Re: memory leak in DBI XS bootstrap code

On Fri, Jun 30, 2006 at 04:08:46AM -0700, Ephraim Dan wrote:
This is why I love the perl community - you're willing to help even though you don't have time, since
you care about your code, and that other people can benefit from it.

But ... you sort of left a dangling "But" in your last post. Do you have a suggestion to fix the
_install_method problem?

I just ran valgrind with your fix - there seems to still be a leak. It is reported on DBI.xs line 216:
INIT_PERINTERP;
That looks to also have a Newz in it - is that what leaks?

Yes. It's basically a plain malloc and there's no mechanism to free it later.

Can you suggest how to fix that one?

The trick is to use newSV to alloc memory in the form of an SV
so that when the interpreter is destroyed the SV memory gets freed
automatically.

The _install_method leak is reported on line 3800 (in my modified DBI.xs) which validates your theory:
ima->usage_msg = savepv( (svp) ? SvPV(*svp,lna) : "");
If you can fix these, I'll be forever in your debt...

As above, and as outlined previously, use an SV.
Something like this (completely untested):

static void *
malloc_using_sv(STRLEN len) {
SV *sv = newSV(len);
return SvPVX(sv);
}

static char *
savepv_using_sv(char *str) {
char *buf = malloc_using_sv(strlen(str));
strcpy(buf, str);
return buf;
}

Tim.

--edan

--------------------------------------------------------------------------------------------------------

From: Tim Bunce [mailto:Tim.Bunce@xxxxxxxxx]
Sent: Fri 6/30/2006 11:35
To: Ephraim Dan
Cc: Tim Bunce; dbi-users@xxxxxxxx
Subject: Re: memory leak in DBI XS bootstrap code

On Fri, Jun 30, 2006 at 12:24:33AM -0700, Ephraim Dan wrote:
> > > I don't see what you mean in the "INSTALL" that comes with perl 5.8.0 (that's what we're using).
> > The file called INSTALL in the perl source code directory.
>
> That I knew. What are the special instructions that I'm supposed to
> find there? Am I supposed to build it with debugging? Can you
> specify what special configuration exactly you meant that I should use?

I'm sorry. I should have checked myself first. You can build perl with
-DPURIFY to enable more precise leak detection.

> It looks like the leak in boot_DBI is on purpose:
> /* Remember the last handle used. BEWARE! Sneaky stuff here! */
> /* We want a handle reference but we don't want to increment */
> /* the handle's reference count and we don't want perl to try */
> /* to destroy it during global destruction. Take care! */
> DBI_UNSET_LAST_HANDLE; /* ensure setup the correct way */
>
> Why is this being done, and does anyone have a way to fix it? Why don't we want perl to destroy it?
Me, that's exactly what I want.

I don't believe that's a leak. It's just using an SV as a reference
without telling perl its a reference. If perl knew it was a reference
we'd get double-free warnings as the referenced value would be freed twice.

The leak is probably the DBIS structure itself:

Newz(dummy, DBIS, 1, dbistate_t);

That should probably be using along the lines of:

sv = newSV(sizeof(struct dbistate_st));
DBIS = (struct dbistate_st*)SvPVX(sv);
memzero(DBIS, sizeof(struct dbistate_st));

So the memory is allocated within an SV so gets freed on global destruction.

> Still looking into the other leak in _install_method... any pointers still appreciated... Thanks for
your help so far, Tim.

I've taken a look at the code. I'd guess that it's due to savepnv:

ima->usage_msg = savepv( (svp) ? SvPV(*svp,lna) : "");

But

Tim.

> -edan
>
> > > -----Original Message-----
> > > From: Tim Bunce [[1]mailto:Tim.Bunce@xxxxxxxxx]
> > > Sent: Thursday, June 29, 2006 14:47
> > > To: Ephraim Dan
> > > Cc: dbi-users@xxxxxxxx
> > > Subject: Re: memory leak in DBI XS bootstrap code
> > >
> > > Try building perl with options to make valgrind leak tracing more
> > > effective (see perl's INSTALL file). That may help you pinpoint
> > > the problem.
> > >
> > > Tim.
> > >
> > > On Thu, Jun 29, 2006 at 04:33:40AM -0700, Ephraim Dan wrote:
> > > > I am experiencing what I believe to be a memory leak in the DBI
> > > bootstrap code. This is a problem for me because I am embedding perl in a
> > > long-running program, and DBI is being loaded over and over, so my program
> > > grows and grows.
> > > >
> > > > The problem appears to be in the following routines:
> > > >
> > > > boot_DBI (in /usr/lib/perl5/vendor_perl/5.8.0/i386-linux-thread-
> > > multi/auto/DBI/DBI.so)
> > > > XS_DBI__install_method (in /usr/lib/perl5/vendor_perl/5.8.0/i386-linux-
> > > thread-multi/auto/DBI/DBI.so)
> > > >
> > > > I am using DBI 1.51
> > > >
> > > > The tool "valgrind" ([2]http://valgrind.org <[3]http://valgrind.org/> ) can be used to reproduce
the
> > > leak using the following code:
> > > >
> > > > ---
> > > > File: embed_test.c
> > > > ---
> > > >
> > > > #include <EXTERN.h> /* from the Perl distribution */
> > > > #include <perl.h> /* from the Perl distribution */
> > > >
> > > > static PerlInterpreter *my_perl; /*** The Perl interpreter ***/
> > > > EXTERN_C void xs_init (pTHX); /*** init dyn. loading ***/
> > > >
> > > > int main(int argc, char **argv, char **env)
> > > > {
> > > > char *embedding[] = { "", "-e", "0" };
> > > > my_perl = perl_alloc();
> > > > perl_construct(my_perl);
> > > > perl_parse(my_perl, xs_init, 3, embedding, (char **)NULL);
> > > > PL_exit_flags |= PERL_EXIT_DESTRUCT_END;
> > > > perl_run(my_perl);
> > > > eval_pv("use DBI", TRUE);
> > > > perl_destruct(my_perl);
> > > > perl_free(my_perl);
> > > > }
> > > >
> > > > ---
> > > > File: Makefile
> > > > ---
> > > >
> > > > CC_OPTS = $(shell perl -MExtUtils::Embed -e ccopts)
> > > > LD_OPTS = $(shell perl -MExtUtils::Embed -e ldopts)
> > > >
> > > > EXE = embed_test
> > > >
> > > > $(EXE): xsinit.o embed_test.o
> > > > gcc -o $(EXE) embed_test.o xsinit.o $(LD_OPTS)
> > > >
> > > > embed_test.o: embed_test.c
> > > > gcc -c embed_test.c $(CC_OPTS)
> > > >
> > > > xsinit.o: xsinit.c
> > > > gcc -c xsinit.c $(CC_OPTS)
> > > >
> > > > xsinit.c:
> > > > perl -MExtUtils::Embed -e xsinit -- -o xsinit.c
> > > >
> > > > clean:
> > > > rm -f *.o xsinit.c $(EXE)
> > > >
> > > > ---
> > > > EOF
> > > > ---
> > > >
> > > > Can anyone suggest a fix for this? I'd be more than willing to take a
> > > patch to DBI 1.51 as soon as someone has one.
> > > >
> > > > Thanks,
> > > > Ephraim Dan
> > > >
>

References

Visible links
1. mailto:Tim.Bunce@xxxxxxxxx
2. http://valgrind.org/
3. http://valgrind.org/
.

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