Fix slow backtracking when parsing strings (no external deps) (#347)

* Fix slow backtracking when parsing strings (no external deps)

Fixes #61

This uses negative lookaheads to avoid ambiguity in how string should be
parsed by the regex.
- https://docs.python.org/2/library/re.html#regular-expression-syntax

- Previously, if it didn't immediately succeed at parsing an escape
  sequence such as `\123`, it would have to try `\1`+`23`, `\12` + `3`,
  and `\123`, which multiplied the time taken by 3 per additional escape
  sequence. This solves that by only allowing `\123`
- The same fix was added for hex escapes.

Also fix a test that relied on the incorrect handling of regexes.
The implementation documentation says that it intends to allow
**decimal** escapes permissively.

* WIP debug

* Fix ambiguity caused by allowing #path directives

Solve this by allowing "\x" when not followed by hex,
in the regular string literal.

In the previous commits,
`\x12` could be parsed both as `\x`+`12` and `\x12`,
which caused exponential options for backtracking.

* Document changes to lexer, remove debug code

* Optimize this for strings

2 files changed, 62 insertions, 8 deletions

diff --git a/pycparser/c_lexer.py b/pycparser/c_lexer.py
index de8445e..371e996 100644
--- a/pycparser/c_lexer.py
+++ b/pycparser/c_lexer.py
@@ -19,7 +19,7 @@ class CLexer(object):
         tokens.
 
         The public attribute filename can be set to an initial
-        filaneme, but the lexer will update it upon #line
+        filename, but the lexer will update it upon #line
         directives.
     """
     def __init__(self, error_func, on_lbrace_func, on_rbrace_func,
@@ -205,12 +205,37 @@ class CLexer(object):
     # parse all correct code, even if it means to sometimes parse incorrect
     # code.
     #
-    simple_escape = r"""([a-zA-Z._~!=&\^\-\\?'"])"""
-    decimal_escape = r"""(\d+)"""
-    hex_escape = r"""(x[0-9a-fA-F]+)"""
-    bad_escape = r"""([\\][^a-zA-Z._~^!=&\^\-\\?'"x0-7])"""
+    # The original regexes were taken verbatim from the C syntax definition,
+    # and were later modified to avoid worst-case exponential running time.
+    #
+    #   simple_escape = r"""([a-zA-Z._~!=&\^\-\\?'"])"""
+    #   decimal_escape = r"""(\d+)"""
+    #   hex_escape = r"""(x[0-9a-fA-F]+)"""
+    #   bad_escape = r"""([\\][^a-zA-Z._~^!=&\^\-\\?'"x0-7])"""
+    #
+    # The following modifications were made to avoid the ambiguity that allowed backtracking:
+    # (https://github.com/eliben/pycparser/issues/61)
+    #
+    # - \x was removed from simple_escape, unless it was not followed by a hex digit, to avoid ambiguity with hex_escape.
+    # - hex_escape allows one or more hex characters, but requires that the next character(if any) is not hex
+    # - decimal_escape allows one or more decimal characters, but requires that the next character(if any) is not a decimal
+    # - bad_escape does not allow any decimals (8-9), to avoid conflicting with the permissive decimal_escape.
+    #
+    # Without this change, python's `re` module would recursively try parsing each ambiguous escape sequence in multiple ways.
+    # e.g. `\123` could be parsed as `\1`+`23`, `\12`+`3`, and `\123`.
+
+    simple_escape = r"""([a-wyzA-Z._~!=&\^\-\\?'"]|x(?![0-9a-fA-F]))"""
+    decimal_escape = r"""(\d+)(?!\d)"""
+    hex_escape = r"""(x[0-9a-fA-F]+)(?![0-9a-fA-F])"""
+    bad_escape = r"""([\\][^a-zA-Z._~^!=&\^\-\\?'"x0-9])"""
 
     escape_sequence = r"""(\\("""+simple_escape+'|'+decimal_escape+'|'+hex_escape+'))'
+
+    # This complicated regex with lookahead might be slow for strings, so because all of the valid escapes (including \x) allowed
+    # 0 or more non-escaped characters after the first character, simple_escape+decimal_escape+hex_escape got simplified to
+
+    escape_sequence_start_in_string = r"""(\\[0-9a-zA-Z._~!=&\^\-\\?'"])"""
+
     cconst_char = r"""([^'\\\n]|"""+escape_sequence+')'
     char_const = "'"+cconst_char+"'"
     wchar_const = 'L'+char_const
@@ -218,10 +243,10 @@ class CLexer(object):
     bad_char_const = r"""('"""+cconst_char+"""[^'\n]+')|('')|('"""+bad_escape+r"""[^'\n]*')"""
 
     # string literals (K&R2: A.2.6)
-    string_char = r"""([^"\\\n]|"""+escape_sequence+')'
+    string_char = r"""([^"\\\n]|"""+escape_sequence_start_in_string+')'
     string_literal = '"'+string_char+'*"'
     wstring_literal = 'L'+string_literal
-    bad_string_literal = '"'+string_char+'*?'+bad_escape+string_char+'*"'
+    bad_string_literal = '"'+string_char+'*'+bad_escape+string_char+'*"'
 
     # floating constants (K&R2: A.2.5.3)
     exponent_part = r"""([eE][-+]?[0-9]+)"""
diff --git a/tests/test_c_lexer.py b/tests/test_c_lexer.py
index 11c7b26..3a70c18 100644
--- a/tests/test_c_lexer.py
+++ b/tests/test_c_lexer.py
@@ -116,6 +116,7 @@ class TestCLexerNoErrors(unittest.TestCase):
         self.assertTokensTypes(r"""'\t'""", ['CHAR_CONST'])
         self.assertTokensTypes(r"""'\''""", ['CHAR_CONST'])
         self.assertTokensTypes(r"""'\?'""", ['CHAR_CONST'])
+        self.assertTokensTypes(r"""'\0'""", ['CHAR_CONST'])
         self.assertTokensTypes(r"""'\012'""", ['CHAR_CONST'])
         self.assertTokensTypes(r"""'\x2f'""", ['CHAR_CONST'])
         self.assertTokensTypes(r"""'\x2f12'""", ['CHAR_CONST'])
@@ -149,6 +150,24 @@ class TestCLexerNoErrors(unittest.TestCase):
         self.assertTokensTypes(
             '"\123\123\123\123\123\123\123\123\123\123\123\123\123\123\123\123"',
             ['STRING_LITERAL'])
+        # Note: a-zA-Z and '.-~^_!=&;,' are allowed as escape chars to support #line
+        # directives with Windows paths as filenames (..\..\dir\file)
+        self.assertTokensTypes(
+            r'"\x"',
+            ['STRING_LITERAL'])
+        self.assertTokensTypes(
+            r'"\a\b\c\d\e\f\g\h\i\j\k\l\m\n\o\p\q\r\s\t\u\v\w\x\y\z\A\B\C\D\E\F\G\H\I\J\K\L\M\N\O\P\Q\R\S\T\U\V\W\X\Y\Z"',
+            ['STRING_LITERAL'])
+        self.assertTokensTypes(
+            r'"C:\x\fa\x1e\xited"',
+            ['STRING_LITERAL'])
+        # The lexer is permissive and allows decimal escapes (not just octal)
+        self.assertTokensTypes(
+            '"jx\9"',
+            ['STRING_LITERAL'])
+        self.assertTokensTypes(
+            '"fo\9999999"',
+            ['STRING_LITERAL'])
 
     def test_mess(self):
         self.assertTokensTypes(
@@ -428,14 +447,24 @@ class TestCLexerErrors(unittest.TestCase):
     def test_char_constants(self):
         self.assertLexerError("'", ERR_UNMATCHED_QUOTE)
         self.assertLexerError("'b\n", ERR_UNMATCHED_QUOTE)
+        self.assertLexerError("'\\xaa\n'", ERR_UNMATCHED_QUOTE)
 
+        self.assertLexerError(r"'\12a'", ERR_INVALID_CCONST)
+        self.assertLexerError(r"'\xabg'", ERR_INVALID_CCONST)
+        self.assertLexerError("''", ERR_INVALID_CCONST)
         self.assertLexerError("'jx'", ERR_INVALID_CCONST)
         self.assertLexerError(r"'\*'", ERR_INVALID_CCONST)
 
     def test_string_literals(self):
-        self.assertLexerError(r'"jx\9"', ERR_STRING_ESCAPE)
+        self.assertLexerError(r'"jx\`"', ERR_STRING_ESCAPE)
         self.assertLexerError(r'"hekllo\* on ix"', ERR_STRING_ESCAPE)
         self.assertLexerError(r'L"hekllo\* on ix"', ERR_STRING_ESCAPE)
+        # Should not suffer from slow backtracking
+        self.assertLexerError(r'"\123\123\123\123\123\123\123\123\123\123\123\123\123\123\123\123\123\123\123\`\123\123\123\123\123\123\123\123\123\123\123\123\123\123\123\123\123\123\123\123"', ERR_STRING_ESCAPE)
+        self.assertLexerError(r'"\xf1\x23\xf1\x23\xf1\x23\xf1\x23\xf1\x23\xf1\x23\xf1\x23\xf1\x23\xf1\x23\x23\`\xf1\x23\xf1\x23\xf1\x23\xf1\x23\xf1\x23\xf1\x23\xf1\x23\xf1\x23\xf1\x23\xf1\x23"', ERR_STRING_ESCAPE)
+        # Should not suffer from slow backtracking when there's no end quote
+        self.assertLexerError(r'"\123\123\123\123\123\123\123\123\123\123\123\123\123\123\123\`\123\123\123\123\123\123\123\123\123\123\123\123\123\123\123\123\123\123\12\123456', ERR_ILLEGAL_CHAR)
+        self.assertLexerError(r'"\x23\x23\x23\x23\x23\x23\x23\x23\x23\x23\x23\x23\x23\x23\x23\`\x23\x23\x23\x23\x23\x23\x23\x23\x23\x23\x23\x23\x23\x23\x23\x23\x23\x23\x2\x23456', ERR_ILLEGAL_CHAR)
 
     def test_preprocessor(self):
         self.assertLexerError('#line "ka"', ERR_FILENAME_BEFORE_LINE)