swarm: add database abstractions (shed package) (#18183)

swarm/shed/schema.go

@@ -0,0 +1,134 @@
+// Copyright 2018 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package shed
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+)
+
+var (
+	// LevelDB key value for storing the schema.
+	keySchema = []byte{0}
+	// LevelDB key prefix for all field type.
+	// LevelDB keys will be constructed by appending name values to this prefix.
+	keyPrefixFields byte = 1
+	// LevelDB key prefix from which indexing keys start.
+	// Every index has its own key prefix and this value defines the first one.
+	keyPrefixIndexStart byte = 2 // Q: or maybe a higher number like 7, to have more space for potential specific perfixes
+)
+
+// schema is used to serialize known database structure information.
+type schema struct {
+	Fields  map[string]fieldSpec `json:"fields"`  // keys are field names
+	Indexes map[byte]indexSpec   `json:"indexes"` // keys are index prefix bytes
+}
+
+// fieldSpec holds information about a particular field.
+// It does not need Name field as it is contained in the
+// schema.Field map key.
+type fieldSpec struct {
+	Type string `json:"type"`
+}
+
+// indxSpec holds information about a particular index.
+// It does not contain index type, as indexes do not have type.
+type indexSpec struct {
+	Name string `json:"name"`
+}
+
+// schemaFieldKey retrives the complete LevelDB key for
+// a particular field form the schema definition.
+func (db *DB) schemaFieldKey(name, fieldType string) (key []byte, err error) {
+	if name == "" {
+		return nil, errors.New("field name can not be blank")
+	}
+	if fieldType == "" {
+		return nil, errors.New("field type can not be blank")
+	}
+	s, err := db.getSchema()
+	if err != nil {
+		return nil, err
+	}
+	var found bool
+	for n, f := range s.Fields {
+		if n == name {
+			if f.Type != fieldType {
+				return nil, fmt.Errorf("field %q of type %q stored as %q in db", name, fieldType, f.Type)
+			}
+			break
+		}
+	}
+	if !found {
+		s.Fields[name] = fieldSpec{
+			Type: fieldType,
+		}
+		err := db.putSchema(s)
+		if err != nil {
+			return nil, err
+		}
+	}
+	return append([]byte{keyPrefixFields}, []byte(name)...), nil
+}
+
+// schemaIndexID retrieves the complete LevelDB prefix for
+// a particular index.
+func (db *DB) schemaIndexPrefix(name string) (id byte, err error) {
+	if name == "" {
+		return 0, errors.New("index name can not be blank")
+	}
+	s, err := db.getSchema()
+	if err != nil {
+		return 0, err
+	}
+	nextID := keyPrefixIndexStart
+	for i, f := range s.Indexes {
+		if i >= nextID {
+			nextID = i + 1
+		}
+		if f.Name == name {
+			return i, nil
+		}
+	}
+	id = nextID
+	s.Indexes[id] = indexSpec{
+		Name: name,
+	}
+	return id, db.putSchema(s)
+}
+
+// getSchema retrieves the complete schema from
+// the database.
+func (db *DB) getSchema() (s schema, err error) {
+	b, err := db.Get(keySchema)
+	if err != nil {
+		return s, err
+	}
+	err = json.Unmarshal(b, &s)
+	return s, err
+}
+
+// putSchema stores the complete schema to
+// the database.
+func (db *DB) putSchema(s schema) (err error) {
+	b, err := json.Marshal(s)
+	if err != nil {
+		return err
+	}
+	return db.Put(keySchema, b)
+}