CVE-2026-54284: sqlparse: TokenList.__init__ materializes O(subtree) value per group, causing CPU DoS before depth/token caps trigger

sqlparse ships hard limits (MAX_GROUPING_DEPTH=100, MAX_GROUPING_TOKENS=10000) intended to bound parsing work on attacker-supplied SQL, but the path that reaches those limits is itself O(n*depth) per token-group construction. A ~1-2 KB SQL payload (e.g. SELECT (((((1))))) ... with 500-2000 nesting levels, or a 200-400-level nested CASE WHEN chain) drives the parser to spend multiple seconds of CPU before the depth cap raises SQLParseError. Concretely: a 2 KB malicious payload consumes ~10 seconds of CPU per request on a single worker (~5000x CPU-to-input amplification), while a benign 1 KB SQL completes in ~3 ms.

The root cause is TokenList.__init__ calling super().__init__(None, str(self)). TokenList.__str__ flattens the entire subtree on every call, and grouping constructs a new TokenList for every parenthesis / CASE / list group, so a tree of depth d with n total tokens performs O(n*d) flatten work just to materialize the cached value field, which is then never read for grouped nodes (they override __str__).

This is a distinct quadratic from the input-size caps added in GHSA-2m57-hf25-phgg / GHSA-27jp-wm6q-gp25: those caps prevent unbounded work, but the time required to trigger the caps is itself superlinear in payload size.