Struct Checkpoint 

pub struct Checkpoint<S: WorkflowState = State> {
    pub checkpoint_id: CheckpointId,
    pub current_node: NodeId,
    pub state: S::Checkpoint,
    pub graph_hash: u64,
    pub created_at: SystemTime,
}

执行检查点 — 物化快照 + 执行游标。

Checkpoint 的唯一职责：恢复（Restore）。 给我一个 Checkpoint，我就能从 current_node 开始，用 state 继续执行。

P0-1: Checkpoint Projection

state 字段使用 S::Checkpoint（关联类型），不是 S（Runtime State）。 这保证：

• Runtime State 可以包含不可序列化字段（Arc<dyn ...>, Sender, Cache）
• Checkpoint 只序列化必要字段
• 编译期保证可序列化

Graph Compatibility

graph_hash 记录创建 Checkpoint 时的图结构指纹。 恢复时必须校验：graph_hash 不匹配 → 拒绝恢复（不允许 silent mismatch）。

Fields

checkpoint_id: CheckpointId

唯一标识

current_node: NodeId

下一个要执行的节点

state: S::Checkpoint

物化状态快照（P0-1: 使用 Checkpoint 关联类型，不是 raw State）

graph_hash: u64

图结构指纹 — 恢复时校验兼容性

created_at: SystemTime

创建时间

Implementations

impl<S: WorkflowState> Checkpoint<S>

pub fn new(current_node: impl Into<String>, state: &S, graph_hash: u64) -> Self

从 Runtime State 创建 Checkpoint（使用 snapshot() 投影）。

pub fn restore_state(self) -> S

从 Checkpoint 恢复 Runtime State（使用 restore()）。

Trait Implementations

impl<S: Clone + WorkflowState> Clone for Checkpoint<S>

where S::Checkpoint: Clone,

fn clone(&self) -> Checkpoint<S>

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<S: Debug + WorkflowState> Debug for Checkpoint<S>

where S::Checkpoint: Debug,

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<'de, S: WorkflowState> Deserialize<'de> for Checkpoint<S>

where S::Checkpoint: Deserialize<'de>,

fn deserialize<__D>(__deserializer: __D) -> Result<Self, __D::Error>

where __D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer.

impl<S: WorkflowState> Serialize for Checkpoint<S>

where S::Checkpoint: Serialize,

fn serialize<__S>(&self, __serializer: __S) -> Result<__S::Ok, __S::Error>

where __S: Serializer,

Serialize this value into the given Serde serializer.