From 62ec3013e8a60ae43129dfa47433a17a9801ecd2 Mon Sep 17 00:00:00 2001
From: Gareth Rees <gdr@ravenbrook.com>
Date: Mon, 5 Nov 2012 23:29:23 +0000
Subject: [PATCH] Need to check hash table for staleness when deleting a key.

Copied from Perforce
 Change: 180354
 ServerID: perforce.ravenbrook.com
---
 mps/example/scheme/scheme.c          |  5 +++-
 mps/manual/source/guide/advanced.rst | 44 ++++++++++++++++++++++++----
 2 files changed, 43 insertions(+), 6 deletions(-)

diff --git a/mps/example/scheme/scheme.c b/mps/example/scheme/scheme.c
index 4f0cf66bd85..69a3089700a 100644
--- a/mps/example/scheme/scheme.c
+++ b/mps/example/scheme/scheme.c
@@ -993,7 +993,10 @@ static void table_delete(obj_t tbl, obj_t key)
 {
   struct bucket_s *b;
   assert(TYPE(tbl) == TYPE_TABLE);
-  b = buckets_find(tbl, tbl->table.buckets, key, &tbl->table.ld);
+  b = buckets_find(tbl, tbl->table.buckets, key, NULL);
+  if ((b == NULL || b->key == NULL) && mps_ld_isstale(&tbl->table.ld, arena, key)) {
+    b = table_rehash(tbl, tbl->table.buckets->buckets.length, key);
+  }
   if (b != NULL && b->key != NULL) {
     b->key = obj_deleted;
     ++ tbl->table.buckets->buckets.deleted;
diff --git a/mps/manual/source/guide/advanced.rst b/mps/manual/source/guide/advanced.rst
index 1f72c9dd020..46e5eaf2ecb 100644
--- a/mps/manual/source/guide/advanced.rst
+++ b/mps/manual/source/guide/advanced.rst
@@ -332,11 +332,6 @@ any of its keys.
     looked up, when the staleness will be discovered. After all, it may
     never be looked up.
 
-.. note::
-
-    Don't forget that deleting a key from an address-based hash table
-    also results in a dependency on its location.
-
 If you look up a key in an address-based hash table and fail to find it
 there, that might be because the table's dependency on the location of
 the key is stale: that is, if the garbage collector moved the key. The
@@ -435,6 +430,45 @@ the location dependency becomes stale and the table has to be rehashed.
     move in the collection, so it's not found in the table, and that
     causes :c:func:`mps_ld_isstale` to be tested.
 
+Don't forget to check the location dependency for staleness if you are
+about to delete a key from a hash table but discover that it's not
+there. In the toy Scheme interpreter, deletion looks like this:
+
+.. code-block:: c
+    :emphasize-lines: 6
+
+    static void table_delete(obj_t tbl, obj_t key)
+    {
+        struct bucket_s *b;
+        assert(TYPE(tbl) == TYPE_TABLE);
+        b = buckets_find(tbl, tbl->table.buckets, key, NULL);
+        if ((b == NULL || b->key == NULL) && mps_ld_isstale(&tbl->table.ld, arena, key)) {
+            b = table_rehash(tbl, tbl->table.buckets->buckets.length, key);
+        }
+        if (b != NULL && b->key != NULL) {
+            b->key = obj_deleted;
+            ++ tbl->table.buckets->buckets.deleted;
+        }
+    }
+
+Again, by adding the line ``puts("stale!");`` after
+:c:func:`mps_ld_isstale` returns true, it's possible to see when the
+location dependency becomes stale and the table has to be rehashed:
+
+.. code-block:: none
+
+    MPS Toy Scheme Example
+    13248, 0> (define ht (make-eq-hashtable))
+    ht
+    13624, 0> (hashtable-set! ht 'one 1)
+    13808, 0> (gc)
+    13832, 1> (hashtable-delete! ht 'one)
+    stale!
+    14112, 1> ht
+    #[hashtable]
+
+
+
 .. topics::
 
     :ref:`topic-location`.