Add snapshot metadata store interface

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

snapshot/storage/metastore.go

@@ -0,0 +1,69 @@
+package storage
+
+import (
+	"context"
+
+	"github.com/docker/containerd/snapshot"
+)
+
+// MetaStore is used to store metadata related to a snapshot driver. The
+// MetaStore is intended to store metadata related to name, state and
+// parentage. Using the MetaStore is not required to implement a snapshot
+// driver but can be used to handle the persistence and transactional
+// complexities of a driver implementation.
+type MetaStore interface {
+	// TransactionContext creates a new transaction context.
+	TransactionContext(ctx context.Context, writable bool) (context.Context, Transactor, error)
+
+	// Stat returns the snapshot stat Info directly from
+	// the metadata.
+	Stat(ctx context.Context, key string) (snapshot.Info, error)
+
+	// Walk iterates through all metadata for the stored
+	// snapshots and calls the provided function for each.
+	Walk(ctx context.Context, fn func(context.Context, snapshot.Info) error) error
+
+	// CreateActive creates a new active snapshot transaction referenced by
+	// the provided key. The new active snapshot will have the provided
+	// parent. If the readonly option is given, the active snapshot will be
+	// marked as readonly and can only be removed, and not committed. The
+	// provided context must contain a writable transaction.
+	CreateActive(ctx context.Context, key, parent string, readonly bool) (Active, error)
+
+	// GetActive returns the metadata for the active snapshot transaction
+	// referenced by the given key.
+	GetActive(ctx context.Context, key string) (Active, error)
+
+	// Remove removes a snapshot from the metastore. The provided context
+	// must contain a writable transaction. The string identifier for the
+	// snapshot is returned.
+	Remove(ctx context.Context, key string) (string, error)
+
+	// Commit renames the active snapshot transaction referenced by `key`
+	// as a committed snapshot referenced by `Name`. The resulting snapshot
+	// will be committed and readonly. The `key` reference will no longer
+	// be available for lookup or removal. The returned string identifier
+	// for the committed snapshot is the same identifier of the original
+	// active snapshot.
+	Commit(ctx context.Context, key, name string) (string, error)
+}
+
+// Transactor is used to finalize an active transaction.
+type Transactor interface {
+	// Commit commits any changes made during the transaction.
+	Commit() error
+
+	// Rollback rolls back any changes made during the transaction.
+	Rollback() error
+}
+
+// Active hold the metadata for an active snapshot transaction. The ParentIDs
+// hold the snapshot identifiers for the committed snapshots this active is
+// based on. The ParentIDs are ordered from the lowest base to highest, meaning
+// they should be applied in order from the first index to the last index. The
+// last index should always be considered the active snapshots immediate parent.
+type Active struct {
+	ID        string
+	ParentIDs []string
+	Readonly  bool
+}