Jack Jansen hit a bug in the new dict code, reported on python-dev.

dictresize() was too aggressive about never ever resizing small dicts.
If a small dict is entirely full, it needs to rebuild it despite that
it won't actually resize it, in order to purge old dummy entries thus
creating at least one virgin slot (lookdict assumes at least one such
exists).

Also took the opportunity to add some high-level comments to dictresize.

Lib/test/test_operations.py

@@ -26,3 +26,18 @@ x2 = BadDictKey()
 d[x1] = 1
 d[x2] = 2
 print "No exception passed through."
+
+# Dict resizing bug, found by Jack Jansen in 2.2 CVS development.
+# This version got an assert failure in debug build, infinite loop in
+# release build.  Unfortunately, provoking this kind of stuff requires
+# a mix of inserts and deletes hitting exactly the right hash codes in
+# exactly the right order, and I can't think of a randomized approach
+# that would be *likely* to hit a failing case in reasonable time.
+
+d = {}
+for i in range(5):
+    d[i] = i
+for i in range(5):
+    del d[i]
+for i in range(5, 9):  # i==8 was the problem
+    d[i] = i

Objects/dictobject.c

@@ -395,14 +395,15 @@ actually be smaller than the old one.
 static int
 dictresize(dictobject *mp, int minused)
 {
-	register int newsize, newpoly;
-	register dictentry *oldtable = mp->ma_table;
-	register dictentry *newtable;
-	register dictentry *ep;
-	register int i;
+	int newsize, newpoly;
+	dictentry *oldtable, *newtable, *ep;
+	int i;
+	int is_oldtable_malloced;
+	dictentry small_copy[MINSIZE];
 
 	assert(minused >= 0);
-	assert(oldtable != NULL);
+
+	/* Find the smallest table size > minused, and its poly[] entry. */
 	newpoly = 0;
 	newsize = MINSIZE;
 	for (i = 0; i < sizeof(polys)/sizeof(polys[0]); ++i) {
@@ -419,10 +420,26 @@ dictresize(dictobject *mp, int minused)
 		PyErr_NoMemory();
 		return -1;
 	}
+
+	/* Get space for a new table. */
+	oldtable = mp->ma_table;
+	assert(oldtable != NULL);
+	is_oldtable_malloced = oldtable != mp->ma_smalltable;
+
 	if (newsize == MINSIZE) {
+		/* Either a large table is shrinking, or we can't get any
+		   smaller. */
 		newtable = mp->ma_smalltable;
-		if (newtable == oldtable)
+		if (newtable == oldtable) {
+			if (mp->ma_fill < mp->ma_size)
 				return 0;
+			/* The small table is entirely full.  We're not
+			   going to resise it, but need to rebuild it
+			   anyway to purge old dummy entries. */
+			assert(mp->ma_fill > mp->ma_used); /* a dummy exists */
+			memcpy(small_copy, oldtable, sizeof(small_copy));
+			oldtable = small_copy;
+		}
 	}
 	else {
 		newtable = PyMem_NEW(dictentry, newsize);
@@ -431,6 +448,8 @@ dictresize(dictobject *mp, int minused)
 			return -1;
 		}
 	}
+
+	/* Make the dict empty, using the new table. */
 	assert(newtable != oldtable);
 	mp->ma_table = newtable;
 	mp->ma_size = newsize;
@@ -455,7 +474,7 @@ dictresize(dictobject *mp, int minused)
 		/* else key == value == NULL:  nothing to do */
 	}
 
-	if (oldtable != mp->ma_smalltable)
+	if (is_oldtable_malloced)
 		PyMem_DEL(oldtable);
 	return 0;
 }