Add namespaced snapshotter implementation

The namespaced snapshotter wraps an existing snapshotter and
enforces namespace.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

metadata/bolt.go

@@ -0,0 +1,40 @@
+package metadata
+
+import (
+	"context"
+
+	"github.com/boltdb/bolt"
+	"github.com/pkg/errors"
+)
+
+type transactionKey struct{}
+
+// WithTransactionContext returns a new context holding the provided
+// bolt transaction. Functions which require a bolt transaction will
+// first check to see if a transaction is already created on the
+// context before creating their own.
+func WithTransactionContext(ctx context.Context, tx *bolt.Tx) context.Context {
+	return context.WithValue(ctx, transactionKey{}, tx)
+}
+
+// view gets a bolt db transaction either from the context
+// or starts a new one with the provided bolt database.
+func view(ctx context.Context, db *bolt.DB, fn func(*bolt.Tx) error) error {
+	tx, ok := ctx.Value(transactionKey{}).(*bolt.Tx)
+	if !ok {
+		return db.View(fn)
+	}
+	return fn(tx)
+}
+
+// update gets a writable bolt db transaction either from the context
+// or starts a new one with the provided bolt database.
+func update(ctx context.Context, db *bolt.DB, fn func(*bolt.Tx) error) error {
+	tx, ok := ctx.Value(transactionKey{}).(*bolt.Tx)
+	if !ok {
+		return db.Update(fn)
+	} else if !tx.Writable() {
+		return errors.Wrap(bolt.ErrTxNotWritable, "unable to use transaction from context")
+	}
+	return fn(tx)
+}