Aborting a make/unmake search by throwing skips every pending unmake

Fact

Recursive search over a mutable structure is normally written as make, recurse, unmake:

apply(move);
const score = -search(depth - 1);
undo();

If the time limit is enforced by throwing from deep inside the recursion, the stack unwinds straight past every undo() on the way out. The structure is left exactly as mutated as the deepest node the search reached — not restored, and not obviously broken either.

Why it matters

The corruption is silent and delayed. The search still returns its best move from the previous completed depth, so the abort looks like it worked. The damage shows up at the next use of the same object: legal moves computed from a position nobody ever played, a move that the caller knows is legal being rejected, an undo stack that no longer matches the move list.

It also hides during development whenever each search gets a fresh copy of the state — a worker that rebuilds the position from a FEN or a serialized board never sees it. The bug only appears once the object is reused, which is usually in the test suite or in a single-threaded fallback path, long after the search was written.

How to apply

Record the undo-stack depth before searching and wind back to it in the handler, rather than trusting the recursion to clean up:

const base = state.history.length;
try { /* iterative deepening */ }
catch (e) { while (state.history.length > base) undo(); if (!(e instanceof TimeUp)) throw e; }

Or do not throw at all: return a sentinel score and check it at every frame, so each undo() still runs. Cheaper to reason about, noisier to write.
Assert the invariant in a test — search a position, then check the serialized state is byte-identical to what went in. A time-limited search that is never actually interrupted in tests will not reveal this, so force a timeout.
The same trap applies to any backtracking algorithm that restores shared state after the recursive call: constraint solvers, graph colouring, puzzle generators, transactional tree walks.