from __future__ import annotations

__all__ = [
    "DatabaseError",
    "ensure_select_only",
    "_ensure_select_only",
]


class DatabaseError(RuntimeError):
    """Raised for database connectivity/query errors."""


def ensure_select_only(query: str) -> None:
    """Ensure a SQL query is read-only.

    Args:
        query: SQL string.

    Raises:
        DatabaseError: If the query does not appear to be a read-only SELECT.

    Notes:
        This is a conservative guard. It intentionally allows only statements
        starting with SELECT or WITH (CTE) after stripping leading whitespace.
    """

    q = query.strip().lower()
    if not (q.startswith("select") or q.startswith("with")):
        raise DatabaseError("Only SELECT queries are allowed")


# Backwards-compatible alias (older call sites use the underscore name).
_ensure_select_only = ensure_select_only