Fix a list GC bug

The presence of an allocated "elts" pointer is determined by capacity>0,
but the GC handler was incorrectly only choosing to visit it if size>0.
Not all code paths free the elts array if size goes down to zero, so
it could happen that capacity>0 and size=0, and then the elts array would
not be tracked even though it was expected to be.

Added a test that illustrates this.

src/gc/collector.cpp

@@ -28,6 +28,9 @@
 #include "valgrind.h"
 #endif
 
+//#undef VERBOSITY
+//#define VERBOSITY(x) 2
+
 namespace pyston {
 namespace gc {
 

src/gc/heap.cpp

@@ -26,6 +26,9 @@
 #include "valgrind.h"
 #endif
 
+//#undef VERBOSITY
+//#define VERBOSITY(x) 2
+
 namespace pyston {
 namespace gc {
 

src/runtime/inline/list.cpp

@@ -65,7 +65,6 @@ void BoxedList::shrink() {
             capacity = new_capacity;
         } else if (size == 0) {
             rt_free(elts);
-            elts = NULL;
             capacity = 0;
         }
     }

src/runtime/types.cpp

@@ -147,10 +147,12 @@ extern "C" void listGCHandler(GCVisitor* v, void* p) {
 
     BoxedList* l = (BoxedList*)p;
     int size = l->size;
-    if (size) {
+    int capacity = l->capacity;
+    assert(capacity >= size);
+    if (capacity)
         v->visit(l->elts);
+    if (size)
         v->visitRange((void**)&l->elts->elts[0], (void**)&l->elts->elts[size]);
-    }
 
     static StatCounter sc("gc_listelts_visited");
     sc.log(size);

test/tests/list_empty_gc_tracking.py

+# Regression test:
+# - create some lists
+# - pop from them to make them empty, but still have capacity allocated
+# - do some stuff to try to force a GC collection
+# -- the old bug was that the element array would get freed because size=0 even though capacity>0
+# - add more stuff to the array to try to get it to realloc more space
+# -- crash
+
+l = []
+for i in xrange(100):
+    l2 = [0]
+    l.append(l2)
+    l2.pop()
+for l2 in l:
+    l2 += range(100)
+
+    # allocate some data to try to force a collection:
+    # TODO shouldn't be too hard to add support for gc.collect()
+    range(10000)