In March I wrote The Perl debug­ger can be your super­pow­er, intro­duc­ing the step debug­ger as a bet­ter way to debug your Perl code rather than lit­ter­ing your source with tem­po­rary print state­ments or log­ging. I use the debug­ger all the time, and I’ve real­ized that some more tech­niques are worth covering.

Although I men­tioned a caveat when debug­ging web appli­ca­tions, our apps at work all adhere to the Perl Web Server Gateway Interface (PSGI) spec­i­fi­ca­tion and thus we can use tools like Test::WWW::Mechanize::PSGI or Plack::Test to run tests and debug­ging ses­sions in the same Perl process. (Mojolicious users can use some­thing like Test::Mojo for the same effect.)

To demon­strate, let’s get start­ed with some­thing like this which tests that a giv­en route (/say-hello) returns a cer­tain JSON struc­ture ({"message": "Hello world!"}):

#!/usr/bin/env perl

use Test::Most;
use Test::WWW::Mechanize::PSGI;
use JSON::MaybeXS;
use Local::MyApp; # name of app's main module

my $mech = Test::WWW::Mechanize::PSGI->new(
    # a Dancer2 app, so to_app returns a PSGI coderef
    app => Local::MyApp->to_app(),
);
$mech->get_ok('/say-hello');
lives_and {
    my $json = decode_json($mech->content);
    cmp_deeply( $json, {message => 'Hello world!'} );
} 'message is Hello world!';

done_testing;

All very fine and well, but what hap­pens if that route starts return­ing a dif­fer­ent mes­sage or worse, invalid out­put that caus­es decode_json to fail? Eventually, you’ll rewrite the test in the script to out­put the offend­ing con­tent when some­thing goes wrong, but right now you want to suss out the root cause.

Debuggers have the con­cept of break­points, which are flags that tell the debug­ger to stop at a cer­tain line of code and wait for instruc­tions. We can set them while run­ning the debug­ger with the b com­mand or con­tin­ue to a one-​time break­point with the c com­mand, or we can insert them into the code our­selves before run­ning it through the debug­ger in the first place.

Add this line right after the lives_and { line:

$DB::single = 1;

This sim­u­lates hav­ing typed the s com­mand in the debug­ger at that line, stop­ping exe­cu­tion at that point. Run our test with per­l’s -d option, and then type c to con­tin­ue to that breakpoint:

$ perl -d -Ilib t/test_psgi.t

Loading DB routines from perl5db.pl version 1.60
Editor support available.

Enter h or 'h h' for help, or 'man perldebug' for more help.

[Local::MyApp:7170] core @2021-07-06 07:33:22> Built config from files: /Users/mgardner/Projects/blog/myapp/config.yml /Users/mgardner/Projects/blog/myapp/environments/development.yml in (eval 310)[/Users/mgardner/.plenv/versions/5.34.0/lib/perl5/site_perl/5.34.0/Sub/Quote.pm:3] l. 910
Test2::API::CODE(0x7ffabea39ee8)(/Users/mgardner/.plenv/versions/5.34.0/lib/perl5/site_perl/5.34.0/Test2/API.pm:71):
71:	    INIT { eval 'END { test2_set_is_end() }; 1' or die [email protected] }

  DB<1> c

[...]
ok 1 - GET /say-hello
main::CODE(0x7f8069caf2c8)(t/test_psgi.t:14):
15:	    my $json = decode_json($mech->content);

  DB<1> 

From here we can exam­ine vari­ables, set oth­er break­points, or even exe­cute arbi­trary lines of code. Let’s see what became of that HTTP GET request:

  DB<1> x $mech->content

0  '{"error":"Undefined subroutine &Local::MyApp::build_frog called at lib/Local/MyApp.pm line 11.\\n"}'

  DB<2> 

Aha, some­thing has returned some dif­fer­ent JSON indi­cat­ing an error. Let’s look at the lines around (10−20) the offend­ing line (11):

  DB<2> f lib/Local/MyApp.pm

  DB<3> l 10-20

10:	        my $method = 'build_frog';
11:	        $method->();
12 	    }
13:	    catch ($e) {
14:	        send_as JSON => {error => $e};
15 	    }
16:	    send_as JSON => {message => 'Hello world!'};
17:	};
18
19 	sub build_frob {
20:	    return;

  DB<4>

Yep, a typo on line 11, and one that was­n’t caught at com­pile time since it’s gen­er­at­ed at runtime.

Just to be sure (and to demon­strate some oth­er cool debug­ger fea­tures), let’s set anoth­er break­point while in the debug­ger and then exer­cise that route again. Then we’ll check that $method vari­able against the list of avail­able meth­ods in the Local::MyApp package.

  DB<4> b 11

  DB<5> $mech->get('/say-hello')

[...]
Local::MyApp::CODE(0x7f8066f2db60)(lib/Local/MyApp.pm:11):
11:	        $method->();

  DB<<6>> x $method

0  'build_frog'

  DB<<7>> m Local::MyApp
any
app
body_parameters
build_frob
captures
config
content
[...]
  DB<<8>>

No doubt about it, that vari­able is being set incorrectly.

Quit out of the debug­ger with the q com­mand, make the fix (we prob­a­bly want errors to give some­thing oth­er than an HTTP 200 OK while we’re at it), and re-​run the test:

$ perl -Ilib t/test_psgi.t

[Local::MyApp:8277] core @2021-07-06 07:48:36> Built config from files: /Users/mgardner/Projects/blog/myapp/config.yml /Users/mgardner/Projects/blog/myapp/environments/development.yml in (eval 309) l. 910
Name "DB::single" used only once: possible typo at t/test_psgi.t line 13.
[...]
ok 1 - GET /say-hello
ok 2 - message is Hello world!
1..2

Note that warn­ing about leav­ing $DB::single in there. While harm­less, it’s a good reminder to remove such lines from your code so that they don’t sur­prise you or your team­mates dur­ing future debug­ging sessions.

And that’s it. Note that because we’re using PSGI, we were able to set break­points in our web app code itself and the debug­ger stopped there and enabled us to have a look around. And as you’ve seen, once you’re at a break­point you can switch to dif­fer­ent files, add/​remove more break­points, run arbi­trary code, and more. The perlde­bug doc­u­men­ta­tion page has all the details.

Happy debug­ging! For your ref­er­ence, here’s the full app mod­ule and test script used in this article:

MyApp.pm

package Local::MyApp;
use Dancer2;
use Feature::Compat::Try;

our $VERSION = '0.1';

get '/say-hello' => sub {
    try {
        no strict 'refs';
        my $method = 'build_frob';
        $method->();
    }
    catch ($e) {
        status 'error';
        send_as JSON => {error => $e};
    }
    send_as JSON => {message => 'Hello world!'};
};

sub build_frob {
    return;
}

true;

test_psgi.t

#!/usr/bin/env perl

use Test::Most;
use Test::WWW::Mechanize::PSGI;
use JSON::MaybeXS;
use Local::MyApp; # name of your app's main module goes here

my $mech = Test::WWW::Mechanize::PSGI->new(
    # a Dancer2 app, so to_app returns a PSGI coderef
    app => Local::MyApp->to_app(),
);
$mech->get_ok('/say-hello');
lives_and {
    my $json = decode_json($mech->content);
    cmp_deeply( $json, {message => 'Hello world!'} );
} 'message is Hello world!';

done_testing;

5 thoughts on “Perl debugger superpowers, part 2

  1. I found I can’t under­stand your last arti­cle. today I know this is because I even don’t know what is the debugger 🙁
    this first time I know perl have this func­tion, thanks.

  2. Great arti­cle as always. You do not know me. But you have helped me a lot in the past. So thank you.

    Btw, I think you should con­sid­er doing a debug­ger review of hdb some­time. I have used it and it’s very easy to use com­pared to some oth­er graph­i­cal debug­gers like ptkdb. Please con­sid­er doing a review of hdb. Thank you again.

    • I guess you mean Devel::hdb? I’m try­ing it out and it looks pret­ty neat. The one thing I miss is being able to exe­cute arbi­trary code dur­ing a break­point. It’s still worth a follow-​up arti­cle, though. Thanks!

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