Simplifying revealed ports

[Ludovic Courtès]

Hello,

While debugging <https://bugs.gnu.org/28784>, I was surprised to see
that revealed ports were not GC’d at all.  The manual suggests otherwise
(info "(guile) Ports and File Descriptors"):

  If a port’s revealed count is greater than zero, the file descriptor
  will not be closed when the port is garbage collected.

That revealed ports are not GC’d makes it easy to shoot oneself in the
foot the way I did (huge memory leak).

So what about the simplification below?

It creates one observable difference: ‘close-port’ does nothing on a
revealed port, whereas until now it would close the port even if it has
a non-zero revealed count.  I would argue that it’s acceptable though.

Thoughts?

Ludo’.

diff --git a/libguile/fports.c b/libguile/fports.c
index 94092b872..ee6ac0bf1 100644
--- a/libguile/fports.c
+++ b/libguile/fports.c
@@ -1,6 +1,6 @@
 /* Copyright (C) 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003,
  *   2004, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013,
- *   2014, 2015 Free Software Foundation, Inc.
+ *   2014, 2015, 2017 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
@@ -467,8 +467,6 @@ fport_input_waiting (SCM port)
 
 #define SCM_REVEALED(x) (SCM_FSTREAM(x)->revealed)
 
-static SCM revealed_ports = SCM_EOL;
-static scm_i_pthread_mutex_t revealed_lock = SCM_I_PTHREAD_MUTEX_INITIALIZER;
 
 /* Find a port in the table and return its revealed count.
    Also used by the garbage collector.
@@ -476,13 +474,7 @@ static scm_i_pthread_mutex_t revealed_lock = 
SCM_I_PTHREAD_MUTEX_INITIALIZER;
 int
 scm_revealed_count (SCM port)
 {
-  int ret;
-
-  scm_i_pthread_mutex_lock (&revealed_lock);
-  ret = SCM_REVEALED (port);
-  scm_i_pthread_mutex_unlock (&revealed_lock);
-
-  return ret;
+  return SCM_REVEALED (port);
 }
 
 SCM_DEFINE (scm_port_revealed, "port-revealed", 1, 0, 0,
@@ -503,25 +495,14 @@ SCM_DEFINE (scm_set_port_revealed_x, 
"set-port-revealed!", 2, 0, 0,
            "The return value is unspecified.")
 #define FUNC_NAME s_scm_set_port_revealed_x
 {
-  int r, prev;
+  int r;
 
   port = SCM_COERCE_OUTPORT (port);
   SCM_VALIDATE_OPFPORT (1, port);
 
   r = scm_to_int (rcount);
-
-  scm_i_pthread_mutex_lock (&revealed_lock);
-
-  prev = SCM_REVEALED (port);
   SCM_REVEALED (port) = r;
 
-  if (r && !prev)
-    revealed_ports = scm_cons (port, revealed_ports);
-  else if (prev && !r)
-    revealed_ports = scm_delq_x (port, revealed_ports);
-
-  scm_i_pthread_mutex_unlock (&revealed_lock);
-
   return SCM_UNSPECIFIED;
 }
 #undef FUNC_NAME
@@ -539,18 +520,7 @@ SCM_DEFINE (scm_adjust_port_revealed_x, 
"adjust-port-revealed!", 2, 0, 0,
   SCM_VALIDATE_OPFPORT (1, port);
 
   a = scm_to_int (addend);
-  if (!a)
-    return SCM_UNSPECIFIED;
-
-  scm_i_pthread_mutex_lock (&revealed_lock);
-
   SCM_REVEALED (port) += a;
-  if (SCM_REVEALED (port) == a)
-    revealed_ports = scm_cons (port, revealed_ports);
-  else if (!SCM_REVEALED (port))
-    revealed_ports = scm_delq_x (port, revealed_ports);
-
-  scm_i_pthread_mutex_unlock (&revealed_lock);
 
   return SCM_UNSPECIFIED;
 }
@@ -668,6 +638,11 @@ fport_close (SCM port)
 {
   scm_t_fport *fp = SCM_FSTREAM (port);
 
+  if (SCM_REVEALED (port) > 0)
+    /* The port has a non-zero revealed count, so don't close the
+       underlying file descriptor.  */
+    return;
+
   scm_run_fdes_finalizers (fp->fdes);
   if (close (fp->fdes) != 0)
     /* It's not useful to retry after EINTR, as the file descriptor is
diff --git a/libguile/fports.h b/libguile/fports.h
index afb8ba771..e397fcc59 100644
--- a/libguile/fports.h
+++ b/libguile/fports.h
@@ -3,7 +3,8 @@
 #ifndef SCM_FPORTS_H
 #define SCM_FPORTS_H
 
-/* Copyright (C) 1995,1996,1997,1998,1999,2000,2001, 2006, 2008, 2009, 2011, 
2012 Free Software Foundation, Inc.
+/* Copyright (C) 1995-2001, 2006, 2008, 2009, 2011, 2012,
+ *   2017 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
@@ -33,9 +34,8 @@
 typedef struct scm_t_fport {
   /* The file descriptor.  */
   int fdes;
-  /* Revealed count; 0 indicates not revealed, > 1 revealed. Revealed
-     ports do not get garbage-collected.  */
-  int revealed;
+  /* Revealed count; 0 indicates not revealed, > 1 revealed.  */
+  unsigned int revealed;
   /* Set of scm_fport_option flags.  */
   unsigned options;
 } scm_t_fport;