From a6f0c72306da0c628be9e180288f79fbf29bfa57 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Tue, 20 Jul 2021 23:17:59 -0700
Subject: hash: change semantics of weak-both hash tables.

From now on, hash tables with both weak keys and values have
dijunctive retention semantics. If either the key or value of
an entry is reachable, then the entry stays. This is subject
to compatibility.

* hash.c (do_weak_tables): Expire an entry if neither the key
nor the value is reachable. In 266 or lower compatibility
mode, expire an entry if either the key or value is
unreachable, like before.

* txr.1: Document the change, with compat notes. Add a
cautionary note about the referencing issue which defeats weak
key or weak value tables.
---
 hash.c | 56 ++++++++++++++++++++++++++++++++++++++++----------------
 1 file changed, 40 insertions(+), 16 deletions(-)

(limited to 'hash.c')

diff --git a/hash.c b/hash.c
index 90e48858..3434b026 100644
--- a/hash.c
+++ b/hash.c
@@ -1274,24 +1274,48 @@ static void do_weak_tables(void)
     case hash_weak_both:
       /* Sweep through all entries. Delete any which have keys
          or values that are garbage. */
-      for (i = 0; i < h->modulus; i++) {
-        val *pchain = &h->table->v.vec[i];
-        val *iter;
-
-        for (iter = pchain; *iter; ) {
-          val entry = us_car(*iter);
-          if (!gc_is_reachable(us_car(entry)) || !gc_is_reachable(us_cdr(entry)))
-          {
-            *iter = us_cdr(*iter);
+      if (opt_compat && opt_compat <= 266) {
+        for (i = 0; i < h->modulus; i++) {
+          val *pchain = &h->table->v.vec[i];
+          val *iter;
+
+          for (iter = pchain; *iter; ) {
+            val entry = us_car(*iter);
+            if (!gc_is_reachable(us_car(entry)) || !gc_is_reachable(us_cdr(entry)))
+            {
+              *iter = us_cdr(*iter);
 #if CONFIG_EXTRA_DEBUGGING
-            if (!gc_is_reachable(us_car(entry)) && us_car(entry) == break_obj)
-              breakpt();
-            if (!gc_is_reachable(us_cdr(entry)) && us_cdr(entry) == break_obj)
-              breakpt();
+              if (!gc_is_reachable(us_car(entry)) && us_car(entry) == break_obj)
+                breakpt();
+              if (!gc_is_reachable(us_cdr(entry)) && us_cdr(entry) == break_obj)
+                breakpt();
 #endif
-          } else {
-            iter = us_cdr_p(*iter);
-            c++;
+            } else {
+              iter = us_cdr_p(*iter);
+              c++;
+            }
+          }
+        }
+      } else {
+        for (i = 0; i < h->modulus; i++) {
+          val *pchain = &h->table->v.vec[i];
+          val *iter;
+
+          for (iter = pchain; *iter; ) {
+            val entry = us_car(*iter);
+            if (!gc_is_reachable(us_car(entry)) && !gc_is_reachable(us_cdr(entry)))
+            {
+              *iter = us_cdr(*iter);
+#if CONFIG_EXTRA_DEBUGGING
+              if (!gc_is_reachable(us_car(entry)) && us_car(entry) == break_obj)
+                breakpt();
+              if (!gc_is_reachable(us_cdr(entry)) && us_cdr(entry) == break_obj)
+                breakpt();
+#endif
+            } else {
+              iter = us_cdr_p(*iter);
+              c++;
+            }
           }
         }
       }
-- 
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