using the if ($0 eq __FILE__) idiom in .psgi scripts and switch to one of these other conditions.

If you suffer from this bug, my apologies. You can either revert to earlier Plack release, or switch to one of those suggested other conditions, since they're meant to be safer and don't get affected by the $0 change.

If you have a really strong opinion that we should revert the change, chime in on a comment or on the irc channel #plack on irc.perl.org.

UPDATE: After another round of discussion, we agreed that this is a good change, and 0.9972 added a quick check in the development environment to see if your .psgi file contains that idiom (with a regular expression :)) to give you a warning in that case.