Fix KeyError for missing refcount

We would sometimes, rarely, have a crash that looked like this:

      File "..refcountstore.py", line 76, in get_refcount
        return self.refcounts[node_id]
    KeyError: 32970

In other words, the object keeping track of reference counts for
B-tree nodes would think it doesn't have the reference count for a
node, when it should have it.

After much debugging, by myself and Itamar Turner-Trauring, and
greatly helped by a repeatable test case provided by Rob Kendrick,
this was tracked to an optimisation in the set_refcount method, which
would remove (to save memory) the refcount for a node (remove the
entry for a node's id in the refcount dict), when the refcount was set
to zero. Unfortunately this conflicted with an assumption in
get_refcount that a refcount for a node that was in the dirty set was
actually present in the refcount dict.

The bug fix is easy: don't remove the node from the refcount dict, and
eat the (very minor) memory consumption hit, in the name of
correctness.

1 files changed, 1 insertions, 5 deletions

diff --git a/larch/refcountstore.py b/larch/refcountstore.py
index 0aea53e..18fb9e7 100644
--- a/larch/refcountstore.py
+++ b/larch/refcountstore.py
@@ -78,11 +78,7 @@ class RefcountStore(object):
     def set_refcount(self, node_id, refcount):
         '''Set the reference count for a given node.'''
         tracing.trace('setting refcoutn for %s to %s' % (node_id, refcount))
-        if refcount == 0:
-            if node_id in self.refcounts:
-                del self.refcounts[node_id]
-        else:
-            self.refcounts[node_id] = refcount
+        self.refcounts[node_id] = refcount
         self.dirty.add(node_id)
 
     def save_refcounts(self):