gh-135854: Fix function email.message.Message.get_boundary strips boundaries twice

Lib/email/message.py

@@ -862,7 +862,7 @@ def get_boundary(self, failobj=None):
         parameter, and it is unquoted.
         """
         missing = object()
-        boundary = self.get_param('boundary', missing)
+        boundary = self.get_param('boundary', missing, unquote=False)
         if boundary is missing:
             return failobj
         # RFC 2046 says that boundaries may begin but not end in w/s

Lib/test/test_email/test_email.py

@@ -2209,6 +2209,42 @@ def test_boundary_with_leading_space(self):
         eq(msg.get_boundary(), '    XXXX')
         eq(len(msg.get_payload()), 2)
 
+    def test_boundary_stripped_only_once(self):
+        eq = self.assertEqual
+        msg = email.message_from_string('''\
+MIME-Version: 1.0
+Content-Type: multipart/mixed; boundary="<>"
+
+--<>
+Content-Type: text/plain
+
+
+--<>
+Content-Type: text/plain
+
+--<>--
+''')
+        self.assertTrue(msg.is_multipart())
+        eq(msg.get_boundary(), '<>')
+        eq(len(msg.get_payload()), 2)
+
+        msg = email.message_from_string('''\
+MIME-Version: 1.0
+Content-Type: multipart/mixed; boundary=<"">
+
+--""
+Content-Type: text/plain
+
+
+--""
+Content-Type: text/plain
+
+--""--
+''')
+        self.assertTrue(msg.is_multipart())
+        eq(msg.get_boundary(), '""')
+        eq(len(msg.get_payload()), 2)
+
     def test_boundary_without_trailing_newline(self):
         m = Parser().parsestr("""\
 Content-Type: multipart/mixed; boundary="===============0012394164=="

Misc/NEWS.d/next/Library/2025-06-24-21-08-26.gh-issue-135854.j0WMfD.rst

@@ -0,0 +1 @@
+Fix :func:`email.message.Message.get_boundary` strips boundaries twice.