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overlay.go
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overlay.go
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package mkvs
import (
"context"
"fmt"
"github.com/tidwall/btree"
"github.com/oasisprotocol/oasis-core/go/storage/mkvs/node"
"github.com/oasisprotocol/oasis-core/go/storage/mkvs/syncer"
)
var _ OverlayTree = (*treeOverlay)(nil)
type treeOverlay struct {
inner KeyValueTree
overlay btree.Map[string, []byte]
dirty map[string]bool
}
// NewOverlay creates a new key-value tree overlay that holds all updates in memory and only commits
// them if requested. This can be used to create snapshots that can be discarded.
//
// While updates (inserts, removes) are stored in the overlay, reads are not cached in the overlay
// as the inner tree has its own cache and double caching makes less sense.
//
// The overlay is not safe for concurrent use.
func NewOverlay(inner KeyValueTree) OverlayTree {
return &treeOverlay{
inner: inner,
dirty: make(map[string]bool),
}
}
// Implements KeyValueTree.
func (o *treeOverlay) Insert(_ context.Context, key, value []byte) error {
o.overlay.Set(string(key), value)
o.dirty[string(key)] = true
return nil
}
// Implements KeyValueTree.
func (o *treeOverlay) Get(ctx context.Context, key []byte) ([]byte, error) {
// For dirty values, check the overlay.
if o.dirty[string(key)] {
value, _ := o.overlay.Get(string(key))
return value, nil
}
// Otherwise fetch from inner tree.
return o.inner.Get(ctx, key)
}
// Implements KeyValueTree.
func (o *treeOverlay) RemoveExisting(ctx context.Context, key []byte) ([]byte, error) {
// For dirty values, remove from the overlay.
if o.dirty[string(key)] {
value, _ := o.overlay.Delete(string(key))
return value, nil
}
value, err := o.inner.Get(ctx, key)
if err != nil {
return nil, err
}
// Do not treat a value as dirty if it was not dirty before and did not exist in the inner tree.
if value != nil {
o.dirty[string(key)] = true
}
return value, nil
}
// Implements KeyValueTree.
func (o *treeOverlay) Remove(_ context.Context, key []byte) error {
// Since we don't care about the previous value, we can just record an update.
o.dirty[string(key)] = true
o.overlay.Delete(string(key))
return nil
}
// Implements KeyValueTree.
func (o *treeOverlay) NewIterator(ctx context.Context, options ...IteratorOption) Iterator {
return &treeOverlayIterator{
tree: o,
inner: o.inner.NewIterator(ctx, options...),
overlay: o.overlay.Iter(),
}
}
// Implements OverlayTree.
func (o *treeOverlay) Copy(inner KeyValueTree) OverlayTree {
if inner == nil {
inner = o.inner
}
o2 := &treeOverlay{
inner: inner,
dirty: make(map[string]bool),
}
for k := range o.dirty {
o2.dirty[k] = true
}
overlay := o.overlay.Copy()
o2.overlay = *overlay
return o2
}
// Implements OverlayTree.
func (o *treeOverlay) Commit(ctx context.Context) (KeyValueTree, error) {
// Insert all items present in the overlay.
it := o.overlay.Iter()
for ok := it.First(); ok; ok = it.Next() {
if err := o.inner.Insert(ctx, []byte(it.Key()), it.Value()); err != nil {
return nil, err
}
delete(o.dirty, it.Key())
}
// Any remaining dirty items must have been removed.
for key := range o.dirty {
if err := o.inner.Remove(ctx, []byte(key)); err != nil {
return nil, err
}
}
o.dirty = make(map[string]bool)
o.overlay.Clear()
return o.inner, nil
}
// Implements ClosableTree.
func (o *treeOverlay) Close() {
if o.inner == nil {
return
}
o.overlay.Clear()
o.inner = nil
o.dirty = nil
}
type treeOverlayIterator struct {
tree *treeOverlay
inner Iterator
overlay btree.MapIter[string, []byte]
overlayValid bool
key node.Key
value []byte
}
func (it *treeOverlayIterator) Valid() bool {
// If either iterator is valid, the merged iterator is valid.
return it.inner.Valid() || it.overlayValid
}
func (it *treeOverlayIterator) Err() error {
return it.inner.Err()
}
func (it *treeOverlayIterator) Rewind() {
it.inner.Rewind()
it.overlayValid = it.overlay.First()
it.updateIteratorPosition()
}
func (it *treeOverlayIterator) Seek(key node.Key) {
it.inner.Seek(key)
it.overlayValid = it.overlay.Seek(string(key))
it.updateIteratorPosition()
}
func (it *treeOverlayIterator) Next() {
if !it.overlayValid || (it.inner.Valid() && it.inner.Key().Compare(node.Key(it.overlay.Key())) <= 0) {
// Key of inner iterator is smaller or equal than the key of the overlay iterator.
it.inner.Next()
} else {
// Key of inner iterator is greater than the key of the overlay iterator.
it.overlayValid = it.overlay.Next()
}
it.updateIteratorPosition()
}
func (it *treeOverlayIterator) updateIteratorPosition() {
// Skip over any dirty entries from the inner iterator.
for it.inner.Valid() && it.tree.dirty[string(it.inner.Key())] {
it.inner.Next()
}
iKey := it.inner.Key()
oKey := node.Key(it.overlay.Key())
if it.inner.Valid() && (!it.overlayValid || iKey.Compare(oKey) < 0) {
// Key of inner iterator is smaller than the key of the overlay iterator.
it.key = iKey
it.value = it.inner.Value()
} else if it.overlayValid {
// Key of overlay iterator is smaller than or equal to the key of the inner iterator.
it.key = oKey
it.value = it.overlay.Value()
} else {
// Both iterators are invalid.
it.key = nil
it.value = nil
}
}
func (it *treeOverlayIterator) Key() node.Key {
return it.key
}
func (it *treeOverlayIterator) Value() []byte {
return it.value
}
func (it *treeOverlayIterator) GetProof() (*syncer.Proof, error) {
panic(fmt.Errorf("tree overlay: proofs are not supported"))
}
func (it *treeOverlayIterator) GetProofBuilder() *syncer.ProofBuilder {
panic(fmt.Errorf("tree overlay: proofs are not supported"))
}
func (it *treeOverlayIterator) Close() {
it.inner.Close()
it.key = nil
it.value = nil
it.tree = nil
}
type treeOverlayWrapper struct {
Tree
}
// Implements OverlayTree.
func (tow *treeOverlayWrapper) Copy(KeyValueTree) OverlayTree {
panic("copy not supported")
}
// Implements OverlayTree.
func (tow *treeOverlayWrapper) Commit(context.Context) (KeyValueTree, error) {
return tow.Tree, nil
}
// NewOverlayWrapper wraps an existing tree so it can behave as an overlay tree without any actual
// overlay overhead.
func NewOverlayWrapper(inner Tree) OverlayTree {
return &treeOverlayWrapper{inner}
}