parser: enforce DepthGuard in DDL and stop trigger-body infinite loop

[chiradip]

Summary

Two related crash / DoS defects on nested CREATE TRIGGER, both in the DDL recursion path. Found during the bottom-up audit (the tokenizer layer came back clean; this is the parser layer).

1. Defeated DepthGuard → stack overflow

Every DDL entry point (12 sites in parser_ddl.cpp) and parse_transaction_stmt (parser_statements.cpp) wrote the recursion guard as an if-init-statement:

if (const DepthGuard guard(this); !guard.is_valid()) return nullptr;

The guard object is destroyed at the end of the if, so depth is incremented then immediately decremented and max_depth is never enforced for the function body. Nested CREATE TRIGGER recurses parse_create_trigger → parse_statement → parse_create_stmt → parse_create_trigger with no participant holding depth → unbounded native recursion → stack-overflow crash (confirmed under ASan). Rewritten as a function-scope local, matching the correct form already used in the select/expression parsers:

DepthGuard guard(this);
if (!guard.is_valid()) return nullptr;

2. No-forward-progress infinite loop

The BEGIN … END trigger-body loop called parse_statement() and, on nullptr, only advanced when the next token was ;. On any token it cannot start a statement from — a stray token such as @, or the depth guard now returning nullptr — it re-called parse_statement() on the same token forever. Once fix (1) stops the stack overflow, deep BEGIN..END input degraded into this hang; it is also independently triggerable by CREATE TRIGGER x AFTER INSERT ON t BEGIN @ END. Added a forward-progress guard: if an iteration neither parsed a statement nor consumed a separator, break (the parser is lenient about the unconsumed remainder).

Both defects were reproduced under ASan/UBSan and now resolve to a graceful "Maximum recursion depth exceeded" (deep nesting) or clean termination (stray token) — never a crash or hang.

Tests

tests/security/test_depth_guard.cpp:

• shallow nested triggers (both body forms) parse;
• deeply nested single-statement and BEGIN..END trigger chains are rejected with the depth error;
• a stray token in a trigger body terminates and leaves the parser reusable.

Falsifiable: reverting (1) restores the ASan stack-overflow; reverting (2) restores the infinite-loop hang. Full suite 37/37; depth-guard suite (11 tests) clean under ASan/UBSan.