Multiprocess mode: using direct assignment for writing into mmap (#315)

Using pack_into can create atomicity problems when writing on cpython, setting the bytes to 0 before writing the value.
When having a lot of writes, some reads would return 0 instead of the real value.

Signed-off-by: David Guerrero <heldroe@gmail.com>

Author: Heldroe

prometheus_client/core.py

@@ -30,8 +30,8 @@
 _MINUS_INF = float("-inf")
 _INITIAL_MMAP_SIZE = 1 << 20
 
-_pack_integer = struct.Struct(b'i').pack_into
-_pack_double = struct.Struct(b'd').pack_into
+_pack_integer_func = struct.Struct(b'i').pack
+_pack_double_func = struct.Struct(b'd').pack
 _unpack_integer = struct.Struct(b'i').unpack_from
 _unpack_double = struct.Struct(b'd').unpack_from
 
@@ -493,6 +493,17 @@ def get(self):
             return self._value
 
 
+# struct.pack_into has atomicity issues because it will temporarily write 0 into
+# the mmap, resulting in false reads to 0 when experiencing a lot of writes.
+# Using direct assignment solves this issue.
+def _pack_double(data, pos, value):
+    data[pos:pos + 8] = _pack_double_func(value)
+
+
+def _pack_integer(data, pos, value):
+    data[pos:pos + 4] = _pack_integer_func(value)
+
+
 class _MmapedDict(object):
     """A dict of doubles, backed by an mmapped file.