Improve INFO GPRO field

Not just the first two bytes should match, it should be the expected
length. Found the hint in dmesg after failing to retrieve the properties
file. Setting all following bytes to 0xff does not seem to affect the
output, so let's keep it like this. The original software does send an
empty buffer though (with all zeroes).

protocol.md

@@ -146,11 +146,12 @@ device (e.g. by writing to an unused partition) and execute:
 ### INFO
 Arguments:
  - arg1: action (`GPRO` - Get Properties, `SPRO` - Set Properties)
-Request body: must begin with two bytes (`08 0b`).
-Response body: 2824 (0xb08) bytes of binary info.
+Request body: a `laf_property` structure.
+Response body: 2824 (0x00000b08) bytes of binary info.
 
 See [scripts/parse-props.py](scripts/parse-props.py) for the structure of the
-property body.
+property body. This structure begins with a DWORD with a version that is
+apparently the same as the expected length (2824 or `\x08\x0b\0\0`).
 
 ### UNLK - Unlink
 Delete a file.

scripts/parse-props.py

@@ -3,7 +3,7 @@
 #
 # Usage:
 #
-#   lglaf.py -c '!INFO GPRO \x08\x0b' > props.bin
+#   lglaf.py -c '!INFO GPRO \x08\x0b\0\0' > props.bin
 #   scripts/parse-props.py props.bin
 
 import argparse, sys, struct
@@ -83,8 +83,10 @@ def print_shadow(shadow):
         print("%03x: %-16s %-16s" % (offset, line1, line2))
 
 def parse_data(data):
-    assert data[0:2] == b'\x08\x0b'
-    assert len(data) == 0xb08
+    version = struct.unpack_from('<I', data)
+    expected_length = 0x00000b08
+    assert version == expected_length, 'Unknown version: 0x%08x' % version
+    assert len(data) == expected_length
 
     # Set to non-None when processed
     shadow = [None] * len(data)
@@ -94,13 +96,21 @@ def parse_data(data):
 
     return data, shadow
 
+def open_local_readable(path):
+    if path == '-':
+        try: return sys.stdin.buffer
+        except: return sys.stdin
+    else:
+        return open(path, "rb")
+
 parser = argparse.ArgumentParser()
-parser.add_argument("--debug", action='store_true')
-parser.add_argument("file", help="2824 byte properties dump file")
+parser.add_argument("--debug", action='store_true', help="Enable debug messages")
+parser.add_argument("file",
+        help="2824 byte properties dump file (or '-' for stdin)")
 
 def main():
     args = parser.parse_args()
-    data = open(args.file, "rb").read()
+    data = open_local_readable(args.file).read()
     data, shadow = parse_data(data)
     if args.debug:
         debug_other(data, shadow)