Test::Refcount(3pm) User Contributed Perl Documentation Test::Refcount(3pm)
NAME
"Test::Refcount" - assert reference counts on objects
SYNOPSIS
use Test::More tests => 2;
use Test::Refcount;
use Some::Class;
my $object = Some::Class->new();
is_oneref( $object, '$object has a refcount of 1' );
my $otherref = $object;
is_refcount( $object, 2, '$object now has 2 references' );
DESCRIPTION
The Perl garbage collector uses simple reference counting during the
normal execution of a program. This means that cycles or unweakened
references in other parts of code can keep an object around for longer
than intended. To help avoid this problem, the reference count of a new
object from its class constructor ought to be 1. This way, the caller
can know the object will be properly DESTROYed when it drops all of its
references to it.
This module provides two test functions to help ensure this property
holds for an object class, so as to be polite to its callers.
If the assertion fails; that is, if the actual reference count is
different to what was expected, either of the following two modules may
be used to assist the developer in finding where the references are.
• If Devel::MAT is installed, this test module will use it to dump
the state of the memory after a failure. It will create a .pmat
file named the same as the unit test, but with the trailing .t
suffix replaced with -TEST.pmat where "TEST" is the number of the
test that failed (in case there was more than one).
• If Devel::FindRef module is installed, a reverse-references trace
is printed to the test output.
See the examples below for more information.
FUNCTIONS
is_refcount
is_refcount( $object, $count, $name )
Test that $object has $count references to it.
is_oneref
is_oneref( $object, $name )
Assert that the $object has only 1 reference to it.
refcount
$count = refcount( $object )
Since version 0.09.
Returns the reference count of the given object as used by the test
functions. This is useful for making tests that don't care what the
count is before they start, but simply assert that the count hasn't
changed by the end.
use Test::Refcount import => [qw( is_refcount refcount )];
{
my $count = refcount( $object );
do_something( $object );
is_refcount( $object, $count, 'do_something() preserves refcount' );
}
EXAMPLE
Suppose, having written a new class "MyBall", you now want to check
that its constructor and methods are well-behaved, and don't leak
references. Consider the following test script:
use Test::More tests => 2;
use Test::Refcount;
use MyBall;
my $ball = MyBall->new();
is_oneref( $ball, 'One reference after construct' );
$ball->bounce;
# Any other code here that might be part of the test script
is_oneref( $ball, 'One reference just before EOF' );
The first assertion is just after the constructor, to check that the
reference returned by it is the only reference to that object. This
fact is important if we ever want "DESTROY" to behave properly. The
second call is right at the end of the file, just before the main scope
closes. At this stage we expect the reference count also to be one, so
that the object is properly cleaned up.
Suppose, when run, this produces the following output (presuming
Devel::MAT::Dumper is available):
1..2
ok 1 - One reference after construct
not ok 2 - One reference just before EOF
# Failed test 'One reference just before EOF'
# at ex.pl line 26.
# expected 1 references, found 2
# SV address is 0x55e14c310278
# Writing heap dump to ex-2.pmat
# Looks like you failed 1 test of 2.
This has written a ex-2.pmat file we can load using the "pmat" shell
and use the "identify" command on the given address to find where it
went:
$ pmat ex-2.pmat
Perl memory dumpfile from perl 5.28.1 threaded
Heap contains 25233 objects
pmat> identify 0x55e14c310278
HASH(0)=MyBall at 0x55e14c310278 is:
├─(via RV) the lexical $ball at depth 1 of CODE() at 0x55e14c3104a0=main_cv, which is:
│ └─the main code
└─(via RV) value {self} of HASH(2) at 0x55e14cacb860, which is (*A):
└─(via RV) value {cycle} of HASH(2) at 0x55e14cacb860, which is:
itself
(This document isn't intended to be a full tutorial on Devel::MAT and
the "pmat" shell; for that see Devel::MAT::UserGuide).
Alternatively, this produces the following output when using
Devel::FindRef instead:
1..2
ok 1 - One reference after construct
not ok 2 - One reference just before EOF
# Failed test 'One reference just before EOF'
# at demo.pl line 16.
# expected 1 references, found 2
# MyBall=ARRAY(0x817f880) is
# +- referenced by REF(0x82c1fd8), which is
# | in the member 'self' of HASH(0x82c1f68), which is
# | referenced by REF(0x81989d0), which is
# | in the member 'cycle' of HASH(0x82c1f68), which was seen before.
# +- referenced by REF(0x82811d0), which is
# in the lexical '$ball' in CODE(0x817fa00), which is
# the main body of the program.
# Looks like you failed 1 test of 2.
From this output, we can see that the constructor was well-behaved, but
that a reference was leaked by the end of the script - the reference
count was 2, when we expected just 1. Reading the trace output, we can
see that there were 2 references that could be found - one stored in
the $ball lexical in the main program, and one stored in a HASH. Since
we expected to find the $ball lexical variable, we know we are now
looking for a leak in a hash somewhere in the code. From reading the
test script, we can guess this leak is likely to be in the bounce()
method. Furthermore, we know that the reference to the object will be
stored in a HASH in a member called "self".
By reading the code which implements the bounce() method, we can see
this is indeed the case:
sub bounce
{
my $self = shift;
my $cycle = { self => $self };
$cycle->{cycle} = $cycle;
}
From reading the tracing output, we find that the HASH this object is
referenced in also contains a reference to itself, in a member called
"cycle". This comes from the last line in this function, a line that
purposely created a cycle, to demonstrate the point. While a real
program probably wouldn't do anything quite this obvious, the trace
would still be useful in finding the likely cause of the leak.
If neither "Devel::MAT::Dumper" nor "Devel::FindRef" are available,
then these detailed traces will not be produced. The basic reference
count testing will still take place, but a smaller message will be
produced:
1..2
ok 1 - One reference after construct
not ok 2 - One reference just before EOF
# Failed test 'One reference just before EOF'
# at demo.pl line 16.
# expected 1 references, found 2
# Looks like you failed 1 test of 2.
BUGS
• Temporaries created on the stack
Code which creates temporaries on the stack, to be released again
when the called function returns does not work correctly on perl
5.8 (and probably before). Examples such as
is_oneref( [] );
may fail and claim a reference count of 2 instead.
Passing a variable such as
my $array = [];
is_oneref( $array );
works fine. Because of the intention of this test module; that is,
to assert reference counts on some object stored in a variable
during the lifetime of the test script, this is unlikely to cause
any problems.
ACKNOWLEDGEMENTS
Peter Rabbitson <ribasushi@cpan.org> - for suggesting using core's "B"
instead of "Devel::Refcount" to obtain refcounts
AUTHOR
Paul Evans <leonerd@leonerd.org.uk>
perl v5.36.0 2022-12-13 Test::Refcount(3pm)
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