Fix echo-content-escaped.py for real (#26655)

See #26645 (comment)

Author: Hexcles

FileAPI/file/resources/echo-content-escaped.py

@@ -1,17 +1,18 @@
+from six import PY3
 from wptserve.utils import isomorphic_encode
 
 # Outputs the request body, with controls and non-ASCII bytes escaped
 # (b"\n" becomes b"\\x0a"), and with backslashes doubled.
 # As a convenience, CRLF newlines are left as is.
 
 def escape_byte(byte):
-    # Iterating over a binary string gives different types in Py2 & Py3.
-    # Py3: bytes -> int
-    # Py2: str -> str (of length 1), so we convert it to int
-    code = byte if type(byte) is int else ord(byte)
-    if 0 <= code <= 0x1F or code >= 0x7F:
-        return b"\\x%02x" % code
-    if code == ord(b"\\"):
+    # We want a single-char binary string, but in Python 3 we may get an int
+    # from iterating over bytes.
+    if PY3:
+        byte = bytes([byte])
+    if b"\0" <= byte <= b"\x1F" or byte >= b"\x7F":
+        return b"\\x%02x" % ord(byte)
+    if byte == b"\\":
         return b"\\\\"
     return byte