lint: Fix lint-format-strings false positives when format specifiers have argument positions

Do not error on valid format specifications like strprintf("arg2=%2$s arg1=%1$s arg2=%2$s", arg1, arg2);

Needed to avoid lint error in upcoming commit: https://cirrus-ci.com/task/4755032734695424?logs=lint#L221

Additionally tested with python -m doctest test/lint/run-lint-format-strings.py

test/lint/run-lint-format-strings.py

@@ -241,20 +241,32 @@ def count_format_specifiers(format_string):
     3
     >>> count_format_specifiers("foo %d bar %i foo %% foo %*d foo")
     4
+    >>> count_format_specifiers("foo %5$d")
+    5
+    >>> count_format_specifiers("foo %5$*7$d")
+    7
     """
     assert type(format_string) is str
     format_string = format_string.replace('%%', 'X')
-    n = 0
-    in_specifier = False
-    for i, char in enumerate(format_string):
-        if char == "%":
-            in_specifier = True
+    n = max_pos = 0
+    for m in re.finditer("%(.*?)[aAcdeEfFgGinopsuxX]", format_string, re.DOTALL):
+        # Increase the max position if the argument has a position number like
+        # "5$", otherwise increment the argument count.
+        pos_num, = re.match(r"(?:(^\d+)\$)?", m.group(1)).groups()
+        if pos_num is not None:
+            max_pos = max(max_pos, int(pos_num))
+        else:
             n += 1
-        elif char in "aAcdeEfFgGinopsuxX":
-            in_specifier = False
-        elif in_specifier and char == "*":
+
+        # Increase the max position if there is a "*" width argument with a
+        # position like "*7$", and increment the argument count if there is a
+        # "*" width argument with no position.
+        star, star_pos_num = re.match(r"(?:.*?(\*(?:(\d+)\$)?)|)", m.group(1)).groups()
+        if star_pos_num is not None:
+            max_pos = max(max_pos, int(star_pos_num))
+        elif star is not None:
             n += 1
-    return n
+    return max(n, max_pos)
 
 
 def main():