gh-70278: Fix PyUnicode_FromFormat() with precision for %s and %V (GH-120365)

PyUnicode_FromFormat() no longer produces the ending \ufffd
character for truncated C string when use precision with %s and %V.
It now truncates the string before the start of truncated multibyte sequences.

Lib/test/test_capi/test_unicode.py

@@ -419,8 +419,29 @@ class CAPITest(unittest.TestCase):
         # truncated string
         check_format('abc',
                      b'%.3s', b'abcdef')
+        check_format('abc[',
+                     b'%.6s', 'abc[\u20ac]'.encode('utf8'))
+        check_format('abc[\u20ac',
+                     b'%.7s', 'abc[\u20ac]'.encode('utf8'))
         check_format('abc[\ufffd',
-                     b'%.5s', 'abc[\u20ac]'.encode('utf8'))
+                     b'%.5s', b'abc[\xff]')
+        check_format('abc[',
+                     b'%.6s', b'abc[\xe2\x82]')
+        check_format('abc[\ufffd]',
+                     b'%.7s', b'abc[\xe2\x82]')
+        check_format('abc[\ufffd',
+                     b'%.7s', b'abc[\xe2\x82\0')
+        check_format('      abc[',
+                     b'%10.6s', 'abc[\u20ac]'.encode('utf8'))
+        check_format('     abc[\u20ac',
+                     b'%10.7s', 'abc[\u20ac]'.encode('utf8'))
+        check_format('     abc[\ufffd',
+                     b'%10.5s', b'abc[\xff]')
+        check_format('      abc[',
+                     b'%10.6s', b'abc[\xe2\x82]')
+        check_format('    abc[\ufffd]',
+                     b'%10.7s', b'abc[\xe2\x82]')
+
         check_format("'\\u20acABC'",
                      b'%A', '\u20acABC')
         check_format("'\\u20",
@@ -433,10 +454,31 @@ class CAPITest(unittest.TestCase):
                      b'%.3S', '\u20acABCDEF')
         check_format('\u20acAB',
                      b'%.3U', '\u20acABCDEF')
+
         check_format('\u20acAB',
                      b'%.3V', '\u20acABCDEF', None)
+        check_format('abc[',
+                     b'%.6V', None, 'abc[\u20ac]'.encode('utf8'))
+        check_format('abc[\u20ac',
+                     b'%.7V', None, 'abc[\u20ac]'.encode('utf8'))
         check_format('abc[\ufffd',
-                     b'%.5V', None, 'abc[\u20ac]'.encode('utf8'))
+                     b'%.5V', None, b'abc[\xff]')
+        check_format('abc[',
+                     b'%.6V', None, b'abc[\xe2\x82]')
+        check_format('abc[\ufffd]',
+                     b'%.7V', None, b'abc[\xe2\x82]')
+        check_format('      abc[',
+                     b'%10.6V', None, 'abc[\u20ac]'.encode('utf8'))
+        check_format('     abc[\u20ac',
+                     b'%10.7V', None, 'abc[\u20ac]'.encode('utf8'))
+        check_format('     abc[\ufffd',
+                     b'%10.5V', None, b'abc[\xff]')
+        check_format('      abc[',
+                     b'%10.6V', None, b'abc[\xe2\x82]')
+        check_format('    abc[\ufffd]',
+                     b'%10.7V', None, b'abc[\xe2\x82]')
+        check_format('     abc[\ufffd',
+                     b'%10.7V', None, b'abc[\xe2\x82\0')
 
         # following tests comes from #7330
         # test width modifier and precision modifier with %S

Misc/NEWS.d/next/C API/2024-06-11-21-38-32.gh-issue-70278.WDE4zM.rst

@@ -0,0 +1,4 @@
+:c:func:`PyUnicode_FromFormat` no longer produces the ending ``\ufffd``
+character for truncated C string when use precision with ``%s`` and ``%V``.
+It now truncates the string before the start of truncated multibyte
+sequences.

Objects/unicodeobject.c

@@ -2581,6 +2581,7 @@ unicode_fromformat_write_utf8(_PyUnicodeWriter *writer, const char *str,
                               Py_ssize_t width, Py_ssize_t precision, int flags)
 {
     /* UTF-8 */
+    Py_ssize_t *pconsumed = NULL;
     Py_ssize_t length;
     if (precision == -1) {
         length = strlen(str);
@@ -2590,15 +2591,23 @@ unicode_fromformat_write_utf8(_PyUnicodeWriter *writer, const char *str,
         while (length < precision && str[length]) {
             length++;
         }
+        if (length == precision) {
+            /* The input string is not NUL-terminated.  If it ends with an
+             * incomplete UTF-8 sequence, truncate the string just before it.
+             * Incomplete sequences in the middle and sequences which cannot
+             * be valid prefixes are still treated as errors and replaced
+             * with \xfffd. */
+            pconsumed = &length;
+        }
     }
 
     if (width < 0) {
         return unicode_decode_utf8_writer(writer, str, length,
-                                          _Py_ERROR_REPLACE, "replace", NULL);
+                                          _Py_ERROR_REPLACE, "replace", pconsumed);
     }
 
     PyObject *unicode = PyUnicode_DecodeUTF8Stateful(str, length,
-                                                     "replace", NULL);
+                                                     "replace", pconsumed);
     if (unicode == NULL)
         return -1;