guile/libguile/guardians.c

/*	Copyright (C) 1998, 1999, 2000 Free Software Foundation, Inc.
 * 
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2, or (at your option)
 * any later version.
 * 
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
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 * along with this software; see the file COPYING.  If not, write to
 * the Free Software Foundation, Inc., 59 Temple Place, Suite 330,
 * Boston, MA 02111-1307 USA
 *
 * As a special exception, the Free Software Foundation gives permission
 * for additional uses of the text contained in its release of GUILE.
 *
 * The exception is that, if you link the GUILE library with other files
 * to produce an executable, this does not by itself cause the
 * resulting executable to be covered by the GNU General Public License.
 * Your use of that executable is in no way restricted on account of
 * linking the GUILE library code into it.
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/* Software engineering face-lift by Greg J. Badros, 11-Dec-1999,
   gjb@cs.washington.edu, http://www.cs.washington.edu/homes/gjb */



/* This is an implementation of guardians as described in
 * R. Kent Dybvig, Carl Bruggeman, and David Eby (1993) "Guardians in
 * a Generation-Based Garbage Collector" ACM SIGPLAN Conference on
 * Programming Language Design and Implementation, June 1993
 * ftp://ftp.cs.indiana.edu/pub/scheme-repository/doc/pubs/guardians.ps.gz
 *
 * By this point, the semantics are actually quite different from
 * those described in the abovementioned paper.  The semantic changes
 * are there to improve safety and intuitiveness.  The interface is
 * still (mostly) the one described by the paper, however.
 *
 * Original design:         Mikael Djurfeldt
 * Original implementation: Michael Livshin
 * Hacked on since by:      everybody
 */


#include "libguile/_scm.h"
#include "libguile/ports.h"
#include "libguile/print.h"
#include "libguile/smob.h"
#include "libguile/validate.h"
#include "libguile/properties.h"
#include "libguile/root.h"

#include "libguile/guardians.h"


/* The live and zombies FIFOs are implemented as tconcs as described
   in Dybvig's paper.  This decouples addition and removal of elements
   so that no synchronization between these needs to take place.
*/

typedef struct tconc_t
{
  SCM head;
  SCM tail;
} tconc_t;

#define TCONC_EMPTYP(tc) (SCM_EQ_P ((tc).head, (tc).tail))

#define TCONC_IN(tc, obj, pair) \
do { \
  SCM_SETCAR ((tc).tail, obj); \
  SCM_SETCAR (pair, SCM_BOOL_F); \
  SCM_SETCDR (pair, SCM_EOL); \
  SCM_SETCDR ((tc).tail, pair); \
  (tc).tail = pair; \
} while (0)

#define TCONC_OUT(tc, res) \
do { \
  (res) = SCM_CAR ((tc).head); \
  (tc).head = SCM_CDR ((tc).head); \
} while (0)


static scm_bits_t tc16_guardian;

typedef struct guardian_t
{
  tconc_t live;
  tconc_t zombies;
  struct guardian_t *next;
  int greedy_p;
  int listed_p;
} guardian_t;

#define GUARDIAN_P(x) SCM_SMOB_PREDICATE(tc16_guardian, x)
#define GUARDIAN(x) ((guardian_t *) SCM_CELL_WORD_1 (x))
#define GUARDIAN_LIVE(x) (GUARDIAN (x)->live)
#define GUARDIAN_ZOMBIES(x) (GUARDIAN (x)->zombies)
#define GUARDIAN_NEXT(x) (GUARDIAN (x)->next)
#define GUARDIAN_GREEDY_P(x) (GUARDIAN (x)->greedy_p)
#define GUARDIAN_LISTED_P(x) (GUARDIAN (x)->listed_p)


/* during the gc mark phase, live guardians are linked into the lists
   here. */
static guardian_t *greedy_guardians = NULL;
static guardian_t *sharing_guardians = NULL;

/* greedily guarded objects have this property set, so that we can
   catch any attempt to greedily guard them again */
static SCM greedily_guarded_prop = SCM_EOL;

/* this is the list of guarded objects that are parts of cycles.  we
   don't know in which order to return them from guardians, so we just
   unguard them and whine about it in after-gc-hook */
static SCM self_centered_zombies = SCM_EOL;


static void
add_to_live_list (SCM g)
{
  if (GUARDIAN_LISTED_P (g))
    return;

  if (GUARDIAN_GREEDY_P (g))
    {
      GUARDIAN_NEXT (g) = greedy_guardians;
      greedy_guardians = GUARDIAN (g);
    }
  else
    {
      GUARDIAN_NEXT (g) = sharing_guardians;
      sharing_guardians = GUARDIAN (g);
    }

  GUARDIAN_LISTED_P (g) = 1;
}

/* mark a guardian by adding it to the live guardian list.  */
static SCM
guardian_mark (SCM ptr)
{
  add_to_live_list (ptr);

  /* the objects protected by the guardian are not marked here: that
     would prevent them from ever getting collected.  instead marking
     is done at the end of the mark phase by guardian_zombify.  */
  return SCM_BOOL_F;
}


static scm_sizet
guardian_free (SCM ptr)
{
  scm_must_free (GUARDIAN (ptr));
  return sizeof (guardian_t);
}


static int
guardian_print (SCM g, SCM port, scm_print_state *pstate)
{
  scm_puts ("#<", port);
  if (GUARDIAN_GREEDY_P (g))
    scm_puts ("greedy ", port);
  scm_puts ("guardian (reachable: ", port);
  scm_display (scm_length (SCM_CDR (GUARDIAN_LIVE (g).head)), port);
  scm_puts (" unreachable: ", port);
  scm_display (scm_length (SCM_CDR (GUARDIAN_ZOMBIES (g).head)), port);
  scm_puts (")>", port);

  return 1;
}


/* This is the Scheme entry point for each guardian: If arg is an object, it's
 * added to the guardian's live list.  If arg is unbound, the next available
 * zombified object (or #f if none) is returned.
 */ 
static SCM
guardian_apply (SCM guardian, SCM arg)
{
  if (!SCM_UNBNDP (arg))
    {
      scm_guard (guardian, arg);
      return SCM_UNSPECIFIED;
    }
  else
    return scm_get_one_zombie (guardian);
}


void
scm_guard (SCM guardian, SCM obj)
{
  if (!SCM_IMP (obj))
    {
      SCM z;

      if (GUARDIAN_GREEDY_P (guardian))
        {
          if (SCM_NFALSEP (scm_primitive_property_ref
                           (greedily_guarded_prop, obj)))
            scm_misc_error ("guard",
                            "object is already greedily guarded", obj);
          else
            scm_primitive_property_set_x (greedily_guarded_prop,
                                          obj, SCM_BOOL_T);
        }

      SCM_NEWCELL (z);
      
      /* This critical section barrier will be replaced by a mutex. */
      SCM_DEFER_INTS;
      TCONC_IN (GUARDIAN_LIVE (guardian), obj, z);
      SCM_ALLOW_INTS;
    }
}


SCM
scm_get_one_zombie (SCM guardian)
{
  SCM res = SCM_BOOL_F;

  /* This critical section barrier will be replaced by a mutex. */
  SCM_DEFER_INTS;
  if (!TCONC_EMPTYP (GUARDIAN_ZOMBIES (guardian)))
    TCONC_OUT (GUARDIAN_ZOMBIES (guardian), res);
  SCM_ALLOW_INTS;

  if (SCM_NFALSEP (res)
      && GUARDIAN_GREEDY_P (guardian)
      && SCM_NFALSEP (scm_primitive_property_ref
                      (greedily_guarded_prop, res)))
    scm_primitive_property_del_x (greedily_guarded_prop, res);
  
  return res;
}


SCM_DEFINE (scm_make_guardian, "make-guardian", 0, 1, 0, 
            (SCM greedy_p),
            "Create a new guardian.\n"
	    "A guardian protects a set of objects from garbage collection,\n"
	    "allowing a program to apply cleanup or other actions.\n\n"

	    "make-guardian returns a procedure representing the guardian.\n"
	    "Calling the guardian procedure with an argument adds the\n"
	    "argument to the guardian's set of protected objects.\n"
	    "Calling the guardian procedure without an argument returns\n"
	    "one of the protected objects which are ready for garbage\n"
	    "collection or @code{#f} if no such object is available.\n"
	    "Objects which are returned in this way are removed from\n"
	    "the guardian.\n\n"

            "make-guardian takes one optional argument that says whether the\n"
            "new guardian should be greedy or not.  if there is any chance\n"
            "that any object protected by the guardian may be resurrected,\n"
            "then make the guardian greedy (this is the default).\n\n"

            "See R. Kent Dybvig, Carl Bruggeman, and David Eby (1993)\n"
            "\"Guardians in a Generation-Based Garbage Collector\".\n"
            "ACM SIGPLAN Conference on Programming Language Design\n"
            "and Implementation, June 1993.\n\n"

            "(the semantics are slightly different at this point, but the\n"
            "paper still (mostly) accurately describes the interface).")
#define FUNC_NAME s_scm_make_guardian
{
  guardian_t *g = SCM_MUST_MALLOC_TYPE (guardian_t);
  SCM z1 = scm_cons (SCM_BOOL_F, SCM_EOL);
  SCM z2 = scm_cons (SCM_BOOL_F, SCM_EOL);
  SCM z;

  /* A tconc starts out with one tail pair. */
  g->live.head = g->live.tail = z1;
  g->zombies.head = g->zombies.tail = z2;
  g->listed_p = 0;

  if (SCM_UNBNDP (greedy_p))
    g->greedy_p = 1;
  else
    g->greedy_p = SCM_NFALSEP (greedy_p);

  SCM_NEWSMOB (z, tc16_guardian, g);

  return z;
}
#undef FUNC_NAME


/* called before gc mark phase begins to initialise the live guardian list. */
static void *
guardian_gc_init (void *dummy1, void *dummy2, void *dummy3)
{
  greedy_guardians = sharing_guardians = NULL;

  return 0;
}

static void
mark_dependencies_in_tconc (tconc_t *tc)
{
  SCM pair, next_pair;
  SCM *prev_ptr;

  /* scan the list for unmarked objects, and mark their
     dependencies */
  for (pair = tc->head, prev_ptr = &tc->head;
       ! SCM_EQ_P (pair, tc->tail);
       pair = next_pair)
    {
      SCM obj = SCM_CAR (pair);
      next_pair = SCM_CDR (pair);
            
      if (! SCM_MARKEDP (obj))
        {
          /* a candidate for finalizing */
          scm_gc_mark_dependencies (obj);

          if (SCM_MARKEDP (obj))
            {
              /* uh oh.  a cycle.  transfer this object (the
                 spine cell, to be exact) to
                 self_centered_zombies, so we'll be able to
                 complain about it later. */
              *prev_ptr = next_pair;
              SCM_SETGCMARK (pair);
              SCM_SETCDR (pair, SCM_CDR (self_centered_zombies));
              SCM_SETCDR (self_centered_zombies, pair);
            }
          else
            {
              /* see if this is a guardian.  if yes, list it (but don't   
                 mark it yet). */
              if (GUARDIAN_P (obj))
                add_to_live_list (obj);

              prev_ptr = SCM_CDRLOC (pair);
            }
        }
    }
}

static void
mark_dependencies (guardian_t *g)
{
  mark_dependencies_in_tconc (&g->zombies);
  mark_dependencies_in_tconc (&g->live);
}

static void
mark_and_zombify (guardian_t *g)
{
  SCM tconc_tail = g->live.tail;
  SCM *prev_ptr = &g->live.head;
  SCM pair = g->live.head;

  while (! SCM_EQ_P (pair, tconc_tail))
    {
      SCM next_pair = SCM_CDR (pair);

      if (SCM_NMARKEDP (SCM_CAR (pair)))
        {
          /* got you, zombie! */

          /* out of the live list! */
          *prev_ptr = next_pair;

          if (g->greedy_p)
            /* if the guardian is greedy, mark this zombie now.  this
               way it won't be zombified again this time around. */
            SCM_SETGCMARK (SCM_CAR (pair));

          /* into the zombie list! */
          TCONC_IN (g->zombies, SCM_CAR (pair), pair);
        }
      else
        prev_ptr = SCM_CDRLOC (pair);

      pair = next_pair;
    }

  /* Mark the cells of the live list (yes, the cells in the list, we
     don't care about objects pointed to by the list cars, since we
     know they are already marked).  */
  for (pair = g->live.head; !SCM_NULLP (pair); pair = SCM_CDR (pair))
    SCM_SETGCMARK (pair);
}


/* this is called by the garbage collector between the mark and sweep
   phases.  for each marked guardian, it moves any unmarked object in
   its live list (tconc) to its zombie list (tconc).  */
static void *
guardian_zombify (void *dummy1, void *dummy2, void *dummy3)
{
  guardian_t *last_greedy_guardian = NULL;
  guardian_t *last_sharing_guardian = NULL;
  guardian_t *first_greedy_guardian = NULL;
  guardian_t *first_sharing_guardian = NULL;
  guardian_t *g;

  /* First, find all newly unreachable objects and mark their
     dependencies.
     
     Note that new guardians may be stuck on the end of the live
     guardian lists as we run this loop, since guardians might be
     guarded too.  When we mark a guarded guardian, its mark function
     sticks in the appropriate live guardian list.  The loop
     terminates when no new guardians are found. */

  do {
    first_greedy_guardian = greedy_guardians;
    first_sharing_guardian = sharing_guardians;

    for (g = greedy_guardians; g != last_greedy_guardian;
         g = g->next)
      mark_dependencies (g);
    for (g = sharing_guardians; g != last_sharing_guardian;
         g = g->next)
      mark_dependencies (g);

    last_greedy_guardian = first_greedy_guardian;
    last_sharing_guardian = first_sharing_guardian;
  } while (first_greedy_guardian != greedy_guardians
           || first_sharing_guardian != sharing_guardians);

  /* now, scan all the guardians that are currently known to be live
     and move their unmarked objects to zombie lists. */

  for (g = greedy_guardians; g; g = g->next)
    {
      mark_and_zombify (g);
      g->listed_p = 0;
    }
  for (g = sharing_guardians; g; g = g->next)
    {
      mark_and_zombify (g);
      g->listed_p = 0;
    }
  
  /* Preserve the zombies in their undead state, by marking to prevent
     collection. */
  for (g = greedy_guardians; g; g = g->next)
    scm_gc_mark (g->zombies.head);
  for (g = sharing_guardians; g; g = g->next)
    scm_gc_mark (g->zombies.head);

  return 0;
}

static void *
whine_about_self_centered_zombies (void *dummy1, void *dummy2, void *dummy3)
{
  if (! SCM_NULLP (SCM_CDR (self_centered_zombies)))
    {
      SCM pair;
      
      scm_puts ("** WARNING: the following guarded objects were unguarded due to cycles:",
                scm_cur_errp);
      scm_newline (scm_cur_errp);
      for (pair = SCM_CDR (self_centered_zombies);
           ! SCM_NULLP (pair); pair = SCM_CDR (pair))
        {
          scm_display (SCM_CAR (pair), scm_cur_errp);
          scm_newline (scm_cur_errp);
        }

      SCM_SETCDR (self_centered_zombies, SCM_EOL);
    }
  
  return 0;
}

void
scm_init_guardians ()
{
  tc16_guardian = scm_make_smob_type ("guardian", 0);
  scm_set_smob_mark (tc16_guardian, guardian_mark);
  scm_set_smob_free (tc16_guardian, guardian_free);
  scm_set_smob_print (tc16_guardian, guardian_print);
  scm_set_smob_apply (tc16_guardian, guardian_apply, 0, 1, 0);

  scm_c_hook_add (&scm_before_mark_c_hook, guardian_gc_init, 0, 0);
  scm_c_hook_add (&scm_before_sweep_c_hook, guardian_zombify, 0, 0);

  greedily_guarded_prop =
    scm_permanent_object (scm_primitive_make_property (SCM_BOOL_F));

  self_centered_zombies =
    scm_permanent_object (scm_cons (SCM_UNDEFINED, SCM_EOL));
  scm_c_hook_add (&scm_after_gc_c_hook,
                  whine_about_self_centered_zombies, 0, 0);

#ifndef SCM_MAGIC_SNARFER
#include "libguile/guardians.x"
#endif
}

/*
  Local Variables:
  c-file-style: "gnu"
  End:
*/