Fix errors with calculating limits when start==end==None

This also has the effect of now handling negative timestamps
correctly.

nilmdb/server/nilmdb.py

@@ -142,6 +142,13 @@ class NilmDB(object):
             with self.con:
                 cur.execute("PRAGMA user_version = {v:d}".format(v=version))
 
+    def _fill_in_limits(self, start, end):
+        if start is None:
+            start = -1e12
+        if end is None:
+            end = 1e12
+        return (start, end)
+
     @nilmdb.utils.lru_cache(size = 16)
     def _get_intervals(self, stream_id):
         """
@@ -303,7 +310,8 @@ class NilmDB(object):
         """
         stream_id = self._stream_id(path)
         intervals = self._get_intervals(stream_id)
-        requested = Interval(start or 0, end or 1e12)
+        (start, end) = self._fill_in_limits(start, end)
+        requested = Interval(start, end)
         result = []
         for n, i in enumerate(intervals.intersection(requested)):
             if n >= self.max_results:
@@ -475,7 +483,8 @@ class NilmDB(object):
         stream_id = self._stream_id(path)
         table = self.data.getnode(path)
         intervals = self._get_intervals(stream_id)
-        requested = Interval(start or 0, end or 1e12)
+        (start, end) = self._fill_in_limits(start, end)
+        requested = Interval(start, end)
         result = []
         matched = 0
         remaining = self.max_results
@@ -521,12 +530,10 @@ class NilmDB(object):
         stream_id = self._stream_id(path)
         table = self.data.getnode(path)
         intervals = self._get_intervals(stream_id)
-        to_remove = Interval(start or 0, end or 1e12)
+        (start, end) = self._fill_in_limits(start, end)
+        to_remove = Interval(start, end)
         removed = 0
 
-        if start == end:
-            return 0
-
         # Can't remove intervals from within the iterator, so we need to
         # remember what's currently in the intersection now.
         all_candidates = list(intervals.intersection(to_remove, orig = True))