学生骑手分时接单怎么管理？时段修改与接单的事务边界

CREATE TABLE rider_policy (
 campus_id text NOT NULL, rider_id text NOT NULL,
 approved boolean NOT NULL, enabled boolean NOT NULL,
 valid_from timestamptz NOT NULL, valid_to timestamptz NOT NULL,
 revision bigint NOT NULL CHECK(revision>0),
 PRIMARY KEY(campus_id,rider_id), CHECK(valid_to>valid_from)
);
CREATE TABLE task (
 campus_id text NOT NULL, task_id text NOT NULL,
 state text NOT NULL CHECK(state IN ('OPEN','ACCEPTED','PICKED','DONE')),
 rider_id text, policy_revision bigint, accepted_at timestamptz,
 PRIMARY KEY(campus_id,task_id),
 CHECK ((state='OPEN' AND rider_id IS NULL AND policy_revision IS NULL
                      AND accepted_at IS NULL)
     OR (state<>'OPEN' AND rider_id IS NOT NULL AND policy_revision IS NOT NULL
                       AND accepted_at IS NOT NULL))
);
-- 认证与campus授权检查通过后；参数只接受已校验输入。
BEGIN;
SELECT * FROM rider_policy WHERE campus_id=:campus AND rider_id=:rider
 FOR UPDATE;
-- 无行：ROLLBACK，返回资格不存在。
-- 获取锁后从数据库读取clock_timestamp()，作为:decision_time。
-- approved/enabled为真且valid_from<=decision_time<valid_to才继续。
-- 同时核对预计完成跨时段风险，按业务契约提示或拒绝。
UPDATE task SET state='ACCEPTED',rider_id=:rider,
 policy_revision=:locked_revision,accepted_at=:decision_time
 WHERE campus_id=:campus AND task_id=:task AND state='OPEN'
 RETURNING *;
-- 返回一行才COMMIT；零行查询原任务，同骑手回放，其他情况冲突。
-- 所有出错路径ROLLBACK，不先发送接单成功通知。