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.


The sysopen hook is currently disabled because of an unknown segv error on some platforms. I am not certain if it will be enabled again. calls to subs, and calls to open are still hooked.


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.

Originally this module only collected the filenames touched by a test. Now in addition to that data it can give you seperate lists of files where subs were called, and files that were touched via open(). Additionally the sub list includes the info about what subs were called. In all of these cases it is also possible to know what secgtions of your test called the subs or opened the files.


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 ...

For yath:

    yath test --cover-files ...


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;


If you use a system like Test::Class, Test::Class::Moose, or Test2::Tools::Spec then you divide your tests into subtests (or similar). In these cases it would be nice to track what subtest (or equivelent) touched what files.

There are 3 methods telated to this, set_from(), get_from(), and clear_from() which you can use to manage this meta-data:

    subtest foo => sub {
        # Note, this is a simple string, but the 'from' data can also be a data
        # structure.

        # subroutine() from will be recorded as having been called by 'foo'.


Doing this manually for all blocks is not ideal, ideally you would hook your tool, such as Test::Class to call set_from() and clear_from() for you. Adding such a hook is left as an exercide to the reader, and if you make one for a popular tool please upload it to cpan and add a ticket or send an email for me to link to it here.

Once you have these hooks in place the data will not only show files and subs that were called, but what called them.

Please see the set_from() documentation for details on values.


$class->touch_source_file($file, $sub)
$class->touch_source_file($file, \@subs)
$class->touch_source_file($file, $subs, $from)

This can be used to manually add coverage data. The first argument is the source file to be "touched" by coverage. The second argument is optional, and may be either a subroutine name, or an arrayref of subroutine names. The third argument is optional and can be used to override the default "from" value, which is normally determined for you automatically.

If no subroutines are specified it will default to using '*', which means the entire file is considered to be touched.

$class->touch_data_file($file, $from)

This can be used to manually add coverage data. The first argument is the file to be "touched" by coverage data. Optionally you can override the 'from' value which is normally determined automatically.

This is the same as calling $class->touch_source_file($file, '<>').

$bool = $class->enabled()

Toggle or check enabled status. When disabled no coverage is recorded.


Reset filter if $0 or __FILE__ have changed. This is advanced usage, you will probably never need this.

$val = $class->get_from()

Get the current 'from' value. The default is '*' when nothing has set a from value.


Set a 'from' value. This can be anything, a string, a hashref, etc. Be advised though that it will usually be serialized to JSON, so make sure anything you put in it will be serializable as json.


Resets the clear value to '*'

$bool = $class->was_from_modified()

This will return true if anything has called set_from() or set_from_manager. This can be reset back to false using reset_from(), which also clears the 'from' and 'from_manager' values.


This should be set to a module that implements the following method:

    sub test_parameters {
        my $class = shift;
        my ($test_file, \@from_values) = @_;


        return {
            # If true - run the test
            # If false - skip the test
            # If not present or undef - run the test
            run => $bool,

            # The following are optional
            argv  => [ ... ],
            env   => { ... },
            stdin => "...",

        # OR
        # If true - run the test
        # If false - skip the test
        # If undef or empty list - run the test
        return $bool;

This will be used by Test2::Harness to determine what data needs to be passed to a test given a set of 'from' values to instruct the test to run the necessary parts/subtests/groups/methods/etc.

The 'argv' data will be prepended befor any other arguments provided to the test.

The 'env' hashref will be merged with any other env vars needed, with these taking priority.

The 'stdin' string will be used as STDIN for the test.

$arrayref = $class->files()
$arrayref = $class->files(root => $path)

This will return an arrayref of all files touched so far.

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

If a root path is provided it MUST be a Path::Tiny instance. This path will be used to filter out any files not under the root directory.

$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.

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.


This will clear the 'from' value, as well as reset the 'was_from_modified' state to false.


Calls both reset_coverage() and reset_from().

$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.