Add interface to disable automatic finalization

* libguile/finalizers.h:
* libguile/finalizers.c (run_finalizers_async_thunk): Call the new
  scm_run_finalizers helper.
  (scm_set_automatic_finalization_enabled, scm_run_finalizers): New
  functions.
  (scm_init_finalizers): Only set a finalizer notifier if automatic
  finalization is enabled.

* doc/ref/libguile-smobs.texi (Garbage Collecting Smobs): Add discussion
  of concurrency.

* doc/ref/api-smobs.texi (Smobs): Document new functions.

doc/ref/api-smobs.texi

@@ -1,6 +1,6 @@
 @c -*-texinfo-*-
 @c This is part of the GNU Guile Reference Manual.
-@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2009, 2013
+@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2009, 2013, 2014
 @c   Free Software Foundation, Inc.
 @c See the file guile.texi for copying conditions.
 
@@ -60,6 +60,36 @@ memory is automatically reclaimed by the garbage collector when it is no
 longer needed (@pxref{Memory Blocks, @code{scm_gc_malloc}}).
 @end deftypefn
 
+Smob free functions must be thread-safe.  @xref{Garbage Collecting
+Smobs}, for a discussion on finalizers and concurrency.  If you are
+embedding Guile in an application that is not thread-safe, and you
+define smob types that need finalization, you might want to disable
+automatic finalization, and arrange to call
+@code{scm_manually_run_finalizers ()} yourself.
+
+@deftypefn {C Function} int scm_set_automatic_finalization_enabled (int enabled_p)
+Enable or disable automatic finalization.  By default, Guile arranges to
+invoke object finalizers automatically, in a separate thread if
+possible.  Passing a zero value for @var{enabled_p} will disable
+automatic finalization for Guile as a whole.  If you disable automatic
+finalization, you will have to call @code{scm_run_finalizers ()}
+periodically.
+
+Unlike most other Guile functions, you can call
+@code{scm_set_automatic_finalization_enabled} before Guile has been
+initialized.
+
+Return the previous status of automatic finalization.
+@end deftypefn
+
+@deftypefn {C Function} int scm_run_finalizers (void)
+Invoke any pending finalizers.  Returns the number of finalizers that
+were invoked.  This function should be called when automatic
+finalization is disabled, though it may be called if it is enabled as
+well.
+@end deftypefn
+
+
 @cindex precise marking
 
 @deftypefn {C Function} void scm_set_smob_mark (scm_t_bits tc, SCM (*mark) (SCM obj))

doc/ref/libguile-smobs.texi

@@ -1,6 +1,6 @@
 @c -*-texinfo-*-
 @c This is part of the GNU Guile Reference Manual.
-@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2005, 2010, 2011, 2013
+@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2005, 2010, 2011, 2013, 2014
 @c   Free Software Foundation, Inc.
 @c See the file guile.texi for copying conditions.
 
@@ -385,6 +385,27 @@ During the sweep phase, the garbage collector will clear the mark bits
 on all live objects.  The code which implements a smob need not do this
 itself.
 
+@cindex finalizer
+@cindex finalization
+
+Note that the free function can be called in any context.  In
+particular, if your Guile is built with support for threads, the
+finalizer may be called from any thread that is running Guile.  In Guile
+2.0, finalizers are invoked via ``asyncs'', which interleaves them with
+running Scheme code; @pxref{System asyncs}.  In Guile 2.2 there will be
+a dedicated finalization thread, to ensure that the finalization doesn't
+run within the critical section of any other thread known to Guile.
+
+In either case, finalizers (free functions) run concurrently with the
+main program, and so they need to be async-safe and thread-safe.  If for
+some reason this is impossible, perhaps because you are embedding Guile
+in some application that is not itself thread-safe, you have a few
+options.  One is to use guardians instead of free functions, and arrange
+to pump the guardians for finalizable objects.  @xref{Guardians}, for
+more information.  The other option is to disable automatic finalization
+entirely, and arrange to call @code{scm_run_finalizers ()} at
+appropriate points.  @xref{Smobs}, for more on these interfaces.
+
 There is no way for smob code to be notified when collection is
 complete.
 

libguile/finalizers.c

@@ -1,4 +1,4 @@
-/* Copyright (C) 2012 Free Software Foundation, Inc.
+/* Copyright (C) 2012, 2014 Free Software Foundation, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public License
@@ -31,6 +31,8 @@
 
 
 
+static int automatic_finalization_p = 1;
+
 static size_t finalization_count;
 
 
@@ -130,7 +132,7 @@ static SCM finalizer_async_cell;
 static SCM
 run_finalizers_async_thunk (void)
 {
-  finalization_count += GC_invoke_finalizers ();
+  scm_run_finalizers ();
   return SCM_UNSPECIFIED;
 }
 
@@ -169,6 +171,43 @@ GC_set_finalizer_notifier (void (*notifier) (void))
 }
 #endif
 
+
+
+
+int
+scm_set_automatic_finalization_enabled (int enabled_p)
+{
+  int was_enabled_p = automatic_finalization_p;
+
+  if (enabled_p == was_enabled_p)
+    return was_enabled_p;
+
+  if (!scm_initialized_p)
+    {
+      automatic_finalization_p = enabled_p;
+      return was_enabled_p;
+    }
+
+  GC_set_finalizer_notifier (enabled_p ? queue_finalizer_async : 0);
+
+  automatic_finalization_p = enabled_p;
+
+  return was_enabled_p;
+}
+
+int
+scm_run_finalizers (void)
+{
+  int finalized = GC_invoke_finalizers ();
+
+  finalization_count += finalized;
+
+  return finalized;
+}
+
+
+
+
 void
 scm_init_finalizers (void)
 {
@@ -178,5 +217,7 @@ scm_init_finalizers (void)
     scm_cons (scm_c_make_gsubr ("%run-finalizers", 0, 0, 0,
                                 run_finalizers_async_thunk),
               SCM_BOOL_F);
+
+  if (automatic_finalization_p)
     GC_set_finalizer_notifier (queue_finalizer_async);
 }

libguile/finalizers.h

@@ -1,7 +1,7 @@
 #ifndef SCM_FINALIZERS_H
 #define SCM_FINALIZERS_H
 
-/* Copyright (C) 2012 Free Software Foundation, Inc.
+/* Copyright (C) 2012, 2014 Free Software Foundation, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public License
@@ -34,6 +34,9 @@ SCM_INTERNAL void scm_i_add_finalizer (void *obj, scm_t_finalizer_proc,
 SCM_INTERNAL void scm_i_add_resuscitator (void *obj, scm_t_finalizer_proc,
                                           void *data);
 
+SCM_API int scm_set_automatic_finalization_enabled (int enabled_p);
+SCM_API int scm_run_finalizers (void);
+
 SCM_INTERNAL void scm_init_finalizers (void);
 
 #endif  /* SCM_FINALIZERS_H */