#16811: Fix folding of headers with no value in provisional policies.

Lib/email/policy.py

@@ -173,7 +173,7 @@ class EmailPolicy(Policy):
         lines = value.splitlines()
         refold = (self.refold_source == 'all' or
                   self.refold_source == 'long' and
-                    (len(lines[0])+len(name)+2 > maxlen or
+                    (lines and len(lines[0])+len(name)+2 > maxlen or
                      any(len(x) > maxlen for x in lines[1:])))
         if refold or refold_binary and _has_surrogates(value):
             return self.header_factory(name, ''.join(lines)).fold(policy=self)

Lib/test/test_email/test_inversion.py

@@ -0,0 +1,45 @@
+"""Test the parser and generator are inverses.
+
+Note that this is only strictly true if we are parsing RFC valid messages and
+producing RFC valid messages.
+"""
+
+import io
+import unittest
+from email import policy, message_from_bytes
+from email.generator import BytesGenerator
+from test.test_email import TestEmailBase, parameterize
+
+# This is like textwrap.dedent for bytes, except that it uses \r\n for the line
+# separators on the rebuilt string.
+def dedent(bstr):
+    lines = bstr.splitlines()
+    if not lines[0].strip():
+        raise ValueError("First line must contain text")
+    stripamt = len(lines[0]) - len(lines[0].lstrip())
+    return b'\r\n'.join(
+        [x[stripamt:] if len(x)>=stripamt else b''
+            for x in lines])
+
+
+@parameterize
+class TestInversion(TestEmailBase, unittest.TestCase):
+
+    def msg_as_input(self, msg):
+        m = message_from_bytes(msg, policy=policy.SMTP)
+        b = io.BytesIO()
+        g = BytesGenerator(b)
+        g.flatten(m)
+        self.assertEqual(b.getvalue(), msg)
+
+    # XXX: spaces are not preserved correctly here yet in the general case.
+    msg_params = {
+        'header_with_one_space_body': (dedent(b"""\
+            From: abc@xyz.com
+            X-Status:\x20
+            Subject: test
+
+            foo
+            """),),
+
+            }