gh-161102: Fix idna incremental decoder after leading empty labels

Count the separating dot for every label after the first, so dots after
leading empty labels are not decoded again by the next call.

Author: gpshead

Lib/encodings/idna.py

@@ -364,6 +364,9 @@ def _buffer_decode(self, input, errors, final):
         result = []
         size = 0
         for label in labels:
+            if result:
+                # Join with U+002E
+                size += 1
             try:
                 u_label = ToUnicode(label)
             except (UnicodeEncodeError, UnicodeDecodeError) as exc:
@@ -376,8 +379,6 @@ def _buffer_decode(self, input, errors, final):
                 )
             else:
                 result.append(u_label)
-            if size:
-                size += 1
             size += len(label)
 
         result = ".".join(result) + trailing_dot

Lib/test/test_codecs.py

@@ -1667,6 +1667,8 @@ class IDNACodecTest(unittest.TestCase):
         (b"xn--pythn-&mua.org", UnicodeDecodeError("idna", b"xn--pythn-&mua.org", 10, 11, "")),
         (b"xn--pythn-m&ua.org", UnicodeDecodeError("idna", b"xn--pythn-m&ua.org", 11, 12, "")),
         (b"xn--pythn-mua&.org", UnicodeDecodeError("idna", b"xn--pythn-mua&.org", 13, 14, "")),
+        (b"bugs.xn--pythn-&mua.org", UnicodeDecodeError("idna", b"bugs.xn--pythn-&mua.org", 15, 16, "")),
+        (b".xn--pythn-&mua.org", UnicodeDecodeError("idna", b".xn--pythn-&mua.org", 11, 12, "")),
     ]
     invalid_encode_testcases = [
         (f"foo.{'\xff'*60}", UnicodeEncodeError("idna", f"foo.{'\xff'*60}", 4, 64, "")),
@@ -1786,6 +1788,22 @@ def test_incremental_decode(self):
         self.assertEqual(decoder.decode(b"rg."), "org.")
         self.assertEqual(decoder.decode(b"", True), "")
 
+    def test_incremental_decode_leading_empty_labels(self):
+        # The dot after an empty label at the start of the input has to be
+        # counted as consumed, or it is decoded again by the next call.
+        for data in b".python.org", b"..python.org.", b"..", b"." * 100:
+            with self.subTest(data=data):
+                decoder = codecs.getincrementaldecoder("idna")()
+                result = decoder.decode(data)
+                self.assertNotIn(b".", decoder.getstate()[0])
+                result += decoder.decode(b"", True)
+                self.assertEqual(result, data.decode("idna"))
+                self.assertEqual(decoder.getstate(), (b"", 0))
+                self.assertEqual(decoder.decode(b"", True), "")
+
+        with io.TextIOWrapper(io.BytesIO(b"." * 100), encoding="idna") as f:
+            self.assertEqual(f.readline(), "." * 100)
+
     def test_incremental_decode_invalid(self):
         iterdecode_testcases = [
             (b"\xFFpython.org", UnicodeDecodeError("idna", b"\xFF", 0, 1, "")),

Misc/NEWS.d/next/Library/2026-10-22-05-20-02.gh-issue-161102.gqDOIM7X.rst

@@ -0,0 +1,4 @@
+Fix the incremental decoder of the :mod:`idna <encodings.idna>` codec
+decoding some dots twice when its input starts with empty labels, for example
+``b".example.com"``.  Reading a run of dots through :class:`io.TextIOWrapper`
+no longer produces quadratically more text than the input holds.