Add better handling of simple zoneinfo files.

This adds support for timezones with no defined transitions (UTC), and
ones that define a single transition for a simple offset. This also
improves the normal search function to properly handle dates that fall
to the future of the last transition point, and thus cannot be
bracketed. Dates in the past before the first transition point give a
more descriptive error.

Fixes #11093

mythtv/bindings/python/MythTV/utility/dt.py

@@ -24,16 +24,22 @@ class basetzinfo( _pytzinfo ):
                              'time utc local offset abbrev isdst')
 
     def _get_transition(self, dt=None):
-        try:
-            index = self.__last
-        except AttributeError:
-            index = 0
+        if len(self._transitions) == 0:
+            self._get_transition = self._get_transition_empty
+        elif len(self._transitions) == 1:
+            self._get_transition = self._get_transition_single
+        else:
+            self.__last = 0
+            self._get_transition = self._get_transition_search
+        return self._get_transition(dt)
 
+    def _get_transition_search(self, dt=None):
         if dt is None:
             dt = _pydatetime.now()
         dt = (dt.year, dt.month, dt.day, dt.hour, dt.minute, dt.second)
 
         direction = 0
+        index = self.__last
         while True:
             if dt < self._transitions[index].local[0:5]:
                 if direction == 1:
@@ -46,11 +52,25 @@ def _get_transition(self, dt=None):
                     break
                 else:
                     direction = 1
+
             index += direction
+            if index >= len(self._transitions):
+                # out of bounds future, use final transition
+                index = len(self._transitions) - 1
+                break
+            elif index < 0:
+                # out of bounds past, undefined time frame
+                raise RuntimeError(("Timezone does not have sufficiently "
+                                    "old data for search: {0}").format(dt))
 
         self.__last = index
         return self._transitions[index]
 
+    def _get_transition_empty(self, dt=None):
+        return self._Transition(0, None, None, 0, 'UTC', False)
+    def _get_transition_single(self, dt=None):
+        return self._transitions[0]
+
     def utcoffset(self, dt=None):
         return timedelta(0, self._get_transition(dt).offset)