fix(rpn): restore stack values in popAll on partial failure (lj)

If stack.Pop() errors midway through the loop, already-popped values
are pushed back in reverse order before returning the error, preventing
stack corruption from concurrent modification or unexpected failures.

1 files changed, 4 insertions, 0 deletions

diff --git a/internal/rpn/operations_helpers.go b/internal/rpn/operations_helpers.go
index afdfa0e..03c0bb1 100644
--- a/internal/rpn/operations_helpers.go
+++ b/internal/rpn/operations_helpers.go
@@ -79,6 +79,10 @@ func popAll(stack *Stack, op string) ([]StackValue, error) {
 	for stack.Len() > 0 {
 		val, err := stack.Pop()
 		if err != nil {
+			// Restore already-popped values in reverse order (top of stack first).
+			for i := len(values) - 1; i >= 0; i-- {
+				stack.Push(values[i])
+			}
 			return nil, fmt.Errorf("%s: failed to pop: %w", op, err)
 		}
 		values = append(values, val)