bug: receive_some might return less then max bytes

also b"" is EOF not None

apns.py

@@ -375,6 +375,16 @@ class APNSField:
         )
 
 
+async def receive_exact(stream: trio.abc.Stream, amount: int):
+        """Reads exactly the given amount of bytes from the given stream"""
+        buffer = b""
+        while len(buffer) < amount:
+            # Check for EOF
+            if (b := await stream.receive_some(1)) == b"":
+                return None # None is how EOF's were represented in the old code, so we'll keep it that way
+            buffer += b
+        return buffer
+
 @dataclass
 class APNSPayload:
     """An APNS payload"""
@@ -385,18 +395,18 @@ class APNSPayload:
     @staticmethod
     async def read_from_stream(stream: trio.abc.Stream) -> APNSPayload:
         """Reads a payload from the given stream"""
-        if not (id_bytes := await stream.receive_some(1)):
+        if not (id_bytes := await receive_exact(stream, 1)):
             raise Exception("Unable to read payload id from stream")
         id: int = int.from_bytes(id_bytes, "big")
 
-        if (length := await stream.receive_some(4)) is None:
+        if (length := await receive_exact(stream, 4)) is None:
             raise Exception("Unable to read payload length from stream")
         length = int.from_bytes(length, "big")
 
         if length == 0:
             return APNSPayload(id, [])
 
-        buffer = await stream.receive_some(length)
+        buffer = await receive_exact(stream, length)
         if buffer is None:
             raise Exception("Unable to read payload from stream")
         fields = []