L2SS-676: Fully support different packet header lengths, avoid parsing a packet multiple times.

tangostationcontrol/tangostationcontrol/devices/sdp/packet.py

@@ -499,16 +499,22 @@ def read_packet(read_func) -> SDPPacket:
         If read_func() returns None, this function will as well.
     """
 
-    # read just the header
-    header = read_func(SDPPacket.header_size)
-    if not header:
+    # read just the marker
+    marker = read_func(1)
+    if not marker:
         return None
 
-    # marker is the first byte of the header
-    marker = header[0]
-
     # read the packet header based on type
     packetClass = PACKET_CLASS_FOR_MARKER[marker]
+
+    # read the rest of the header
+    header = read_func(packetClass.header_size - len(marker))
+    if not header:
+        return None
+
+    header = marker + header
+
+    # parse the packet header size
     packet = packetClass(header)
 
     # read the payload
@@ -534,7 +540,7 @@ def main(args=None, **kwargs):
     offset = 0
 
     while True:
-        # read just the header
+        # read the packet from input
         packet = read_packet(sys.stdin.buffer.read)
 
         # print header