As a software engineer, I often find myself deep in the trenches of
testing, ensuring the code I write is not just functional but also
resilient. Recently, I embarked on a refactoring project that involved
moving a script’s functionality into a new Perl module,
App::Workflow::Controller. This module’s job was to
orchestrate a few tasks: query a database via an external CLI tool,
fetch more details based on the results, and write outputs. This
naturally called for mocking external dependencies in the unit tests,
and that’s where the real fun began.

My go-to tool for this is usually Test::MockModule.
The module under test, App::Workflow::Controller, used:

• IPC::Run to call the
  external CLI tool.
• File::ShareDir
  to locate SQL files.
• Internal modules like App::Data::Fetcher and
  App::Utils.

Understanding
the Mocking Challenge: Exported Functions

The core challenge revolves around how Perl handles imported
functions. When a module like App::Workflow::Controller
uses use IPC::Run qw(run);, it copies the run
subroutine into its own namespace at compile time.

Key Insight 1: Mock in the Calling Namespace

As the Test::MockModule
documentation clearly states, to affect the run used
by App::Workflow::Controller, you must mock
App::Workflow::Controller::run. Mocking
IPC::Run::run after App::Workflow::Controller
is loaded has no effect on the already imported symbol.

# t/controller.t
use Test::More;
use Test::MockModule;
use App::Workflow::Controller; # Load the module first

# Mock run within App::Workflow::Controller's namespace
my $runner_mock = Test::MockModule->new('App::Workflow::Controller', no_auto => 1);
$runner_mock->mock('run', sub {
    my ($cmd_array_ref, @redirections) = @_;
    # Mocked logic to handle the command and return expected results
    # Example:
    if ($cmd_array_ref->[0] eq '/usr/bin/my_cli_tool') {
        my $out_ref = $redirections[1]; # Assuming '>'
        $$out_ref = "mocked output";
        return 1; # Simulate success
    }
    die "Unexpected command";
});

# ... tests calling methods of App::Workflow::Controller that use run ...

Key Insight 2: Timing Matters – BEGIN Blocks

For dependencies used during the compilation of
App::Workflow::Controller (e.g., in attribute defaults or
builders using File::ShareDir), the mocks must be
active before use App::Workflow::Controller; is
executed. This requires placing the Test::MockModule setup
for those specific dependencies inside a BEGIN block.

# t/controller.t
use strict;
use warnings;
use Test::More;
use Test::MockModule;
# ... other includes ...

my $mock_sql_file_path = '...'; # Path to mock SQL

BEGIN {
    # Mock File::ShareDir BEFORE App::Workflow::Controller is loaded
    my $fsd_mock = Test::MockModule->new('File::ShareDir');
    $fsd_mock->redefine('dist_file', sub { return $mock_sql_file_path; });
}

use App::Workflow::Controller;

# ... Mock IPC::Run as shown above ...

The IPC::Run Enigma: $self
Corruption

In this specific journey, despite correctly mocking
App::Workflow::Controller::run, I encountered bizarre
errors where the $self object within my module’s methods
seemed to be corrupted, sometimes appearing as the command path. This
indicated a deep, complex interaction between
Test::MockModule’s override mechanism and the way
IPC::Run manipulates the call stack or environment.

After extensive debugging, it seems that directly mocking the
imported run sub from IPC::Run can be
unstable, potentially due to the intricacies of how
IPC::Run works internally.

The
Pragmatic Solution: Mocking an Internal Wrapper

The most robust solution was to avoid mocking the imported
run directly. Instead, I refactored
App::Workflow::Controller to have a small, internal method
that wraps the call to IPC::Run::run:

# In App::Workflow::Controller.pm
sub _execute_cli {
    my ($self, @cmd_list) = @_;
    my $stdout;
    run \@cmd_list, '>', \$stdout or die "Failed";
    return $stdout;
}

sub some_other_method {
    my $self = shift;
    # ...
    my $output = $self->_execute_cli('/usr/bin/my_cli_tool', 'arg1');
    # ...
}

Then, in the test, I mock _execute_cli:

# t/controller.t
use Test::More;
use Test::MockModule;
use App::Workflow::Controller;

my $controller_mock = Test::MockModule->new('App::Workflow::Controller');

subtest 'Successful run' => sub {
    $controller_mock->redefine('_execute_cli', sub {
        my ($self, @cmd) = @_;
        # Check @cmd if necessary
        return "mocked cli output";
    });

    my $controller = App::Workflow::Controller->new();
    is($controller->some_other_method(), "expected result", "Test with mocked CLI");

    $controller_mock->unmock('_execute_cli');
};

This approach tests the logic around the CLI call without
interfering with IPC::Run itself.

Lessons Learned:

1. Mocking Exported Functions: Always mock in the
   namespace of the importing module.
2. Compile-Time Dependencies: Use BEGIN
   blocks to mock dependencies needed when the module under test is
   loaded.
3. IPC::Run is Special: Directly mocking
   the imported run function from IPC::Run can be
   fragile and lead to unexpected side effects like $self
   corruption.
4. Wrap and Mock: Encapsulating external calls like
   IPC::Run::run within internal methods provides a stable
   seam for mocking, avoiding direct interference with the complex external
   library.
5. State Management: our variables with
   local hash key assignments in subtests remain a good
   pattern for managing mock state across test cases.

This debugging journey was a deep dive into the nuances of Perl’s
symbol table, compile-time vs. runtime, and the limits of mocking. The
wrapper method pattern proved to be the most effective strategy for
handling complex dependencies like IPC::Run.