Add decode support for new binary VDF

The new Steam beta introduced a new `appinfo.vdf` version. This
appinfo.vdf V29 introduces a new binary VDF format which does not
include field keys in binary VDF segments as-is. Instead, each key is
represented by a 32-bit integer which needs to be mapped to an actual
string using a table are stored at the end of the `appinfo.vdf` file.

This also means the binary VDF segments in this case are no longer
self-contained.

Also see SteamDatabase/SteamAppinfo#56b1fec7f5ce6be961c3e44cf9baf117e363ad91

Refs ValvePython/steam#462

vdf/__init__.py

@@ -295,7 +295,7 @@ class COLOR(BASE_INT):
 BIN_INT64       = b'\x0A'
 BIN_END_ALT     = b'\x0B'
 
-def binary_loads(b, mapper=dict, merge_duplicate_keys=True, alt_format=False, raise_on_remaining=True):
+def binary_loads(b, mapper=dict, merge_duplicate_keys=True, alt_format=False, field_names=None, raise_on_remaining=True):
     """
     Deserialize ``b`` (``bytes`` containing a VDF in "binary form")
     to a Python object.
@@ -307,13 +307,17 @@ def binary_loads(b, mapper=dict, merge_duplicate_keys=True, alt_format=False, ra
     ``merge_duplicate_keys`` when ``True`` will merge multiple KeyValue lists with the
     same key into one instead of overwriting. You can se this to ``False`` if you are
     using ``VDFDict`` and need to preserve the duplicates.
+
+    ``field_names`` contains a list of field keys. Newer `appinfo.vdf` format
+    stores this at the very end of the file, and it is needed to deserialize
+    the binary VDF segments in that file
     """
     if not isinstance(b, bytes):
         raise TypeError("Expected s to be bytes, got %s" % type(b))
 
-    return binary_load(BytesIO(b), mapper, merge_duplicate_keys, alt_format, raise_on_remaining)
+    return binary_load(BytesIO(b), mapper, merge_duplicate_keys, alt_format, field_names, raise_on_remaining)
 
-def binary_load(fp, mapper=dict, merge_duplicate_keys=True, alt_format=False, raise_on_remaining=False):
+def binary_load(fp, mapper=dict, merge_duplicate_keys=True, alt_format=False, field_names=None, raise_on_remaining=False):
     """
     Deserialize ``fp`` (a ``.read()``-supporting file-like object containing
     binary VDF) to a Python object.
@@ -325,7 +329,13 @@ def binary_load(fp, mapper=dict, merge_duplicate_keys=True, alt_format=False, ra
     ``merge_duplicate_keys`` when ``True`` will merge multiple KeyValue lists with the
     same key into one instead of overwriting. You can se this to ``False`` if you are
     using ``VDFDict`` and need to preserve the duplicates.
+
+    ``field_names`` contains a list of field keys. Newer `appinfo.vdf` format
+    stores this at the very end of the file, and it is needed to deserialize
+    the binary VDF segments in that file
     """
+    use_field_indices = bool(field_names)
+
     if not hasattr(fp, 'read') or not hasattr(fp, 'tell') or not hasattr(fp, 'seek'):
         raise TypeError("Expected fp to be a file-like object with tell()/seek() and read() returning bytes")
     if not issubclass(mapper, Mapping):
@@ -337,7 +347,14 @@ def binary_load(fp, mapper=dict, merge_duplicate_keys=True, alt_format=False, ra
     int64 = struct.Struct('<q')
     float32 = struct.Struct('<f')
 
-    def read_string(fp, wide=False):
+    def read_string(fp, wide=False, use_field_indices=False):
+        if use_field_indices:
+            # Newer appinfo.vdf has the list of field names in an out-of-band
+            # table
+            index = int32.unpack(fp.read(int32.size))[0]
+
+            return field_names[index]
+
         buf, end = b'', -1
         offset = fp.tell()
 
@@ -382,7 +399,9 @@ def read_string(fp, wide=False):
                 continue
             break
 
-        key = read_string(fp)
+        # If 'field_names' was provided, each key is an int32 value that
+        # corresponds to an entry in the field name array.
+        key = read_string(fp, use_field_indices=use_field_indices)
 
         if t == BIN_NONE:
             if merge_duplicate_keys and key in stack[-1]: