Move metadata to core/metadata

Signed-off-by: Derek McGowan <derek@mcg.dev>

core/metadata/bolt.go

@@ -0,0 +1,61 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package metadata
+
+import (
+	"context"
+	"fmt"
+
+	bolt "go.etcd.io/bbolt"
+)
+
+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)
+}
+
+type transactor interface {
+	View(fn func(*bolt.Tx) error) error
+	Update(fn func(*bolt.Tx) error) error
+}
+
+// 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 transactor, 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 transactor, fn func(*bolt.Tx) error) error {
+	tx, ok := ctx.Value(transactionKey{}).(*bolt.Tx)
+	if !ok {
+		return db.Update(fn)
+	} else if !tx.Writable() {
+		return fmt.Errorf("unable to use transaction from context: %w", bolt.ErrTxNotWritable)
+	}
+	return fn(tx)
+}