Test::Refcount

Section: User Contributed Perl Documentation (3pm)
Updated: 2015-10-31
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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::FindRef module is installed, a reverse-references trace is printed to the test output.
•: 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).

See the examples below for more information.

FUNCTIONS

is_refcount( $object, $count, $name )

Test that $object has $count references to it.

is_oneref( $object, $name )

Assert that the $object has only 1 reference to it.

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::FindRef" 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 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 "Devel::FindRef" could find - 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 "Devel::FindRef" 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 "Devel::FindRef" is unavailable, 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>