GHSA-f2ff-p2ww-7p4p

### Summary A comment-only statement (`-- c\n`*n) may cause a Denial of Service (DoS).

### Details Location: [sqlparse/engine/grouping.py:331-341](https://github.com/andialbrecht/sqlparse/blob/f80af6a4007f11ada847218df8c29dc859238290/sqlparse/engine/grouping.py#L332) (`group_comments`), invoked first in `group()` at `grouping.py:439`. Reachable via `sqlparse.parse()` and `sqlparse.format(sql, strip_comments=True)`.

A statement made of many single-line comments (`'-- c\n'` repeated) lexes in O(n) but `group_comments` is O(n²):

```python def group_comments(tlist): tidx, token = tlist.token_next_by(t=T.Comment) while token: eidx, end = tlist.token_not_matching( lambda tk: imt(tk, t=T.Comment) or tk.is_newline, idx=tidx) ... tidx, token = tlist.token_next_by(t=T.Comment, idx=tidx) ```

The `while` loop runs n times and each `token_next_by` / `token_not_matching` rescans the O(n) remaining tokens. When all tokens are comments/newlines nothing ever groups, yet the full scan is repeated per token.

Two following factors increase the severity:

1. `group_comments` runs first in `group()` (`grouping.py:439`), before the `_group_matching` token-count guard (`grouping.py:34-39`). So the entire quadratic cost is paid even on oversized input. `MAX_GROUPING_TOKENS` does not provide protection on this vector. 2. It sits on the primary sanitizer path: `format(sql, strip_comments=True)`, used by query loggers, SQL firewalls, ORMs, and migration tools.

### PoC Tested using Python 3.14:

```python import time, sqlparse for n in (1000, 2000, 4000): s = "-- c\n" * n t = time.perf_counter() sqlparse.format(s, strip_comments=True) print(f"n={n:5d} format(strip_comments)={1000*(time.perf_counter()-t):7.1f} ms") ```

Output:

``` n= 1000 format(strip_comments)= 106.0 ms n= 2000 format(strip_comments)= 403.3 ms n= 4000 format(strip_comments)= 1602.8 ms ```

Time increase of ~4× per 2× input (quadratic). `parse()` shows the identical curve. Instrumented scan counts are exactly 1.0M / 4.0M / 16.0M tokens for n=1000/2000/4000. A ~250 KB comment-only payload forces minutes of CPU regardless of the 10000 token cap.

### Impact Denial of Service