Re: [ANNOUNCE] libblkio v0.1.0 preview release

[Daniel P . Berrangé]

On Thu, Apr 29, 2021 at 04:31:41PM +0100, Richard W.M. Jones wrote:
> On Thu, Apr 29, 2021 at 04:08:22PM +0100, Daniel P. Berrangé wrote:
> > On Thu, Apr 29, 2021 at 04:00:38PM +0100, Richard W.M. Jones wrote:
> > > On Thu, Apr 29, 2021 at 03:41:29PM +0100, Stefan Hajnoczi wrote:
> > > > On Thu, Apr 29, 2021 at 03:22:59PM +0100, Richard W.M. Jones wrote:
> > > > > On Thu, Apr 29, 2021 at 03:05:50PM +0100, Stefan Hajnoczi wrote:
> > > > > > The purpose of this preview release is to discuss both the API 
> > > > > > design
> > > > > > and general direction of the project. API documentation is available
> > > > > > here:
> > > > > > 
> > > > > >   https://gitlab.com/libblkio/libblkio/-/blob/v0.1.0/docs/blkio.rst
> > > > > 
> > > > > libvirt originally, and now libnbd, keep a per-thread error message
> > > > > (stored in thread-local storage).  It's a lot nicer than having to
> > > > > pass &errmsg to every function.  You can just write:
> > > > > 
> > > > >  if (nbd_connect_tcp (nbd, "remote", "nbd") == -1) {
> > > > >    fprintf (stderr,
> > > > >             "failed to connect to remote server: %s (errno = %d)\n",
> > > > >             nbd_get_error (), nbd_get_errno ());
> > > > >    exit (EXIT_FAILURE);
> > > > >  }
> > > > > 
> > > > > (https://libguestfs.org/libnbd.3.html#ERROR-HANDLING)
> > > > > 
> > > > > It means you can extend the range of error information available in
> > > > > future.  Also you can return a 'const char *' and the application
> > > > > doesn't have to worry about lifetimes, at least in the common case.
> > > > 
> > > > Thanks for sharing the idea, I think it would work well for libblkio
> > > > too.
> > > > 
> > > > Do you ignore the dlclose(3) memory leak?
> > > 
> > > I believe this mechanism in libnbd ensures there is no leak in the
> > > ordinary shared library (not dlopen/dlclose) case:
> > > 
> > > https://gitlab.com/nbdkit/libnbd/-/blob/f9257a9fdc68706a4079deb4ced61e1d468f28d6/lib/errors.c#L35
> > > 
> > > However I'm not sure what happens in the dlopen case, so I'm going to
> > > test that out now ...
> > 
> > pthread_key destructors are a disaster waiting to happen with
> > dlclose, if the dlclose happens while non-main threads are
> > still running. When those threads exit, they'll run the
> > destructor which points to a function that is no longer
> > resident in memory.
> > 
> > IOW if you have destrutors, then you need to make sure your
> > library uses "-z nodelete" when linking, to turn dlclose()
> > into a no-op.
> 
> I suspect letting the string leak is a better idea for libnbd.

Is dlclose() really that important ? libnbd is such a small
thing that i doubt anyone will even notice the space being
consumed if you mark it nodelete, and if an app is repeatedly
doing an op that triggers dlopen+dlclose many times, then
dlclose is especially useless.

Personally I think removing memory leaks on thread exit is
more important than honouring dlclose, as some apps can do
pathelogical things creating *many* very short lived
threads.

Regards,
Daniel
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