Where is the memory leak in my Perl XS code?

Question

I have a large program written in C++ and executed from Perl using Inline::CPP. The code seems to be using a lot of memory, so I am assuming there is a leak somehow. I have written the following code that reproduces the same issue, but is much simpler. I found the leak when I looped the code 1,000,000 times to test the performance of the code. This simple Perl script uses 828MiB, and my full program uses 1.3GiB.

I have tried a lot of things, like using SvREFCNT_dec, newRV_noinc, sv_free, and sv_2mortal on various variables in the code, but I haven't been able to bring the memory usage down.

Here's my example code:

use Data::Dumper;

print Dumper test ();

use Inline 'CPP' => << 'CPP';
    #include <array>

    using namespace std;

    AV *array_to_av (const array<int,3> &v)
    {
        AV *array = newAV ();

        for (int i : v) {
            av_push (array, newSViv (i));
        }

        return array;
    }

    SV *test_leak ()
    {
        HV *hash = newHV ();

        AV *array1 = array_to_av ({1,2,3});
        AV *array2 = array_to_av ({1,2,3});
        AV *array3 = array_to_av ({1,2,3});

        SV *value1 = newRV_noinc ((SV *)(array1));
        SV *value2 = newRV_noinc ((SV *)(array2));
        SV *value3 = newRV_noinc ((SV *)(array3));

        hv_stores (hash, "Test1", value1);
        hv_stores (hash, "Test2", value2);
        hv_stores (hash, "Test3", value3);

        return newRV_noinc ((SV *)(hash));
    }

    SV *test ()
    {
        SV *hash;

        for (int i = 0; i < 1000000; i++) {
            hash = test_leak ();
        }

        return hash;
    }
CPP

sleep 10;

Answer 1

You need to free up the hashes that are not returned to the Perl script in the for loop. This loop:

for (int i = 0; i < 1000000; i++) {
    hash = test_leak ();
}

Should be something like:

for (int i = 0; i < 1000000; i++) {
    hash = test_leak ();
    SvREFCNT_dec(hash);  // Free the memory not returned to Perl
 }
 hash = test_leak();  // The final hashref is returned to Perl