Sorry if I miss these are already stated, but I couldn't find in draft #4.

   The underlying system call may return EINTR when the call is interrupted

   by a signal.  When it happens, unless otherwise noted, the SRFI procedures first

   call the registered Scheme signal handlers if necessary, then restart the system

   call as if it is  never interrupted.  In other words, they never raise syscall-error

   exception with errno/intr. 

In 3.9 Signal system, after the first paragraph:

   Inside a signal handler, you can execute any Scheme procedures, including

   raising an exceptions.  In general, this is realized by the runtime to record the incoming

   signal in the system's signal handler, and processing it at a "safe point" where

   execution of arbitrary Scheme expression is safe; for there's a severe restriciton

   in what you can do within the system's signal handler.   When a Scheme signal

   handler is executed, the same rule of signal masking as POSIX is applied.