From: Yusuke ENDOH <mame@...>
Date: 2009-02-07T00:56:13+09:00
Subject: [ruby-dev:37933] Re: [Bug:1.9] lack consistency in hash iteration

遠藤です。

2009/02/06 1:01 Yusuke ENDOH <mame@tsg.ne.jp>:
> 同じ要素を複数回 yield する可能性があります。
>
> $ ./ruby -e 'h = {1=>1,2=>2}; h.each {|x| p x; h.delete(2) }'
> [1, 1]
> [1, 1]
>
> すぐには解決策が思いつかないので、ちょっと考えます。


現状の st_foreach だと、Hash のイテレート中にハッシュの要素を削除すると
削除されていない要素でも取りこぼすことがあるようです。


a = (1..3).map do |i|
  x = Object.new
  def x.hash; 1; end
  x
end

h = {}
a.each {|x| h[x] = true }

p h  #=> {#<Object:0x81fcdb0>=>true, #<Object:0x81fcce8>=>true,
#<Object:0x81fcc5c>=>true}

h.each do |x|
  p x
  h.delete(a[0])
  h.delete(a[1])
end
#=> [#<Object:0x81fcdb0>, true] だけしか出ない、[#<Object:0x81fcc5c>=>true, true] は出るはず

p h  #=> {#<Object:0x81fcc5c>=>true}


原因はイテレートの終了条件が正しくないためですが、Hash の順序を
ループ構造で管理している限り解決できないような気がしました。

順序を双方向リストで管理すれば直りました。

Index: include/ruby/st.h
===================================================================
--- include/ruby/st.h	(revision 22101)
+++ include/ruby/st.h	(working copy)
@@ -75,7 +75,7 @@
 #endif
     st_index_t num_entries : ST_INDEX_BITS - 1;
     struct st_table_entry **bins;
-    struct st_table_entry *head;
+    struct st_table_entry *head, *back;
 };

 #define st_is_member(table,key) st_lookup(table,key,(st_data_t *)0)
Index: st.c
===================================================================
--- st.c	(revision 22101)
+++ st.c	(working copy)
@@ -176,6 +176,7 @@
     tbl->num_bins = size;
     tbl->bins = (st_table_entry **)Calloc(size, sizeof(st_table_entry*));
     tbl->head = 0;
+    tbl->back = 0;

     return tbl;
 }
@@ -244,6 +245,7 @@
     }
     table->num_entries = 0;
     table->head = 0;
+    table->back = 0;
 }

 void
@@ -335,7 +337,7 @@

 #define ADD_DIRECT(table, key, value, hash_val, bin_pos)\
 do {\
-    st_table_entry *entry, *head;\
+    st_table_entry *entry;\
     if (table->num_entries/(table->num_bins) > ST_DEFAULT_MAX_DENSITY) {\
 	rehash(table);\
         bin_pos = hash_val % table->num_bins;\
@@ -347,13 +349,14 @@
     entry->key = key;\
     entry->record = value;\
     entry->next = table->bins[bin_pos];\
-    if ((head = table->head) != 0) {\
-	entry->fore = head;\
-	(entry->back = head->back)->fore = entry;\
-	head->back = entry;\
+    if (table->head != 0) {\
+	entry->fore = 0;\
+	(entry->back = table->back)->fore = entry;\
+	table->back = entry;\
     }\
     else {\
-	table->head = entry->fore = entry->back = entry;\
+	table->head = table->back = entry;\
+	entry->fore = entry->back = 0;\
     }\
     table->bins[bin_pos] = entry;\
     table->num_entries++;\
@@ -455,7 +458,7 @@
 	    hash_val = ptr->hash % new_num_bins;
 	    ptr->next = new_bins[hash_val];
 	    new_bins[hash_val] = ptr;
-	} while ((ptr = ptr->fore) != table->head);
+	} while ((ptr = ptr->fore) != 0);
     }
 }

@@ -502,10 +505,8 @@
 	    entry->back = prev;
 	    *tail = prev = entry;
 	    tail = &entry->fore;
-	} while ((ptr = ptr->fore) != old_table->head);
-	entry = new_table->head;
-	entry->back = prev;
-	*tail = entry;
+	} while ((ptr = ptr->fore) != 0);
+	new_table->back = prev;
     }

     return new_table;
@@ -513,14 +514,16 @@

 #define REMOVE_ENTRY(table, ptr) do					\
     {									\
-	if (ptr == ptr->fore) {						\
+	if (ptr->fore == 0 && ptr->back == 0) {				\
 	    table->head = 0;						\
+	    table->back = 0;						\
 	}								\
 	else {								\
 	    st_table_entry *fore = ptr->fore, *back = ptr->back;	\
-	    fore->back = back;						\
-	    back->fore = fore;						\
+	    if (fore) fore->back = back;				\
+	    if (back) back->fore = fore;				\
 	    if (ptr == table->head) table->head = fore;			\
+	    if (ptr == table->back) table->back = back;			\
 	}								\
 	table->num_entries--;						\
     } while (0)
@@ -613,7 +616,7 @@
 {
     st_table_entry *ptr, **last, *tmp;
     enum st_retval retval;
-    int i, end;
+    int i;

     if (table->entries_packed) {
         for (i = 0; i < table->num_entries; i++) {
@@ -651,7 +654,6 @@

     if ((ptr = table->head) != 0) {
 	do {
-	    end = ptr->fore == table->head;
 	    retval = (*func)(ptr->key, ptr->record, arg);
 	    switch (retval) {
 	      case ST_CHECK:	/* check if hash is modified during iteration */
@@ -683,7 +685,7 @@
 		    }
 		}
 	    }
-	} while (!end && table->head);
+	} while (ptr && table->head);
     }
     return 0;
 }
@@ -694,7 +696,7 @@
 {
     st_table_entry *ptr, **last, *tmp;
     enum st_retval retval;
-    int i, end;
+    int i;

     if (table->entries_packed) {
         for (i = table->num_entries-1; 0 <= i; i--) {
@@ -732,7 +734,6 @@
     if ((ptr = table->head) != 0) {
 	ptr = ptr->back;
 	do {
-	    end = ptr == table->head;
 	    retval = (*func)(ptr->key, ptr->record, arg, 0);
 	    switch (retval) {
 	      case ST_CHECK:	/* check if hash is modified during iteration */
@@ -766,7 +767,7 @@
 		free(tmp);
 		table->num_entries--;
 	    }
-	} while (!end && table->head);
+	} while (ptr && table->head);
     }
     return 0;
 }

-- 
Yusuke ENDOH <mame@tsg.ne.jp>