Subject: bug#28590: [PATCH 5/7] weak-table: 'move_weak_entry' reports disappeared links.

From: Ludovic Courtès <[email-masked]>
Date: Tue, 3 Oct 2017 13:43:50 +0200

* libguile/weak-table.c (move_weak_entry): Change to return a Boolean.
(give_to_poor): Remove 'copy_weak_entry' call and adjust accordingly.
---
 libguile/weak-table.c | 46 +++++++++++++++++++++++++++++-----------------
 1 file changed, 29 insertions(+), 17 deletions(-)

diff --git a/libguile/weak-table.c b/libguile/weak-table.c
index b5db3ef48..5c4b3d30a 100644
--- a/libguile/weak-table.c
+++ b/libguile/weak-table.c
@@ -199,28 +199,45 @@ move_disappearing_links (scm_t_weak_entry *from, scm_t_weak_entry *to,
     }
 }
 
-static void
+/* Move weak entry FROM to TO.  Return non-zero on success, and zero if
+   the entry at FROM disappeared in the meantime.  */
+static int
 move_weak_entry (scm_t_weak_entry *from, scm_t_weak_entry *to,
                  scm_t_weak_table_kind kind)
 {
   if (from->hash)
     {
       scm_t_weak_entry copy;
-      
+
       copy_weak_entry (from, &copy);
-      to->hash = copy.hash;
-      to->key = copy.key;
-      to->value = copy.value;
 
-      move_disappearing_links (from, to,
-                               SCM_PACK (copy.key), SCM_PACK (copy.value),
-                               kind);
+      if (copy.key != 0 && copy.value != 0)
+	{
+	  to->hash = copy.hash;
+	  to->key = copy.key;
+	  to->value = copy.value;
+
+	  move_disappearing_links (from, to,
+				   SCM_PACK (copy.key), SCM_PACK (copy.value),
+				   kind);
+	  return 1;
+	}
+      else
+	{
+	  /* Lost weak reference: make sure all the disappearing links
+	     are registered (in the case of a doubly-weak table, one of
+	     the disappearing links could still be there.)  */
+	  memset (to, '\0', sizeof *to);
+	  unregister_disappearing_links (from, kind);
+	  return 0;
+	}
     }
   else
     {
+      unregister_disappearing_links (from, kind);
       to->hash = 0;
-      to->key = 0;
-      to->value = 0;
+      to->key = to->value = 0;
+      return 0;
     }
 }
 
@@ -301,16 +318,14 @@ give_to_poor (scm_t_weak_table *table, unsigned long k)
     {
       unsigned long next = (k + 1) % size;
       unsigned long hash;
-      scm_t_weak_entry copy;
 
       hash = table->entries[next].hash;
 
       if (!hash || hash_to_index (hash, size) == next)
         break;
 
-      copy_weak_entry (&table->entries[next], &copy);
-
-      if (!copy.key || !copy.value)
+      if (!move_weak_entry (&table->entries[next], &table->entries[k],
+			    table->kind))
         /* Lost weak reference.  */
         {
           give_to_poor (table, next);
@@ -318,9 +333,6 @@ give_to_poor (scm_t_weak_table *table, unsigned long k)
           continue;
         }
 
-      move_weak_entry (&table->entries[next], &table->entries[k],
-                       table->kind);
-
       k = next;
     }
 
-- 
2.14.2