feat: speed up unmarshalling by skipping unused unix_fds header (#246)

src/dbus_fast/_private/unmarshaller.pxd

@@ -22,6 +22,8 @@ cdef unsigned int HEADER_SIGNATURE_SIZE
 cdef unsigned int LITTLE_ENDIAN
 cdef unsigned int BIG_ENDIAN
 cdef unsigned int PROTOCOL_VERSION
+cdef unsigned int HEADER_UNIX_FDS_IDX
+cdef cython.list HEADER_IDX_TO_ARG_NAME
 
 cdef str UINT32_CAST
 cdef str INT16_CAST

src/dbus_fast/_private/unmarshaller.py

@@ -108,18 +108,19 @@
 EAGAIN = errno.EAGAIN
 EWOULDBLOCK = errno.EWOULDBLOCK
 
-
-HEADER_MESSAGE_ARG_NAME = {
-    1: "path",
-    2: "interface",
-    3: "member",
-    4: "error_name",
-    5: "reply_serial",
-    6: "destination",
-    7: "sender",
-    8: "signature",
-    9: "unix_fds",
-}
+HEADER_IDX_TO_ARG_NAME = [
+    "",
+    "path",
+    "interface",
+    "member",
+    "error_name",
+    "reply_serial",
+    "destination",
+    "sender",
+    "signature",
+    "unix_fds",
+]
+HEADER_UNIX_FDS_IDX = HEADER_IDX_TO_ARG_NAME.index("unix_fds")
 
 _SignatureType = SignatureType
 _int = int
@@ -581,9 +582,11 @@ def _header_fields(self, header_length: _int) -> Dict[str, Any]:
             signature_len = buf[self._pos]  # byte
             o = self._pos + 1
             self._pos += signature_len + 2  # one for the byte, one for the '\0'
+            if field_0 == HEADER_UNIX_FDS_IDX:  # defined by self._unix_fds
+                continue
             token_as_int = buf[o]
             # Now that we have the token we can read the variant value
-            key = HEADER_MESSAGE_ARG_NAME[field_0]
+            key = HEADER_IDX_TO_ARG_NAME[field_0]
             # Strings and signatures are the most common types
             # so we inline them for performance
             if token_as_int == TOKEN_O_AS_INT or token_as_int == TOKEN_S_AS_INT:
@@ -661,7 +664,6 @@ def _read_body(self) -> None:
         self._pos = HEADER_ARRAY_OF_STRUCT_SIGNATURE_POSITION
         header_fields = self._header_fields(self._header_len)
         self._pos += -self._pos & 7  # align 8
-        header_fields.pop("unix_fds", None)  # defined by self._unix_fds
         signature = header_fields.pop("signature", "")
         if not self._body_len:
             tree = SIGNATURE_TREE_EMPTY