ADR 049: State Sync Hooks

// SnapshotItem is an item contained in a rootmulti.Store snapshot.
// On top of the exsiting SnapshotStoreItem and SnapshotIAVLItem, we add two new options for the item.
message SnapshotItem {
  // item is the specific type of snapshot item.
  oneof item {
    SnapshotStoreItem        store             = 1;
    SnapshotIAVLItem         iavl              = 2 [(gogoproto.customname) = "IAVL"];
    SnapshotExtensionMeta    extension         = 3;
    SnapshotExtensionPayload extension_payload = 4;
  }
}

// SnapshotExtensionMeta contains metadata about an external snapshotter.
// One module may need multiple snapshotters, so each module may have multiple SnapshotExtensionMeta.
message SnapshotExtensionMeta {
  // the name of the ExtensionSnapshotter, and it is registered to snapshotter manager when setting up the application
  // name should be unique for each ExtensionSnapshotter as we need to alphabetically order their snapshots to get
  // deterministic snapshot stream.
  string name   = 1;
  // this is used by each ExtensionSnapshotter to decide the format of payloads included in SnapshotExtensionPayload message
  // it is used within the snapshotter/namespace, not global one for all modules
  uint32 format = 2;
}

// SnapshotExtensionPayload contains payloads of an external snapshotter.
message SnapshotExtensionPayload {
  bytes payload = 1;
}

type Manager struct {
    store      *Store
	multistore types.Snapshotter
	extensions map[string]types.ExtensionSnapshotter
	mtx                sync.Mutex
	operation          operation
	chRestore          chan<- io.ReadCloser
	chRestoreDone      <-chan restoreDone
	restoreChunkHashes [][]byte
	restoreChunkIndex  uint32
}

// ExtensionPayloadReader read extension payloads,
// it returns io.EOF when reached either end of stream or the extension boundaries.
type ExtensionPayloadReader = func() ([]byte, error)

// ExtensionPayloadWriter is a helper to write extension payloads to underlying stream.
type ExtensionPayloadWriter = func([]byte)

error

// ExtensionSnapshotter is an extension Snapshotter that is appended to the snapshot stream.
// ExtensionSnapshotter has an unique name and manages it's own internal formats.
type ExtensionSnapshotter interface {
	// SnapshotName returns the name of snapshotter, it should be unique in the manager.
	SnapshotName()

string

	// SnapshotFormat returns the default format used to take a snapshot.
	SnapshotFormat()

uint32

	// SupportedFormats returns a list of formats it can restore from.
	SupportedFormats() []uint32

	// SnapshotExtension writes extension payloads into the underlying protobuf stream.
	SnapshotExtension(height uint64, payloadWriter ExtensionPayloadWriter)

error

	// RestoreExtension restores an extension state snapshot,
	// the payload reader returns `io.EOF` when reached the extension boundaries.
	RestoreExtension(height uint64, format uint32, payloadReader ExtensionPayloadReader)

error
}