scsh-0.5/scsh/procobj.scm

;;; Unix wait & process objects for scsh
;;; Copyright (c) 1993, 1994, 1995 by Olin Shivers.

;;; This is a GC'd abstraction for Unix process id's.
;;; The problem with Unix pids is (a) they clutter up the kernel
;;; process table until you wait(2) them, and (b) you can only
;;; wait(2) them once. Scsh's process objects are similar, but
;;; allow the storage to be allocated in the scsh address space,
;;; and out of the kernel process table, and they can be waited on
;;; multiple times.

;;; Process objects
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(define-record proc		; A process object
  pid		; Proc's pid.
  (%status #f)	; The cached exit status of the process; 
                ; #f if we haven't wait(2)'d the process yet.

  ;; Make proc objects print like #{proc 2318}.
  ((disclose p) (list "proc" (proc:pid p))))


;;; Indexing this table by pid requires a linear scan. 
;;; Probably not an important op, tho.

(define process-table (make-population))

(define (maybe-pid->proc pid)
  (call/cc (lambda (quit)
	     ;; Search the table.
	     (walk-population (lambda (p)
				(if (= (proc:pid p) pid) (quit p)))
			      process-table)
	     #f)))

(define (pid->proc pid . maybe-probe?)
  (let ((probe? (:optional maybe-probe? #f)))
    (or (maybe-pid->proc pid)
	(case probe?
	  ((#f)     (error "Pid has no corresponding process object" pid))
	  ((create) (let ((p (make-proc pid))) 	; Install a new one.
		      (add-to-population! p  process-table)
		      p))
	  (else     #f)))))
	     
;;; Coerce pids and procs to procs.

(define (->proc proc/pid)
  (cond ((proc? proc/pid) proc/pid)
	((and (integer? proc/pid) (>= proc/pid 0))
	 (pid->proc proc/pid))
	(else (error "Illegal parameter" ->proc proc/pid))))


;;; Is X a pid or a proc?

(define (pid/proc? x) (or (proc? x) (and (integer? x) (>= pid 0))))


;;; Process reaping
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; "Reaping" a process means using wait(2) to move its exit status from the
;;; kernel's process table into scsh, thus cleaning up the kernel's process
;;; table and saving the value in a gc'd data structure, where it can be
;;; referenced multiple times.
;;;
;;; - Stopped processes are never reaped, only dead ones.
;;; 
;;; - Stopped process status codes are never cached in proc objects, 
;;;   only status codes for dead processes. So you can wait for a
;;;   dead process multiple times, but only once per process-stop.
;;; 
;;; - Unfortunately, reaping a process loses the information specifying its
;;;   process group, so if a process is reaped into scsh, it cannot be
;;;   waited for by WAIT-PROCESS-GROUP. Notice that only dead processes are
;;;   reaped, not suspended ones. Programs almost never use WAIT-PROCESS-GROUP
;;;   to wait for dead processes, so this is not likely to be a problem. If
;;;   it is, turn autoreaping off with (autoreap-policy #f).
;;; 
;;; - Reaping can be encouraged by calling (REAP-ZOMBIES).

;;; (autoreap-policy [new-policy])

(define *autoreap-policy* 'early) ; Not exported from this module.

(define (autoreap-policy . maybe-policy)
  (let ((old-policy *autoreap-policy*))
    (if (pair? maybe-policy)
	(let ((new-policy (car maybe-policy)))
	  (cond ((pair? (cdr maybe-policy))
		 (error "Too many args to autoreap-policy" maybe-policy))
		((not (memq new-policy '(early #f)))
		 (error "Illegal autoreap policy." new-policy))
		(else (set! *autoreap-policy* new-policy)))))
    old-policy))


;;; (reap-zombies)  => bool
;;;   Move any zombies from the kernel process table into scsh.
;;;   Return true if no more outstanding children; #f if some still live.

(define (reap-zombies)
  (let lp ()
    (receive (pid status) (%wait-any wait/poll)
      (if pid
	  (begin (add-reaped-proc! pid status)
		 (lp))
	  status))))

;;; This list contains procs that haven't exited yet. FORK adds new
;;; procs to the list. When a proc exits, it is removed from the list.
;;; Being on this list keeps live children's proc objects from being gc'd.

(define unexited-procs '())

(define (new-child-proc pid)
  (let ((proc (make-proc pid)))
    (add-to-population! proc process-table)
    (set! unexited-procs (cons proc unexited-procs))
    proc))

(define (mark-proc-exited proc)
  (set! unexited-procs (del proc unexited-procs)))


;;; (WAIT proc/pid [flags])
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;;; (wait proc/pid [flags]) => status or #f
;;;
;;; FLAGS (default 0) is the exclusive or of the following:
;;;     wait/poll	
;;;		Return #f immediately if there are no 
;;;		unwaited children available. 
;;; 	wait/stopped-children
;;; 		Report on suspended children as well.
;;;
;;;     If the process hasn't terminated (or suspended, if wait/stopped 
;;; 	is set) and wait/poll is set, return #f.

;;; WAIT waits for a specific process. Currently, if the autoreap policy is
;;; 'early, it also does a (reap-zombies) Before performing a waitpid(2)
;;; systcall, wait first consults the proc object to see if a/the process has
;;; been reaped already. If so, its saved status is returned immediately.
;;;

;;; (wait-any [flags]) => [proc status]
;;;     [#f #f] => non-blocking, none ready.
;;;     [#f #t] => no more.

;;; (wait-process-group [pid/proc flags]) => [proc status]
;;;     [#f #f] => non-blocking, none ready.
;;;     [#f #t] => no more.

(define (wait pid/proc . maybe-flags)
  (if (not *autoreap-policy*) (reap-zombies))
  (let ((flags (:optional maybe-flags 0))
	(proc (->proc pid/proc)))
    (cond ((proc:%status proc) =>	; Already reaped.
	   (lambda (status)
	     (mark-proc-waited! proc)	; Not eligible for a WAIT-ANY.
	     status))
	  (else				; Really wait.
	   (cache-wait-status proc (%wait-pid (proc:pid proc)
					      flags))))))

(define (cache-wait-status proc status)
  (cond ((and (integer? status)
	      (not (status:stop-sig status)))	; He's dead, Jim.
	 (set-proc:%status proc status)	; Cache exit status.
	 (mark-proc-exited proc)))	; We're now gc'able.
  status)


;;; (wait-any [flags]) -> [proc status]

(define (wait-any . maybe-flags)
  (if (not *autoreap-policy*) (reap-zombies))
  (cond ((get-reaped-proc!) =>			; Check internal table.
	 (lambda (proc) (values proc (proc:%status proc))))	; Hit.
	(else
	 (receive (pid status) (%wait-any (:optional maybe-flags 0)) ; Wait.
	   (if pid
	       (let ((proc (pid->proc pid)))
		 (cache-wait-status proc status)
		 (values proc status))
	       (values pid status))))))	; pid = #f -- Empty poll.


;;; (wait-process-group [proc-group flags])
;;; 
;;; If you are doing process-group waits, you do *not* want to use 
;;; early autoreaping, since the reaper loses process-group information.

(define (wait-process-group . args)
  (let-optionals args ((proc-group 0) (flags 0))
    (if (not *autoreap-policy*) (reap-zombies))
    (let ((proc-group (cond ((integer? proc-group) proc-group)
			     ((proc? proc-group)    (proc:pid proc-group))
			     (else (error "Illegal argument" wait-process-group
					  proc-group)))))
      (receive (pid status) (%wait-process-group proc-group flags)
	(if pid
	    (let ((proc (pid->proc pid)))
	      (cache-wait-status proc status)
	      (values proc status))
	    (values pid status))))))		; pid = #f -- Empty poll.



;;; (%wait-any flags) (%wait-pid pid flags) (%wait-process-group pgrp flags)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Direct interfaces to waitpid(2) call.
;;; [#f #f] means no processes ready on a non-blocking wait.
;;; [#f #t] means no waitable process on wait-any.

(define (%wait-pid pid flags)
  (let lp ()
    (receive (err pid status) (%wait-pid/errno pid flags)
      (cond ((not err) (and (not (zero? pid)) status))	; pid=0 => none ready.
	    ((= err errno/intr) (lp))
	    (else (errno-error err %wait-pid pid flags))))))

(define (%wait-any flags)
  (let lp ()
    (receive (err pid status) (%wait-pid/errno -1 flags)
      (cond (err (cond ((= err errno/child) (values #f #t))	; No more.
		       ((= err errno/intr)  (lp))
		       (else (errno-error err %wait-any flags))))
	    ((zero? pid) (values #f #f))			; None ready.
	    (else (values pid status))))))

(define (%wait-process-group pgrp flags)
  (let lp ()
    (receive (err pid status) (%wait-pid/errno (- pgrp) flags)
      (cond (err (cond ((= err errno/child) (values #f #t))	; No more.
		       ((= err errno/intr) (lp))
		       (else (errno-error err %wait-process-group pgrp flags))))
	    ((zero? pid) (values #f #f))			; None ready.
	    (else (values pid status))))))


;;; Reaped process table
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; We keep track of procs that have been reaped but not yet waited on by
;;; the user's code. These proces are eligible for return by WAIT-ANY.
;;; We keep track of these so that WAIT-ANY will hand them out exactly once.
;;; Whenever WAIT, WAIT-ANY, WAIT-PROCESS-GROUP waits on a process to exit,
;;; it removes the process from this table if it's in it.
;;; This code is bogus -- we use weak pointers. We need populations that
;;; support deletion or filtering.

(define reaped-procs '())	; Reaped, but not yet waited. 

(define (filter-weak-ptr-list pred lis)
  (reverse (reduce (lambda (wptr result)
			 (let ((val (weak-pointer-ref wptr)))
			   (if (and val (pred val))
			       (cons wptr result)
			       result)))
		   '()
		   lis)))

;;; Add a newly-reaped proc to the list.
(define (add-reaped-proc! pid status)
  (cond ((maybe-pid->proc pid) =>
         (lambda (proc)
	   (set-proc:%status proc status)
	   (set! reaped-procs (cons (make-weak-pointer proc)
				    reaped-procs))))
	(else (error "Child pid mysteriously missing proc object." pid))))
  
;;; Pop one off the list.
(define (get-reaped-proc!)
  (and (pair? reaped-procs)
       (let ((proc (weak-pointer-ref (car reaped-procs))))
	 (set! reaped-procs (cdr reaped-procs))
	 (or proc (get-reaped-proc!)))))

;;; PROC no longer eligible to be in the list. Delete it.
(define (mark-proc-waited! proc)
  (set! reaped-procs (filter-weak-ptr-list (lambda (elt) (not (eq? proc elt)))
					   reaped-procs)))

;;; The mark-proc-waited! machinery above is a crock. It is inefficient --
;;; we should have a flag in the proc saying if it's eligible for a WAIT-ANY.
;;; Starts off #t, changes to #f after a wait. On a #t->#f transition, we
;;; delete it from the WAIT-ANY population. Right now, every time the user
;;; waits on the proc, we re-delete it from the population -- which is
;;; a no-op after the first time.