Snapshotter 快照原理与接口设计

// 创建 Active 快照
mounts, err := snapshotter.Prepare(ctx, "container-1-layer", "image-layer-3")

// 将 Active 提交为 Committed
err := snapshotter.Commit(ctx, "image-layer-4", "container-1-layer")

// 创建只读视图
mounts, err := snapshotter.View(ctx, "view-1", "image-layer-3")

// core/snapshots/snapshotter.go

type Snapshotter interface {
    // Stat 返回快照信息
    Stat(ctx context.Context, key string) (Info, error)

    // Update 更新快照信息（如 Labels）
    Update(ctx context.Context, info Info, fieldpaths ...string) (Info, error)

    // Usage 返回快照的磁盘使用情况
    Usage(ctx context.Context, key string) (Usage, error)

    // Mounts 返回快照的挂载点配置
    Mounts(ctx context.Context, key string) ([]mount.Mount, error)

    // Prepare 创建 Active 快照
    Prepare(ctx context.Context, key, parent string, opts ...Opt) ([]mount.Mount, error)

    // View 创建只读视图
    View(ctx context.Context, key, parent string, opts ...Opt) ([]mount.Mount, error)

    // Commit 将 Active 快照提交为 Committed
    Commit(ctx context.Context, name, key string, opts ...Opt) error

    // Remove 删除快照
    Remove(ctx context.Context, key string) error

    // Walk 遍历所有快照
    Walk(ctx context.Context, fn WalkFunc, filters ...string) error

    // Close 关闭 Snapshotter
    Close() error
}

// Info 描述快照信息
type Info struct {
    Kind    Kind              // Active, Committed, View
    Name    string            // 快照名称/key
    Parent  string            // 父快照
    Labels  map[string]string // 标签
    Created time.Time         // 创建时间
    Updated time.Time         // 更新时间
}

// Kind 快照类型
type Kind uint8

const (
    KindUnknown Kind = iota
    KindView        // 只读视图
    KindActive      // 可写快照
    KindCommitted   // 已提交快照
)

// Usage 描述磁盘使用
type Usage struct {
    Inodes int64 // inode 数量
    Size   int64 // 字节大小
}

// mount/mount.go

type Mount struct {
    Type    string   // 挂载类型 (overlay, bind, etc.)
    Source  string   // 源路径
    Target  string   // 目标路径（通常由调用者设置）
    Options []string // 挂载选项
}

func unpackImage(ctx context.Context, sn snapshots.Snapshotter, layers []ocispec.Descriptor) error {
    var parent string

    for i, layer := range layers {
        key := fmt.Sprintf("extract-%d-%s", i, layer.Digest.Encoded()[:12])
        name := fmt.Sprintf("layer-%d", i)

        // 创建 Active 快照
        mounts, err := sn.Prepare(ctx, key, parent)
        if err != nil {
            return err
        }

        // 挂载并解压层内容
        if err := mount.WithTempMount(ctx, mounts, func(root string) error {
            return extractLayer(layer, root)
        }); err != nil {
            sn.Remove(ctx, key)
            return err
        }

        // 提交为 Committed
        if err := sn.Commit(ctx, name, key); err != nil {
            return err
        }

        parent = name
    }

    return nil
}

// 对于 3 层镜像 + 1 层容器
// parent chain: layer-0 <- layer-1 <- layer-2 <- container-layer

mounts := []mount.Mount{
    {
        Type:   "overlay",
        Source: "overlay",
        Options: []string{
            "workdir=/var/lib/containerd/.../work",
            "upperdir=/var/lib/containerd/.../snapshots/container-layer/fs",
            "lowerdir=/var/lib/containerd/.../snapshots/layer-2/fs:" +
                     "/var/lib/containerd/.../snapshots/layer-1/fs:" +
                     "/var/lib/containerd/.../snapshots/layer-0/fs",
        },
    },
}

// 只有一个 Committed 层，没有 upperdir
mounts := []mount.Mount{
    {
        Type:   "bind",
        Source: "/var/lib/containerd/.../snapshots/layer-0/fs",
        Options: []string{"ro", "rbind"},
    },
}

// 创建带标签的快照
mounts, err := snapshotter.Prepare(ctx, key, parent,
    snapshots.WithLabels(map[string]string{
        "my-label": "my-value",
    }),
)

// 更新标签
info, err := snapshotter.Update(ctx, snapshots.Info{
    Name: key,
    Labels: map[string]string{
        "new-label": "new-value",
    },
}, "labels.new-label")

// GC 会删除未被引用的快照
// 删除顺序：叶子节点优先

// 例如：删除 container 后
// 1. active-1 变为无引用
// 2. GC 删除 active-1
// 3. 如果 layer-3 无其他引用，也会被删除
// 4. 依此类推