syncthing/lib/db/smallindex.go

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// Copyright (C) 2018 The Syncthing Authors.
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this file,
// You can obtain one at https://mozilla.org/MPL/2.0/.
package db
import (
"encoding/binary"
"sort"
"github.com/syncthing/syncthing/lib/db/backend"
"github.com/syncthing/syncthing/lib/sync"
)
// A smallIndex is an in memory bidirectional []byte to uint32 map. It gives
// fast lookups in both directions and persists to the database. Don't use for
// storing more items than fit comfortably in RAM.
type smallIndex struct {
db backend.Backend
prefix []byte
id2val map[uint32]string
val2id map[string]uint32
nextID uint32
mut sync.Mutex
}
func newSmallIndex(db backend.Backend, prefix []byte) *smallIndex {
idx := &smallIndex{
db: db,
prefix: prefix,
id2val: make(map[uint32]string),
val2id: make(map[string]uint32),
mut: sync.NewMutex(),
}
idx.load()
return idx
}
// load iterates over the prefix space in the database and populates the in
// memory maps.
func (i *smallIndex) load() {
it, err := i.db.NewPrefixIterator(i.prefix)
if err != nil {
panic("loading small index: " + err.Error())
}
defer it.Release()
for it.Next() {
val := string(it.Value())
id := binary.BigEndian.Uint32(it.Key()[len(i.prefix):])
if val != "" {
// Empty value means the entry has been deleted.
i.id2val[id] = val
i.val2id[val] = id
}
if id >= i.nextID {
i.nextID = id + 1
}
}
}
// ID returns the index number for the given byte slice, allocating a new one
// and persisting this to the database if necessary.
func (i *smallIndex) ID(val []byte) (uint32, error) {
i.mut.Lock()
// intentionally avoiding defer here as we want this call to be as fast as
// possible in the general case (folder ID already exists). The map lookup
// with the conversion of []byte to string is compiler optimized to not
// copy the []byte, which is why we don't assign it to a temp variable
// here.
if id, ok := i.val2id[string(val)]; ok {
i.mut.Unlock()
return id, nil
}
id := i.nextID
i.nextID++
valStr := string(val)
i.val2id[valStr] = id
i.id2val[id] = valStr
key := make([]byte, len(i.prefix)+8) // prefix plus uint32 id
copy(key, i.prefix)
binary.BigEndian.PutUint32(key[len(i.prefix):], id)
if err := i.db.Put(key, val); err != nil {
i.mut.Unlock()
return 0, err
}
i.mut.Unlock()
return id, nil
}
// Val returns the value for the given index number, or (nil, false) if there
// is no such index number.
func (i *smallIndex) Val(id uint32) ([]byte, bool) {
i.mut.Lock()
val, ok := i.id2val[id]
i.mut.Unlock()
if !ok {
return nil, false
}
return []byte(val), true
}
func (i *smallIndex) Delete(val []byte) error {
i.mut.Lock()
defer i.mut.Unlock()
// Check the reverse mapping to get the ID for the value.
if id, ok := i.val2id[string(val)]; ok {
// Generate the corresponding database key.
key := make([]byte, len(i.prefix)+8) // prefix plus uint32 id
copy(key, i.prefix)
binary.BigEndian.PutUint32(key[len(i.prefix):], id)
// Put an empty value into the database. This indicates that the
// entry does not exist any more and prevents the ID from being
// reused in the future.
if err := i.db.Put(key, []byte{}); err != nil {
return err
}
// Delete reverse mapping.
delete(i.id2val, id)
}
// Delete forward mapping.
delete(i.val2id, string(val))
return nil
}
// Values returns the set of values in the index
func (i *smallIndex) Values() []string {
// In principle this method should return [][]byte because all the other
// methods deal in []byte keys. However, in practice, where it's used
// wants a []string and it's easier to just create that here rather than
// having to convert both here and there...
i.mut.Lock()
vals := make([]string, 0, len(i.val2id))
for val := range i.val2id {
vals = append(vals, val)
}
i.mut.Unlock()
sort.Strings(vals)
return vals
}