is_utf8_valid_partial_char_flags: Use DFA

The DFA macro for determining if a sequence is valid UTF-8 was
deliberately made general enough to accommodate this use-case, in which
only a partial character is acceptable.  Change the code to use the DFA.

The helper function's name is changed to indicate it is private

embed.fnc

@@ -1203,9 +1203,9 @@ ATidp	|bool	|is_utf8_fixed_width_buf_loclen_flags			    \
 		|NN const U8 * const s|STRLEN len			    \
 		|NULLOK const U8 **ep|NULLOK STRLEN *el|const U32 flags
 AmTdP	|bool	|is_utf8_valid_partial_char				    \
-		|NN const U8 * const s|NN const U8 * const e
+		|NN const U8 * const s0|NN const U8 * const e
 ATidRp	|bool	|is_utf8_valid_partial_char_flags			    \
-		|NN const U8 * const s|NN const U8 * const e|const U32 flags
+		|NN const U8 * const s0|NN const U8 * const e|const U32 flags
 CpR     |bool   |_is_uni_FOO|const U8 classnum|const UV c
 CpR     |bool   |_is_utf8_FOO|const U8 classnum|NN const U8 *p     \
 		|NN const U8 * const e

inline.h

@@ -2217,18 +2217,47 @@ determined from just the first one or two bytes.
  */
 
 PERL_STATIC_INLINE bool
-Perl_is_utf8_valid_partial_char_flags(const U8 * const s, const U8 * const e, const U32 flags)
+Perl_is_utf8_valid_partial_char_flags(const U8 * const s0, const U8 * const e, const U32 flags)
 {
     PERL_ARGS_ASSERT_IS_UTF8_VALID_PARTIAL_CHAR_FLAGS;
-
     assert(0 == (flags & ~(UTF8_DISALLOW_ILLEGAL_INTERCHANGE
                           |UTF8_DISALLOW_PERL_EXTENDED)));
 
-    if (s >= e || s + UTF8SKIP(s) <= e) {
-        return FALSE;
+    PERL_IS_UTF8_CHAR_DFA(s0, e, PL_extended_utf8_dfa_tab,
+                          DFA_RETURN_FAILURE_,
+                          DFA_TEASE_APART_FF_,
+                          NOOP);
+
+    /* The NOOP above causes the DFA to drop down here iff the input was a
+     * partial character.  flags=0 => can return TRUE immediately; otherwise we
+     * need to check (not inline) if the partial character is the beginning of
+     * a disallowed one */
+    if (flags == 0) {
+        return TRUE;
+    }
+
+    return cBOOL(is_utf8_char_helper(s0, e, flags));
+
+#ifdef HAS_EXTRA_LONG_UTF8
+
+  tease_apart_FF:
+
+    /* Getting here means the input is either malformed, or, in the case of
+     * PL_extended_utf8_dfa_tab, was for the largest possible start byte.  The
+     * latter case has to be extended UTF-8, so can fail immediately if that is
+     * forbidden */
+
+    if (   *s0 != I8_TO_NATIVE_UTF8(0xFF)
+        || (flags & (UTF8_DISALLOW_SUPER|UTF8_DISALLOW_PERL_EXTENDED)))
+    {
+        return 0;
     }
 
-    return cBOOL(is_utf8_char_helper(s, e, flags));
+    return is_utf8_FF_helper_(s0, e,
+                              TRUE /* Require to be a partial character */
+                             );
+#endif
+
 }
 
 /*

proto.h

@@ -1780,16 +1780,16 @@ PERL_STATIC_INLINE bool	Perl_is_utf8_string_loclen_flags(const U8 *s, STRLEN len
 #define PERL_ARGS_ASSERT_IS_UTF8_STRING_LOCLEN_FLAGS	\
 	assert(s)
 #endif
-/* PERL_CALLCONV bool	is_utf8_valid_partial_char(const U8 * const s, const U8 * const e)
+/* PERL_CALLCONV bool	is_utf8_valid_partial_char(const U8 * const s0, const U8 * const e)
 			__attribute__warn_unused_result__
 			__attribute__pure__; */
 #define PERL_ARGS_ASSERT_IS_UTF8_VALID_PARTIAL_CHAR
 
 #ifndef PERL_NO_INLINE_FUNCTIONS
-PERL_STATIC_INLINE bool	Perl_is_utf8_valid_partial_char_flags(const U8 * const s, const U8 * const e, const U32 flags)
+PERL_STATIC_INLINE bool	Perl_is_utf8_valid_partial_char_flags(const U8 * const s0, const U8 * const e, const U32 flags)
 			__attribute__warn_unused_result__;
 #define PERL_ARGS_ASSERT_IS_UTF8_VALID_PARTIAL_CHAR_FLAGS	\
-	assert(s); assert(e)
+	assert(s0); assert(e)
 #endif
 
 PERL_CALLCONV bool	Perl_isinfnan(NV nv)