Avoiding ref leaks in perl callback code
So, one of the big problems I’m having with kt3 is memory leaks. This is probably no surprise to anyone who has done XS programming before.
I found one particularly pernicious one by using Devel::Leak.. this is in the log handler.
Originally I had this, which I’m fairly sure I got from the internet somewhere:
static void _kt_log_callback(int iLogLevel, char *log_subsystem, char *msg)
{
dTHX;
dSP;
HV *rv = newHV();
sv_2mortal((SV *)rv);
hv_store(rv, "loglevel", 8, newSViv(iLogLevel),0);
hv_store(rv, "message", 7, newSVpv(msg, strnlen(msg, KT_LOGBUFSIZE)),0);
hv_store(rv, "system", 6, newSVpv(log_subsystem, strnlen(log_subsystem,32)),0);
PUSHMARK(SP);
XPUSHs(newRV_noinc((SV *)rv));
PUTBACK;
call_pv("KittenTrader::KittenBrain::Log", G_DISCARD);
}
This worked well enough – it created a hash ref and passed it to the function – but the hash ref kept leaking.
Eventually – after much digging through the documentation – I figured out that what I needed to do on the XPUSHs was this:
XPUSHs(sv_2mortal(newRV((SV *)rv)));
If anyone is curious how I debugged.. I used this relatively simple function to invoke the callback:
my $count = Devel::Leak::NoteSV($handle);
for($i=0;$i<1000;$i++) {
KittenTrader::KittenBrain::testLog(4, "THIS IS A MESSAGE");
}
my $count2 = Devel::Leak::NoteSV($handle);
Devel::Leak::CheckSV($handle);
print "Count: $count\n";
print "Count2: $count2\n";
This gave me a count of SVs, which I could clearly see my leaking SVs in.
Then I added returns everywhere along the path until I could isolate the leaking SV to a few lines of code. In particular I could definitely see that as soon as I created the hash, I was leaking a SV, but not if I destroyed it before passing it in. I realized the problem was that the callee didn't realize that the SV was mortal - I had made the hash mortal but not the *reference* to the hash. ANd of course the hash could never be destroyed until the reference to it was.