Test2::Plugin::Cover - Fast and Minimal file coverage info.


This plugin will collect minimal file coverage info, and will do so with minimal performance impact.

Every time a subroutine is called this tool will do its best to find the filename the subroutine was defined in, and add it to a list. Also, anytime you attempt to open a file with open() or sysopen() the file will be added to the list. This list will be attached to a test2 event just before the test exits. In most formaters the event will only show up as a comment on STDOUT # This test covered N source files. . However tools such as Test2::Harness::UI can make full use of the coverage information contained in the event.


This tool is not intended to record comprehensive coverage information, if you want that use Devel::Cover.

This tool is intended to obtain and maintain lists of files that were opened, or which define subs which were executed by any given test. This information is useful if you want to determine what test files to run after any given code change.

The collected coverage data is contained in test2 events, if you use Test2::Harness aka yath then this data can be logged and consumed by other tools such as Test2::Harness::UI.


Unlike tools that need to record comprehensive coverage (Devel::Cover), This module is only concerned about what files you open, or defined subs executed directly or indirectly by a given test file. As a result this module can get away with a tiny bit of XS code that only fires when a subroutine is called. Most coverage tools fire off XS for every statement.


This tool uses XS to inject a little bit of C code that runs every time a subroutine is called, or every time open() or sysopen() is called. This C code obtains the next op that will be run and tries to pull the filename from it. eval, XS, Moose, and other magic can sometimes mask the filename, this module only makes a minimal attempt to find the filename in these cases.

This tool DOES NOT cover anything beyond files in which subs executed by the test were defined. If you want sub names, lines executed, and more, use Devel::Cover.


The following data was gathered using prove to run the full Moose test suite:

    # Prove on its own
    Files=478, Tests=17326, 64 wallclock secs ( 1.62 usr  0.46 sys + 57.27 cusr  4.92 csys = 64.27 CPU)

    # Prove with Test2::Plugin::Cover (no coverage event)
    Files=478, Tests=17326, 67 wallclock secs ( 1.61 usr  0.46 sys + 60.98 cusr  5.31 csys = 68.36 CPU)

    # Prove with Devel::Cover
    Files=478, Tests=17324, 963 wallclock secs ( 2.39 usr  0.58 sys + 929.12 cusr 31.98 csys = 964.07 CPU)

no coverage event - No report was generated. This was done to only measure the effect of the XS that adds the data collection overhead, and not the cost of the perl code that generates the report event at the end of every test.

The Moose test suite was also run using Test2::Harness aka yath

    # Without Test2::Plugin::Cover
    Wall Time: 62.51 seconds CPU Time: 69.13 seconds (usr: 1.84s | sys: 0.08s | cusr: 60.77s | csys: 6.44s)

    # With Test2::Plugin::Cover (no coverage event)
    Wall Time: 75.46 seconds CPU Time: 82.00 seconds (usr: 1.96s | sys: 0.05s | cusr: 72.64s | csys: 7.35s)

As you can see, there is a performance hit, but it is fairly small, specially compared to Devel::Cover. This is not to say anything bad about Devel::Cover which is amazing, but a bad choice for the use case Test2::Plugin::Cover was written to address.



    use Test2::Plugin::Cover;


    # Arrayref of files covered so far
    my $covered_files = Test2::Plugin::Cover->files;


You can tell prove to use the module this way:

    HARNESS_PERL_SWITCHES=-MTest2::Plugin::Cover prove ...

This also works for Test2::Harness aka yath, but yath may have a flag to enable this for you by the time you are reading these docs.


You can suppess the final report (only collect data, do not send the Test2 event)


    HARNESS_PERL_SWITCHES=-MTest2::Plugin::Cover=no_event,1 prove ...


    use Test2::Plugin::Cover no_event => 1;


$arrayref = $class->files()
$arrayref = $class->files(filter => \&filter, extract => \&extract)

This will return an arrayref of all files touched so far. If no filter or extract callbacks are provided then $class->filter() and $class->extract() will be used as defaults.

The list of files will be sorted alphabetically, and duplicates will be removed.

Custom filter callbacks should match the interface for $class->filter().

Custom extract callbacks should match the interface for $class->extract().

$event = $class->report(%options)

This will send a Test2 event containing coverage information. It will also return the event.


root => Path::Tiny->new("...")

Normally this is set to the current directory at module load-time. This is used to filter out any source files that do not live under the current directory. This MUST be a Path::Tiny instance, passing a string will not work.

filter => sub { ... }

Normally $class->filter() is used.

extract => sub { ... }

Normally $class->extract() is used.

verbose => $BOOL

If this is set to true then the comment stating how many source files were touched will be printed as a diagnostics message instead so that it shows up without a verbose harness.

ctx => DO NOT USE

This is used ONLY when the Test2::API is doing its final book-keeping. Most users will never want to use this.


This will completely clear all coverage data so far.

$file_or_undef = $class->filter($file)
$file_or_undef = $class->filter($file, root => Path::Tiny->new('...'))

This method is used as a callback when getting the final list of covered source files. The default implementation removes any files that are not under the current directory which lets you focus on files in the distribution you are testing. You may return a modified filename if you wish to normalize it here, the default implementation will turn it into a relative path.

If you provide a custom root parameter, it MUST be a Path::Tiny instance, passing a string will not work.

A custom filter callback should look something like this:

    sub {
        my $class = shift;
        my ($file, %params) = @_;

        # clean_filename() does not exist, it is just an example
        $file = clean_filename($file, %params);

        # should_show() does not exist, it is just an example
        return $file if should_show(%params);

        # Return undef or an empty list if you do NOT want to show the file.

Please take a look at the source to see what and how filter() is implemented if you want all the details on how it works.

$file_or_undef = $class->extract($file)
$file_or_undef = $class->extract($file, %params)

This method is responsible for extracting a sensible filename from whatever the XS found. Some magic such as eval or Moose can set the filename to strings like '(eval 123)' or 'foo bar (defined at FILE line LINE)' or even nonsensical strings, or text with no filenames.

If a sensible file name can be extracted it will be returned, otherwise undef (or an empty list) is returned.

The default implementation does not use any parameters, but they are passed in for custom implementations to use.

A custom extract callback should look something like this:

    sub {
        my $class = shift;
        my ($file, %params) = @_;

        # It is a valid file
        return $file if -e $file;

        # Do not use this, just an example
        return $1 if $file =~ m/($VALID_FILE_REGEX)/;

        # Cannot find a file here


Devel::Cover is by far the best and most complete coverage tool for perl. If you need comprehensive coverage use Devel::Cover. Test2::Plugin::Cover is only better for a limited use case.


The source code repository for Test2-Plugin-Cover can be found at


Chad Granum <>


Chad Granum <>


Copyright 2020 Chad Granum <>.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.