string-util: introduce strspn_from_end()

src/basic/string-util.c

@@ -1174,3 +1174,18 @@ char *string_replace_char(char *str, char old_char, char new_char) {
 
         return str;
 }
+
+size_t strspn_from_end(const char *str, const char *accept) {
+        size_t n = 0;
+
+        if (isempty(str))
+                return 0;
+
+        if (isempty(accept))
+                return 0;
+
+        for (const char *p = str + strlen(str); p > str && strchr(accept, p[-1]); p--)
+                n++;
+
+        return n;
+}

src/basic/string-util.h

@@ -235,3 +235,5 @@ static inline int string_contains_word(const char *string, const char *separator
 bool streq_skip_trailing_chars(const char *s1, const char *s2, const char *ok);
 
 char *string_replace_char(char *str, char old_char, char new_char);
+
+size_t strspn_from_end(const char *str, const char *accept);