Fix a selector parsing bug

Parsing e.g. `:is(:is(foo))` (as a selector) does not consume the last `)`, which is obviously erroneous behaviour.

The bug stems from the body of the `parse_any_value` procedure in the `selectors` module which does not increment the counter number corresponding to parentheses when encountering a function token (which features these). With the wrong counter value, the first subsequently encountered `)` token (a `CloseParenToken`) would break the `any-value` parsing procedure before the latter token is consumed, as it would be considered "non-matching" (borrowing the term from the "Selectors" spec.).

This change fixes the issue by adding a `case` for `FunctionToken`, and supplements it all with an additional test, that was missing as well (meaning lack of coverage hid the issue all up until now, unfortunately).

Author: amn

src/csspring/selectors.py

@@ -8,7 +8,7 @@
 """
 
 from .syntax.parsing import Input as TokenStream, Product, tokens # Conveniently reusing some of the helpful constructs offered by the [CSS] syntax-level parsing module
-from .syntax.tokenizing import Token, BadStringToken, BadURLToken, CloseBraceToken, CloseBracketToken, CloseParenToken, ColonToken, DelimToken, FunctionToken, HashToken, IdentToken, OpenBraceToken, OpenBracketToken, OpenParenToken, StringToken
+from .syntax.tokenizing import Token, token_value, BadStringToken, BadURLToken, CloseBraceToken, CloseBracketToken, CloseParenToken, ColonToken, DelimToken, FunctionToken, HashToken, IdentToken, OpenBraceToken, OpenBracketToken, OpenParenToken, StringToken
 
 from .syntax.grammar import any_value
 from .values import Production, AlternativesProduction, CommaSeparatedRepetitionProduction, ConcatenationProduction, NonEmptyProduction, OptionalProduction, ReferenceProduction, RepetitionProduction, TokenProduction, OWS
@@ -56,6 +56,9 @@ def parse_any_value(input: TokenStream) -> Product | None:
                 break
             case OpenParenToken() | OpenBracketToken() | OpenBraceToken():
                 count[type(token)] += 1
+            case FunctionToken():
+                assert token.source[-1] == token_value(OpenParenToken)
+                count[OpenParenToken] += 1
             case CloseParenToken() | CloseBracketToken() | CloseBraceToken():
                 if count[token.mirror_type] == 0:
                     break

tests/test_selectors.py

@@ -26,7 +26,8 @@
     'foo[ bar ]',
     'foo[bar=baz]'
     'foo[bar= baz ]'
-    'foo[bar= "baz" ]'
+    'foo[bar= "baz" ]',
+    ':is(:is(foo))'
     ))
 def test_selector_parsing(text: str) -> None:
     """Test whether the text recovered from the product of parsing given text (as a selector [list]) equals given text.