#3196: if needed pad a short base64 encoded word before trying to decode.

The RFCs encourage following Postel's law: be liberal in what you accept.
So if someone forgot to pad the base64 encoded word payload to an
even four bytes, we add the padding before handing it to base64mime.decode.
Previously, missing padding resulted in a HeaderParseError.

Patch by Jason Williams.

Lib/email/header.py

@@ -94,6 +94,9 @@ def decode_header(header):
             word = email.quoprimime.header_decode(encoded_string)
             decoded_words.append((word, charset))
         elif encoding == 'b':
+            paderr = len(encoded_string) % 4   # Postel's law: add missing padding
+            if paderr:
+                encoded_string += '==='[:4 - paderr]
             try:
                 word = email.base64mime.decode(encoded_string)
             except binascii.Error:

Lib/email/test/test_email.py

@@ -1649,6 +1649,15 @@ Re: =?mac-iceland?q?r=8Aksm=9Arg=8Cs?= baz foo bar =?mac-iceland?q?r=8Aksm?=
                               (b'rg', None), (b'\xe5', 'iso-8859-1'),
                               (b'sbord', None)])
 
+    def test_rfc2047_B_bad_padding(self):
+        s = '=?iso-8859-1?B?%s?='
+        data = [                                # only test complete bytes
+            ('dm==', b'v'), ('dm=', b'v'), ('dm', b'v'),
+            ('dmk=', b'vi'), ('dmk', b'vi')
+          ]
+        for q, a in data:
+            dh = decode_header(s % q)
+            self.assertEqual(dh, [(a, 'iso-8859-1')])
 
 
 # Test the MIMEMessage class
@@ -3176,7 +3185,7 @@ A very long line that must get split to something other than at the
 
     def test_broken_base64_header(self):
         raises = self.assertRaises
-        s = 'Subject: =?EUC-KR?B?CSixpLDtKSC/7Liuvsax4iC6uLmwMcijIKHaILzSwd/H0SC8+LCjwLsgv7W/+Mj3IQ?='
+        s = 'Subject: =?EUC-KR?B?CSixpLDtKSC/7Liuvsax4iC6uLmwMcijIKHaILzSwd/H0SC8+LCjwLsgv7W/+Mj3I ?='
         raises(errors.HeaderParseError, decode_header, s)
 
 

Misc/ACKS

@@ -854,6 +854,7 @@ Felix Wiemann
 Gerry Wiener
 Frank Wierzbicki
 Bryce "Zooko" Wilcox-O'Hearn
+Jason Williams
 John Williams
 Sue Williams
 Gerald S. Williams

Misc/NEWS

@@ -37,6 +37,9 @@ Extensions
 Library
 -------
 
+- Issue #3196: email header decoding is now forgiving if an RFC2047
+  encoded word encoded in base64 is lacking padding.
+
 - Issue #9444: Argparse now uses the first element of prefix_chars as
   the option character for the added 'h/help' option if prefix_chars
   does not contain a '-', instead of raising an error.