Re: patch 1/2 debuginfod client

[Mark Wielaard <mark at klomp dot org>]

Hi,

On Mon, 2019-11-18 at 15:33 -0500, Frank Ch. Eigler wrote:
> > > > Do you know how other libraries that use libcurl deal with this?
> > > 
> > > I looked over some other libcurl users in fedora.  Some don't worry
> > > about the issue at all, relying on implicit initialization, which is
> > > only safe if single-threaded.  Others (libvirtd) do an explicitly
> > > invoked initialization function, which is also only safe if invoked
> > > from a single-threaded context.
> > > 
> > > I think actually our way here, running the init function from the
> > > shared library constructor, is about the best possible.  As long as
> > > the ld.so process is done as normal, it should be fine.  Programs that
> > > use the elfutils trick of manual dlopen'ing libdebuginfod.so should do
> > > so only if they are single-threaded.
> > 
> > But they cannot really control that... Since they might not know (and
> > really shouldn't care) that libdw lazily tries to dlopen
> > libdebuginfod.so which then pulls in libcurl and calls that global init
> > function...
> > 
> > Could we do try to do the dlopen and global curl initialization from
> > libdw _init, or a ctor, to make sure it is done as early as possible? 
> 
> Doing a redundant initialization later is not a problem; there is a
> counter in there.  The problematic case would be
> - a multithreaded application
> - loading debuginfod.so multiply concurrently somehow
> - and calling the solib ctor concurrently somehow
> - and all of this concurrently enough to defeat libcurl's init-counter
> 
> IMHO, not worth worrying about.  Someday libcurl will do the right
> thing (tm) and plop this initialization into their solib ctor.

I do worry about this because any multi-threaded app that uses libdw.so
might cause trouble because we dlopen libdebuginfod.so lazily and then
it will call curl_global_init () which explicitly says:

   This function is not thread safe. You must not call it when any
   other thread in the program (i.e. a thread sharing the same memory)
   is running. This doesn't just mean no other thread that is using
   libcurl.  Because curl_global_init calls functions of other
   libraries that are similarly thread unsafe, it could conflict with
   any other thread that uses these other libraries.

That is why I think doing the dlopen of libdebuginfod.so eagerly from a
libdw.so constructor function or _init might be necessary to make sure
no other threads are running yet.

   > > > > I was more thinking zero == infinity (no timeout).
> > > 
> > > An unset environment variable should do that.
> > 
> > Are you sure? If DEBUGINFOD_TIMEOUT isn't set, then it seems it
> > defaults to 5 seconds:
> > 
> > /* Timeout for debuginfods, in seconds.
> >    This env var must be set for debuginfod-client to run.  */
> > static const char *server_timeout_envvar =
> > DEBUGINFOD_TIMEOUT_ENV_VAR;
> > static int server_timeout = 5;
> > [...]
> > 
> >   if (getenv(server_timeout_envvar))
> >     server_timeout = atoi (getenv(server_timeout_envvar));
> 
> OK, hm, we could make an -empty- but set environment variable mean
> 'infinity'.  Then again, a user can also say =99999.

Yes. In this case even setting it to 600 probably feels like forever
anyway since someone is waiting on the file... :)

Cheers,

Mark