I added the above patch and the result is as follows:

After the following two lines:

    nread = read_process_output (proc, channel);

    pp = XPROCESS(proc);

nread==4095, pp->pid=1234 repeatedly. (Actually 1234 seems to be an
arbitrary, but constant number between 1000 and 2000).

This seems strange, as obviously the sub-process does not produce 4095
characters repeatedly.

Thanks,
Dov

On Mon, May 7, 2012 at 9:26 AM, Chong Yidong <cyd@gnu.org> wrote:

> Actually, try the following patch instead (apparently gdb has some
> issues with printing errno).  Apply the patch, then when Emacs is taking
> 100% CPU do an interrupt and set the breakpoint at process.c:4855, then
> when the breakpoint triggers do
>
> n
> p nread
> p errno
>
> and step through the subsequent if/else blocks.  Thanks.
>
> Basically, the 100% CPU appears to be because Emacs' select() call keeps
> getting worken up by the pty attached to your program.  But, for some
> reason, no actual output being read from that pty.  These debugging
> steps are trying to figure out if some uncaught errno is being reported
> by the pty read.
>
>
> === modified file 'src/process.c'
> *** src/process.c       2012-04-20 06:39:29 +0000
> --- src/process.c       2012-05-07 06:21:39 +0000
> ***************
> *** 4822,4827 ****
> --- 4822,4829 ----
>                && !FD_ISSET (channel, &non_process_wait_mask))
>            {
>              int nread;
> +             int saved_errno = 0;
> +             struct Lisp_Process *pp;
>
>              /* If waiting for this channel, arrange to return as
>                 soon as no more input to be processed.  No more
> ***************
> *** 4847,4852 ****
> --- 4849,4859 ----
>                  buffered-ahead character if we have one.  */
>
>              nread = read_process_output (proc, channel);
> +
> +             pp = XPROCESS (proc);
> +             if (pp->pid == -2)
> +               saved_errno = errno;
> +
>              if (nread > 0)
>                {
>                  /* Since read_process_output can run a filter,
>
>