_XtWaitForSomething calls either select or poll to wait.
On failure, it handles EINTR and EAGAIN fine, but drops to a generic error
handler on any other type of error.  The generic handler calls XtAppWarningMsg
and then continues the loop.

This behavior is acceptable for transient errors, e.g. ENOMEM, but for others,
unless there is a mechanism to correct the error on the fly, the result is
an infinite loop.

You can either assume that the user's XtAppWarningMsg routine will correct the
error in which case you must at least set app->rebuild_fdlist = TRUE before
continuing the loop
OR you assume that the error is permanent and call XtAppErrorMsg instead.
Errors like EINVAL and EFAULT should always be fatal in any case.