How does siglongjmp work with multiple signal handlers

Question

I am following the APUE textbook on siglongjump(), and there is this piece of code which is confusing me.

• pr_mask() is simply a function which prints the signals which are masked.
• According to this, if USR1 signal comes, it gets added to the process mask and this is the handler.

  static void
  sig_usr1(int signo)
  {
      time_t  starttime;
  if (canjump == 0)
      return;     /* unexpected signal, ignore */
  
  pr_mask("starting sig_usr1: ");
  
  alarm(3);               /* SIGALRM in 3 seconds */
  starttime = time(NULL);
  for ( ; ; )             /* busy wait for 5 seconds */
      if (time(NULL) > starttime + 5)
          break;
  
  pr_mask("finishing sig_usr1: ");
  
  canjump = 0;
  siglongjmp(jmpbuf, 1);  /* jump back to main, don't return */
  
  }
  

• Now while USR1 is processing, SIG_ALARM comes, it will call the handler

  static void
  sig_alrm(int signo)
  {
      pr_mask("in sig_alrm: ");
  }
  

  and this SIG_ALARM will also get added to the proc mask. However, when sig_alarm() is done, then SIG_ALARM is going to be removed from the proc mask. This is illustrated when the call to pr_mask("finishing sig_usr1: "); is made in sig_usr1().

How is SIG_ALARM not masked when pr_mask("finishing sig_usr1") is called? siglongjump() should only restore the old mask when siglongjump() is called, which happens after this call.