feat(recipe): add support for curator SharedCount recipe (#559)

* feat(recipe): add support for curator SharedCount recipe

This feature allows Java clients using curator's SharedCount recipe
and python clients using kazoo's Counter recipe to read and
write from the same path without receiving type errors.

example use:

counter = zk.Counter("/curator", support_curator=True)
counter += 2
counter -= 1
counter.value == 1
counter.pre_value == 2
counter.post_value == 1

Closes #558

kazoo/recipe/counter.py

@@ -6,7 +6,7 @@
 """
 from kazoo.exceptions import BadVersionError
 from kazoo.retry import ForceRetryError
-
+import struct
 
 class Counter(object):
     """Kazoo Counter
@@ -19,6 +19,12 @@ class Counter(object):
     `type(counter.default)(value)` both using an ascii encoding. As
     such other data types might be used for the counter value.
 
+    If you would like to support clients updating the same znode path using
+    either kazoo's counter recipe or curator's SharedCount recipe, you will
+    need to enable the support_curator flag. This flag limits
+    support to integers only and does not use ascii encoding as described
+    above.
+
     Counter changes can raise
     :class:`~kazoo.exceptions.BadVersionError` if the retry policy
     wasn't able to apply a change.
@@ -42,22 +48,35 @@ class Counter(object):
         counter.pre_value == 1.0
         counter.post_value == 3.0
 
+        counter = zk.Counter("/curator", support_curator=True)
+        counter += 2
+        counter -= 1
+        counter.value == 1
+        counter.pre_value == 2
+        counter.post_value == 1
+
     """
-    def __init__(self, client, path, default=0):
+    def __init__(self, client, path, default=0, support_curator=False):
         """Create a Kazoo Counter
 
         :param client: A :class:`~kazoo.client.KazooClient` instance.
         :param path: The counter path to use.
-        :param default: The default value.
+        :param default: The default value to use for new counter paths.
+        :param support_curator: Enable if support for curator's SharedCount
+                                recipe is desired.
 
         """
         self.client = client
         self.path = path
         self.default = default
         self.default_type = type(default)
+        self.support_curator = support_curator
         self._ensured_path = False
         self.pre_value = None
         self.post_value = None
+        if self.support_curator and not isinstance(self.default, int):
+            raise TypeError("when support_curator is enabled the default "
+                            "type must be an int")
 
     def _ensure_node(self):
         if not self._ensured_path:
@@ -68,7 +87,10 @@ def _ensure_node(self):
     def _value(self):
         self._ensure_node()
         old, stat = self.client.get(self.path)
-        old = old.decode('ascii') if old != b'' else self.default
+        if self.support_curator:
+            old = struct.unpack(">i", old)[0] if old != b'' else self.default
+        else:
+            old = old.decode('ascii') if old != b'' else self.default
         version = stat.version
         data = self.default_type(old)
         return data, version
@@ -86,7 +108,10 @@ def _change(self, value):
     def _inner_change(self, value):
         self.pre_value, version = self._value()
         post_value = self.pre_value + value
-        data = repr(post_value).encode('ascii')
+        if self.support_curator:
+            data = struct.pack(">i", post_value)
+        else:
+            data = repr(post_value).encode('ascii')
         try:
             self.client.set(self.path, data, version=version)
         except BadVersionError:  # pragma: nocover

kazoo/tests/test_counter.py

@@ -21,6 +21,22 @@ def test_int_counter(self):
         counter - 1
         eq_(counter.value, -1)
 
+    def test_int_curator_counter(self):
+        counter = self._makeOne(support_curator=True)
+        eq_(counter.value, 0)
+        counter += 2
+        counter + 1
+        eq_(counter.value, 3)
+        counter -= 3
+        counter - 1
+        eq_(counter.value, -1)
+        counter += 1
+        counter += 2147483647
+        eq_(counter.value, 2147483647)
+        counter -= 2147483647
+        counter -= 2147483647
+        eq_(counter.value, -2147483647)
+
     def test_float_counter(self):
         counter = self._makeOne(default=0.0)
         eq_(counter.value, 0.0)
@@ -33,6 +49,8 @@ def test_errors(self):
         counter = self._makeOne()
         self.assertRaises(TypeError, counter.__add__, 2.1)
         self.assertRaises(TypeError, counter.__add__, b"a")
+        with self.assertRaises(TypeError):
+            counter = self._makeOne(default=0.0, support_curator=True)
 
     def test_pre_post_values(self):
         counter = self._makeOne()