Stack overflow when dropping a flat operator chain AST (parse succeeds) #2909

Summary

Parser::parse_sql accepts a flat left-associative operator chain and returns successfully, but dropping the returned Vec<Statement> then overflows the stack: the derived drop glue for the left-deep Box<Expr> tree recurses once per level with no guard. The process aborts (rc=134), and abort cannot be caught by the caller.

On a 2 MB stack (the default for spawned threads, including tokio workers) roughly 35,000 + operators — about 70 KB of input — is enough:

The parsed ... line is printed before the abort, so the overflow is in drop, not in parsing.

thread 'main' has overflowed its stack
fatal runtime error: stack overflow, aborting

Root cause

Expr and the rest of the AST derive Drop (src/ast/mod.rs). The only recursion guard on these types is

#[cfg_attr(feature = "recursive-protection", recursive::recursive)]
impl Display for Expr

at src/ast/mod.rs:1795; there is no impl Drop and no ManuallyDrop.

A flat chain such as SELECT 1+1+1... is built by the iterative precedence-climbing loop in parse_subexpr, so the recursion counter never fires, and the result is a left-deep BinaryOp { left: Box<Expr>, .. } spine. Dropping it walks that spine recursively, one stack frame per operator. Parser::with_recursion_limit does not help because parsing never recursed.

Reproduction

[dependencies]
sqlparser = "=0.63.0"

fn main() {
    let n: usize = std::env::args().nth(1).and_then(|s| s.parse().ok()).unwrap_or(132_000);
    let mut sql = String::from("SELECT 1");
    for _ in 0..n { sql.push_str("+1"); }
    let dialect = sqlparser::dialect::GenericDialect {};
    let stmts = sqlparser::parser::Parser::parse_sql(&dialect, &sql).expect("parse");
    println!("parsed {} statement(s), {} bytes", stmts.len(), sql.len());
    drop(stmts);
    println!("dropped ok");
}

$ cargo run --release -- 130625
parsed 1 statement(s), 261258 bytes
dropped ok
$ cargo run --release -- 132000
parsed 1 statement(s), 264008 bytes
thread 'main' has overflowed its stack
fatal runtime error: stack overflow, aborting   # rc=134

SELECT 1=1 AND 1=1 ... shows the same behaviour.

Suggested fix

Count the chain length against the existing recursion accounting: when the precedence loop folds another operator onto the same chain, add it to RecursionCounter and return RecursionLimitExceeded past the limit, matching what deeply nested parentheses already do. Alternatively, implement an iterative Drop for Expr with an explicit work stack, which also covers trees built programmatically.