#19772: Do not mutate message when downcoding to 7bit.

This is a bit of an ugly hack because of the way generator pieces together the
output message.  The deepcopys aren't too expensive, though, because we know it
is only called on messages that are not multiparts, and the payload (the thing
that could be large) is an immutable object.

Test and preliminary work on patch by Vajrasky Kok.

1 files changed, 14 insertions, 0 deletions

diff --git a/Lib/email/generator.py b/Lib/email/generator.py
index 4ea0b559b9..07a97c7e27 100644
--- a/Lib/email/generator.py
+++ b/Lib/email/generator.py
@@ -12,6 +12,7 @@ import time
 import random
 import warnings
 
+from copy import deepcopy
 from io import StringIO, BytesIO
 from email._policybase import compat32
 from email.header import Header
@@ -173,10 +174,18 @@ class Generator:
         # necessary.
         oldfp = self._fp
         try:
+            self._munge_cte = None
             self._fp = sfp = self._new_buffer()
             self._dispatch(msg)
         finally:
             self._fp = oldfp
+            munge_cte = self._munge_cte
+            del self._munge_cte
+        # If we munged the cte, copy the message again and re-fix the CTE.
+        if munge_cte:
+            msg = deepcopy(msg)
+            msg.replace_header('content-transfer-encoding', munge_cte[0])
+            msg.replace_header('content-type', munge_cte[1])
         # Write the headers.  First we see if the message object wants to
         # handle that itself.  If not, we'll do it generically.
         meth = getattr(msg, '_write_headers', None)
@@ -225,9 +234,14 @@ class Generator:
         if _has_surrogates(msg._payload):
             charset = msg.get_param('charset')
             if charset is not None:
+                # XXX: This copy stuff is an ugly hack to avoid modifying the
+                # existing message.
+                msg = deepcopy(msg)
                 del msg['content-transfer-encoding']
                 msg.set_payload(payload, charset)
                 payload = msg.get_payload()
+                self._munge_cte = (msg['content-transfer-encoding'],
+                                   msg['content-type'])
         if self._mangle_from_:
             payload = fcre.sub('>From ', payload)
         self._write_lines(payload)