clanker-saddle/src/Data/Migrations/M0007_ParkingGuard.vala

using InvercargillSql.Migrations;
using Sqlite;

namespace ClankerSaddle.Data.Migrations {

    /**
     * Goal 27 refinements — the parking guard. `saddle suspend` on a
     * goal run that owns NO live task is refused by the web (there is
     * nothing whose terminal transition could ever wake the parked
     * agent): the park mechanics still run, but the run carries a
     * one-shot `pending_resume_notice`, so the daemon's next ask hands
     * it the refusal instruction instead of a sleep nothing ends.
     *
     * tasks.pending_resume_notice is that one-shot channel. It is set
     * by the refused park and consumed by the allocation that hands
     * the assignment out (the resume_notice discipline): while set,
     * a suspended task is immediately offerable — the exact lift the
     * task-level force_wake flag gives a plain parked task, but
     * carrying the guard's own instruction text. Nullable, NULL when
     * no refusal is pending.
     *
     * IDEMPOTENCE: same contract as M0002–M0006 — up() runs before the
     * apply transaction opens (the builder only queues), so it
     * introspects sqlite_master through a separate read-only handle
     * and adds the column only when the schema still lacks it.
     */
    public class M0007_ParkingGuard : Migration {

        public override string name { get { return "Parking guard"; } }
        public override uint64 serial { get { return 7; } }
        public override string migration_namespace { get { return "clanker-saddle"; } }

        // Same resolution as M0006: the runner never hands up() a
        // connection, so the schema is read through a read-only handle
        // on the SPRY_DB_PATH main.vala connects to.
        private string db_path() {
            var path = Environment.get_variable("SPRY_DB_PATH") ?? "";
            if (path.strip().length == 0) {
                return "./clanker-saddle.sqlite";
            }
            return path;
        }

        private bool column_exists() {
            Database db;
            int rc = Database.open_v2(db_path(), out db, Sqlite.OPEN_READONLY);
            if (rc != Sqlite.OK) {
                return false;
            }
            Statement statement;
            rc = db.prepare_v2(
                "select 1 from pragma_table_info('tasks') where name = 'pending_resume_notice'",
                -1, out statement);
            if (rc != Sqlite.OK) {
                return false;
            }
            return statement.step() == Sqlite.ROW;
        }

        public override void up(MigrationBuilder b) {
            if (column_exists()) {
                return;
            }
            b.alter_table("tasks", (t) => {
                t.add_column<string>("pending_resume_notice");
            });
        }

        public override void down(MigrationBuilder b) {
            // SQLite cannot drop a column through the builder's
            // alter_table path; the raw statement is the honest form.
            b.execute_sql("alter table tasks drop column pending_resume_notice");
        }
    }
}