From 6f1c6a84c39f67fa1ab841af039d9f672a0b15ba Mon Sep 17 00:00:00 2001
From: Yu Feng <rainwoodman@gmail.com>
Date: Mon, 4 Mar 2019 10:09:02 -0800
Subject: [PATCH] Protect against empty heap.

I do not understand how this can happen, but we are seeing this stack trace:

```
../testenv/lib/python3.6/site-packages/nbodykit/tests/test_cache.py:19: in test_cache
    cache.cache.shrink()
/home/travis/miniconda/envs/test/lib/python3.6/site-packages/cachey/cache.py:139: in shrink
    self._shrink_one()
/home/travis/miniconda/envs/test/lib/python3.6/site-packages/cachey/cache.py:126: in _shrink_one
    key, score = self.heap.popitem()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
self = <heapdict.heapdict object at 0x7f24b44e46d8>
    def popitem(self):
        """D.popitem() -> (k, v), remove and return the (key, value) pair with lowest\nvalue; but raise KeyError if D is empty."""
>       wrapper = self.heap[0]
E       IndexError: list index out of range
/home/travis/miniconda/envs/test/lib/python3.6/site-packages/heapdict.py:91: IndexError
```

This should fix the stack trace, but it is not clear to me how it can happen when the heap is empty yet total_bytes is not zero.
---
 cachey/cache.py | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/cachey/cache.py b/cachey/cache.py
index b291c1c..d84ded8 100644
--- a/cachey/cache.py
+++ b/cachey/cache.py
@@ -123,9 +123,13 @@ def retire(self, key):
         self.total_bytes -= self.nbytes.pop(key)
 
     def _shrink_one(self):
-        key, score = self.heap.popitem()
+        try:
+            key, score = self.heap.popitem()
+        except IndexError:
+            return
         self.retire(key)
         
+        
     def resize(self, available_bytes):
         """ Resize the cache.