added a check for POA extraData

Author: voith

web3/middleware/pythonic.py

@@ -64,6 +64,24 @@ def bytes_to_ascii(value):
 is_not_null = complement(is_null)
 
 
+@curry
+def check_extradata_length(val, length):
+    if not isinstance(val, (str, int, bytes)):
+        return val
+    result = HexBytes(val)
+    if len(result) > length:
+        raise ValueError(
+            "The field extraData: %r is %d bytes, but should be %d. "
+            "It is quite likely that you are connected to a POA chain. "
+            "Refer "
+            "http://web3py.readthedocs.io/en/latest/middleware.html#geth-style-proof-of-authority "
+            "for more details" % (
+                result, len(result), length
+            )
+        )
+    return val
+
+
 @curry
 def to_hexbytes(num_bytes, val, variable_length=False):
     if isinstance(val, (str, int, bytes)):
@@ -151,7 +169,7 @@ def to_hexbytes(num_bytes, val, variable_length=False):
 receipt_formatter = apply_formatters_to_dict(RECEIPT_FORMATTERS)
 
 BLOCK_FORMATTERS = {
-    'extraData': to_hexbytes(32, variable_length=True),
+    'extraData': compose(to_hexbytes(32, variable_length=True), check_extradata_length(length=32)),
     'gasLimit': to_integer_if_hex,
     'gasUsed': to_integer_if_hex,
     'size': to_integer_if_hex,