golang_str,strconv: Fix decoding of rune-error

Error rune (u+fffd) is returned by _utf8_decode_rune to indicate an
error in decoding. But the error rune itself is valid unicode codepoint:

   >>> x = u"�"
   >>> x
   u'\ufffd'
   >>> x.encode('utf-8')
   '\xef\xbf\xbd'

This way only (r=_rune_error, size=1) should be treated by the caller as
utf8 decoding error.

But e.g. strconv.quote was not careful to also inspect the size, and this way
was quoting � into just "\xef" instead of "\xef\xbf\xbd".
_utf8_decode_surrogateescape was also subject to similar error.

-> Fix it.

Without the fix e.g. added test for strconv.quote fails as

    >           assert quote(tin) == tquoted
    E           assert '"\xef"' == '"�"'
    E             - "\xef"
    E             + "�"

/reviewed-by @jerome
/reviewed-at nexedi/pygolang!18

golang/_golang_str.pyx

@@ -242,7 +242,7 @@ def _utf8_decode_surrogateescape(s): # -> unicode
 
     while len(s) > 0:
         r, width = _utf8_decode_rune(s)
-        if r == _rune_error:
+        if r == _rune_error  and  width == 1:
             b = ord(s[0])
             assert 0x80 <= b <= 0xff
             emit(unichr(0xdc00 + b))

golang/golang_str_test.py

@@ -75,6 +75,9 @@ def test_strings():
         # some characters with U >= 0x10000
         (b'\xf0\x9f\x99\x8f',           u'\U0001f64f'),    # 🙏
         (b'\xf0\x9f\x9a\x80',           u'\U0001f680'),    # 🚀
+
+        # invalid rune
+        (b'\xef\xbf\xbd',               u'�'),
     )
 
     for tbytes, tunicode in testv:

golang/strconv.py

@@ -83,7 +83,7 @@ def _quote(s):
             isize = i + size
 
             # decode error - just emit raw byte as escaped
-            if r == _rune_error:
+            if r == _rune_error  and  size == 1:
                 emit(br'\x%02x' % ord(c))
 
             # printable utf-8 characters go as is

golang/strconv_test.py

 # -*- coding: utf-8 -*-
-# Copyright (C) 2018-2021  Nexedi SA and Contributors.
+# Copyright (C) 2018-2022  Nexedi SA and Contributors.
 #                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
@@ -67,6 +67,9 @@ def test_quote():
 
         # non-printable utf-8
         (u"\u007f\u0080\u0081\u0082\u0083\u0084\u0085\u0086\u0087", u"\\x7f\\xc2\\x80\\xc2\\x81\\xc2\\x82\\xc2\\x83\\xc2\\x84\\xc2\\x85\\xc2\\x86\\xc2\\x87"),
+
+        # invalid rune
+        (u'\ufffd',         u'�'),
     )
 
     for tin, tquoted in testv: