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gh-93671: Avoid exponential backtracking in deeply nested sequence patterns in match statements #93680

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Merged
ambv merged 2 commits into from
Jun 10, 2022
+36 −3
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gh-93671: Avoid exponential backtracking in deeply nested sequence patterns in match statements #93680

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e042cbb
gh-93671: Avoid exponential backtracking in deeply nested sequence pa…
pablogsal Jun 10, 2022
9907737
s/patters/patterns/
ambv Jun 10, 2022
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gh-93671: Avoid exponential backtracking in deeply nested sequence pa… 
…tterns in match statements
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@pablogsal
pablogsal committed Jun 10, 2022
commit e042cbbe9e148ca9d9434a2f7621b49131f79597
6 changes: 3 additions & 3 deletions Grammar/python.gram
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Original file line number Diff line number Diff line change
Expand Up @@ -471,7 +471,7 @@ or_pattern[pattern_ty]:
| patterns[asdl_pattern_seq*]='|'.closed_pattern+ {
asdl_seq_LEN(patterns) == 1 ? asdl_seq_GET(patterns, 0) : _PyAST_MatchOr(patterns, EXTRA) }

closed_pattern[pattern_ty]:
closed_pattern[pattern_ty] (memo):
| literal_pattern
| capture_pattern
| wildcard_pattern
Expand Down Expand Up @@ -558,7 +558,7 @@ maybe_star_pattern[pattern_ty]:
| star_pattern
| pattern

star_pattern[pattern_ty]:
star_pattern[pattern_ty] (memo):
| '*' target=pattern_capture_target {
_PyAST_MatchStar(target->v.Name.id, EXTRA) }
| '*' wildcard_pattern {
Expand Down Expand Up @@ -1312,4 +1312,4 @@ invalid_kvpair:
| a=expression !(':') {
RAISE_ERROR_KNOWN_LOCATION(p, PyExc_SyntaxError, a->lineno, a->end_col_offset - 1, a->end_lineno, -1, "':' expected after dictionary key") }
| expression ':' a='*' bitwise_or { RAISE_SYNTAX_ERROR_STARTING_FROM(a, "cannot use a starred expression in a dictionary value") }
| expression a=':' {RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "expression expected after dictionary key and ':'") }
| expression a=':' {RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "expression expected after dictionary key and ':'") }
21 changes: 21 additions & 0 deletions Lib/test/test_patma.py
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Expand Up @@ -3151,6 +3151,27 @@ def f(command): # 0
self.assertListEqual(self._trace(f, "go x"), [1, 2, 3])
self.assertListEqual(self._trace(f, "spam"), [1, 2, 3])

def test_parser_deeply_nested_patters(self):
# Deeply nested patterns can cause exponential backtracking when parsing.
# See gh-93671 for more information.

levels = 100

patterns = [
"A" + "(" * levels + ")" * levels,
"{1:" * levels + "1" + "}" * levels,
"[" * levels + "1" + "]" * levels,
]

for pattern in patterns:
with self.subTest(pattern):
code = inspect.cleandoc("""
match None:
case {}:
pass
""".format(pattern))
compile(code, "<string>", "exec")


if __name__ == "__main__":
"""
Expand Down
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core and Builtins/2022-06-10-12-03-17.gh-issue-93671.idkQqG.rst
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@@ -0,0 +1,2 @@
Fix some exponential backtrace case happening with deeply nested sequence
patterns in match statements. Patch by Pablo Galindo
10 changes: 10 additions & 0 deletions Parser/parser.c
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