Fix for email.generator.Generator with whitespace between encoded words.

email.generator.Generator currently does not handle whitespace between encoded words correctly when the encoded words span multiple lines. The current generator will create an encoded word for each line. If the end of the line happens to correspond with the end real word in the plaintext, the generator will place an unencoded space at the start of the subsequent lines to represent the whitespace between the plaintext words. A compliant decoder will strip all the whitespace from between two encoded words which leads to missing spaces in the round-tripped output. The fix for this is to make sure that whitespace between two encoded words ends up inside of one or the other of the encoded words. This fix places the space inside of the second encoded word. A second problem happens with continuation lines. A continuation line that starts with whitespace and is followed by a non-encoded word is fine because the newline between such continuation lines is defined as condensing to a single space character. When the continuation line starts with whitespace followed by an encoded word, however, the RFCs specify that the word is run together with the encoded word on the previous line. This is because normal words are filded on syntactic breaks by encoded words are not. The solution to this is to add the whitespace to the start of the encoded word on the continuation line. Test cases are from #92081
python · warsaw · May 20, 2024 · May 20, 2024 · May 20, 2024 · May 20, 2024
commit 5071b527827c29b6afcf36be8a2343c454e327f8
@@ -2784,11 +2784,15 @@ def _refold_parse_tree(parse_tree, *, policy):
    # max_line_length 0/None means no limit, ie: infinitely long.
    maxlen = policy.max_line_length or sys.maxsize
    encoding = 'utf-8' if policy.utf8 else 'us-ascii'
-    lines = ['']
-    last_ew = None
+    lines = ['']  # Folded lines to be output
+    leading_whitespace = ''  # When we have whitespace between two encoded
+                             # words, we may need to encode the whitespace
+                             # at the beginning of the second word.
+    last_ew = None  # Points to the last encoded character if there's an ew on
+                    # the line
    last_charset = None
    wrap_as_ew_blocked = 0
-    want_encoding = False
+    want_encoding = False  # This is set to True if we need to encode this part
    end_ew_not_allowed = Terminal('', 'wrap_as_ew_blocked')
    parts = list(parse_tree)
    while parts:
@@ -2812,10 +2816,12 @@ def _refold_parse_tree(parse_tree, *, policy):
                # 'charset' property on the policy.
                charset = 'utf-8'
            want_encoding = True
+
        if part.token_type == 'mime-parameters':
            # Mime parameter folding (using RFC2231) is extra special.
            _fold_mime_parameters(part, lines, maxlen, encoding)
            continue
+
        if want_encoding and not wrap_as_ew_blocked:
            if not part.as_ew_allowed:
                want_encoding = False
@@ -2847,21 +2853,38 @@ def _refold_parse_tree(parse_tree, *, policy):
                     last_charset == 'utf-8' and charset != 'us-ascii')):
                    last_ew = None
                last_ew = _fold_as_ew(tstr, lines, maxlen, last_ew,
-                                      part.ew_combine_allowed, charset)
+                                      part.ew_combine_allowed, charset, leading_whitespace)
+                # This whitespace has been added to the lines in _fold_as_ew()
+                # so clear it now.
+                leading_whitespace = ''
                last_charset = charset
            want_encoding = False
            continue
+
        if len(tstr) <= maxlen - len(lines[-1]):
            lines[-1] += tstr
            continue
+
        # This part is too long to fit.  The RFC wants us to break at
        # "major syntactic breaks", so unless we don't consider this
        # to be one, check if it will fit on the next line by itself.
+        leading_whitespace = ''
        if (part.syntactic_break and
                len(tstr) + 1 <= maxlen):
            newline = _steal_trailing_WSP_if_exists(lines)
            if newline or part.startswith_fws():
+                # We're going to fold the data onto a new line here.  Due to
+                # the way encoded strings handle continuation lines, we need to
+                # be prepared to encode any whitespace if the next line turns
+                # out to start with an encoded word.
                lines.append(newline + tstr)
+
+                leading_whitespace = []
+                for char in lines[-1]:
+                    if char not in WSP:
+                        break
+                    leading_whitespace.append(char)
+                leading_whitespace = ''.join(leading_whitespace)
                last_ew = None
                continue
        if not hasattr(part, 'encode'):
@@ -2885,9 +2908,10 @@ def _refold_parse_tree(parse_tree, *, policy):
        else:
            # We can't fold it onto the next line either...
            lines[-1] += tstr
+
    return policy.linesep.join(lines) + policy.linesep

-def _fold_as_ew(to_encode, lines, maxlen, last_ew, ew_combine_allowed, charset):
+def _fold_as_ew(to_encode, lines, maxlen, last_ew, ew_combine_allowed, charset, leading_whitespace):
    """Fold string to_encode into lines as encoded word, combining if allowed.
    Return the new value for last_ew, or None if ew_combine_allowed is False.

@@ -2902,14 +2926,15 @@ def _fold_as_ew(to_encode, lines, maxlen, last_ew, ew_combine_allowed, charset):
        to_encode = str(
            get_unstructured(lines[-1][last_ew:] + to_encode))
        lines[-1] = lines[-1][:last_ew]
-    if to_encode[0] in WSP:
+    elif to_encode[0] in WSP:
        # We're joining this to non-encoded text, so don't encode
        # the leading blank.
        leading_wsp = to_encode[0]
        to_encode = to_encode[1:]
        if (len(lines[-1]) == maxlen):
            lines.append(_steal_trailing_WSP_if_exists(lines))
        lines[-1] += leading_wsp
+
    trailing_wsp = ''
    if to_encode[-1] in WSP:
        # Likewise for the trailing space.
@@ -2929,11 +2954,20 @@ def _fold_as_ew(to_encode, lines, maxlen, last_ew, ew_combine_allowed, charset):

    while to_encode:
        remaining_space = maxlen - len(lines[-1])
-        text_space = remaining_space - chrome_len
+        text_space = remaining_space - chrome_len - len(leading_whitespace)
        if text_space <= 0:
            lines.append(' ')
            continue

+        # If we are at the start of a continuation line, prepend whitespace
+        # (we only want to do this when the line starts with an encoded word
+        # but if we're folding in this helper function, then we know that we
+        # are going to be writing out an encoded word.)
+        if len(lines) > 1 and len(lines[-1]) == 1 and leading_whitespace:
+            encoded_word = _ew.encode(leading_whitespace, charset=encode_as)
+            lines[-1] += encoded_word
+            leading_whitespace = ''
+
        to_encode_word = to_encode[:text_space]
        encoded_word = _ew.encode(to_encode_word, charset=encode_as)
        excess = len(encoded_word) - remaining_space

@@ -281,6 +281,41 @@ class TestBytesGenerator(TestGeneratorBase, TestEmailBase):
    ioclass = io.BytesIO
    typ = lambda self, x: x.encode('ascii')

+    def test_defaults_handle_spaces_between_encoded_words_when_folded(self):
+        source = ("Уведомление о принятии в работу обращения для"
+                  " подключения услуги")
+        expected = ('Subject: =?utf-8?b?0KPQstC10LTQvtC80LvQtdC90LjQtSDQviDQv9GA0LjQvdGP0YLQuNC4?=\n'
+                    ' =?utf-8?b?INCyINGA0LDQsdC+0YLRgyDQvtCx0YDQsNGJ0LXQvdC40Y8g0LTQu9GPINC/0L4=?=\n'
+                    ' =?utf-8?b?0LTQutC70Y7Rh9C10L3QuNGPINGD0YHQu9GD0LPQuA==?=\n\n').encode('ascii')
+        msg = EmailMessage()
+        msg['Subject'] = source
+        s = io.BytesIO()
+        g = BytesGenerator(s)
+        g.flatten(msg)
+        self.assertEqual(s.getvalue(), expected)
+
+    def test_defaults_handle_spaces_at_start_of_subject(self):
+        source = " Уведомление"
+        expected = b"Subject:  =?utf-8?b?0KPQstC10LTQvtC80LvQtdC90LjQtQ==?=\n\n"
+        msg = EmailMessage()
+        msg['Subject'] = source
+        s = io.BytesIO()
+        g = BytesGenerator(s)
+        g.flatten(msg)
+        self.assertEqual(s.getvalue(), expected)
+
+    def test_defaults_handle_spaces_at_start_of_continuation_line(self):
+        source = " ф ффффффффффффффффффф ф ф"
+        expected = (b"Subject:  "
+                    b"=?utf-8?b?0YQg0YTRhNGE0YTRhNGE0YTRhNGE0YTRhNGE0YTRhNGE0YTRhNGE0YQ=?=\n"
+                    b" =?utf-8?b?INGEINGE?=\n\n")
+        msg = EmailMessage()
+        msg['Subject'] = source
+        s = io.BytesIO()
+        g = BytesGenerator(s)
+        g.flatten(msg)
+        self.assertEqual(s.getvalue(), expected)
+
    def test_cte_type_7bit_handles_unknown_8bit(self):
        source = ("Subject: Maintenant je vous présente mon "
                 "collègue\n\n").encode('utf-8')

@@ -7,6 +7,7 @@
 from test.test_email import TestEmailBase, parameterize
 from email import headerregistry
 from email.headerregistry import Address, Group
+from email.header import decode_header
 from test.support import ALWAYS_EQ


@@ -1648,7 +1649,7 @@ def test_address_display_names(self):
                    'Lôrem ipsum dôlôr sit amet, cônsectetuer adipiscing. '
                    'Suspendisse pôtenti. Aliquam nibh. Suspendisse pôtenti.',
                    '=?utf-8?q?L=C3=B4rem_ipsum_d=C3=B4l=C3=B4r_sit_amet=2C_c'
-                    '=C3=B4nsectetuer?=\n =?utf-8?q?adipiscing=2E_Suspendisse'
+                    '=C3=B4nsectetuer?=\n =?utf-8?q?_adipiscing=2E_Suspendisse'
                    '_p=C3=B4tenti=2E_Aliquam_nibh=2E?=\n Suspendisse =?utf-8'
                    '?q?p=C3=B4tenti=2E?=',
                    ),

diff --git a/Misc/NEWS.d/next/Library/2023-04-26-22-24-17.gh-issue-92081.V8xMot.rst b/Misc/NEWS.d/next/Library/2023-04-26-22-24-17.gh-issue-92081.V8xMot.rst
@@ -0,0 +1 @@
+Fix missing spaces in email headers when the spaces are mixed with encoded 8-bit characters.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Fix missing spaces in email headers when the spaces are mixed with encoded 8-bit characters.