Re: Testers wanted: reentrant resolver

On Fri, 20 Feb 2004, Brian F. Feldman wrote:

> Daniel Eischen <eischen_at_vigrid.com> wrote:
> > On Fri, 20 Feb 2004, Brian F. Feldman wrote:
> > 
> > > Daniel Eischen <eischen_at_vigrid.com> wrote:
> > > > Ugh, can you put h_errno inside the per-thread res stuff.
> > > > We shouldn't need to have to add special hooks in the
> > > > threads libraries for this.
> > > 
> > > Please explain what you're saying further.  On correctly-threaded operating 
> > > systems, h_errno is just like errno -- and I made it act EXACTLY as errno 
> > > acts, and is per-thread storage for everything but the first thread.  It's 
> > > absolutely necessary if we want to return the correct errors; even if 
> > > everything else in the world is totally reentrant, if h_errno isn't, the 
> > > wrong errors can be returned!  What "special hooks" do you mean?  There's no 
> > > way to not change probably hundreds of lines of code without actually doing 
> > > the work to make h_errno thread-safe.  It's the only proper thing to do.
> > 
> > The implementation of __h_errno() need not depend on something
> > special stuffed in struct pthread.  Use thread-local storage
> > (pthread_getspecific()) like you did for the res_send_private
> > stuff.  Especially since these interfaces should be deprecated
> > in favor of what looks to be BIND 8.2.2 interfaces (according
> > to the Solaris man pages).
> 
> Other APIs have the option of failing.  __h_errno() does not have the option 
> of failing, so what do I do if pthread_key_create() fails?  Also, if 
> malloc() fails each time pthread_getspecific() returns NULL for the thread?

The API isn't thread-safe by design, so if malloc() fails,
just use the global errno.  A better design would be to
add the thread-safe interfaces I mention above, and have
the non-thread-safe interfaces first do the pthread_once(),
pthread_[gs]etspecific() thing and then call the thread-safe
interfaces.  Since the malloc() will be the first thing
in the entry point, you can fail right away:


	/* MT-safe */
	int
	res_nsend(res_state statp, const u_char *msg, int msglen,
	    u_char *answer, int anslen)
	{
		...
	}

	/* Not MT-safe */
	int
	res_send(const u_char *msg, int msglen, u_char *answer,
	    int anslen)
	{
		res_state statp;

		if (pthread_once(...)) {
			...
		}
		statp = pthread_getspecific(res_key);
		if (statp == NULL) {
			/* Allocate statp. */
			pthread_setspecific(res_key, statp);
		}
		return (res_nsend(statp, msg, msglen, answer, anslen));
	}

-- 
Dan Eischen