Fantom-foundation/go-lachesis

package flushable

import (
    "github.com/Fantom-foundation/go-lachesis/kvdb"
    "github.com/Fantom-foundation/go-lachesis/kvdb/devnulldb"
)

// LazyFlushable is a Flushable with delayed DB producer
type LazyFlushable struct {
    *Flushable
    producer func() kvdb.KeyValueStore
}

var (
    devnull = devnulldb.New()
)

// NewLazy makes flushable with real db producer.
// Real db won't be produced until first .Flush() is called.
// All the writes into the cache won't be written in parent until .Flush() is called.
func NewLazy(producer func() kvdb.KeyValueStore, drop func()) *LazyFlushable {
    if producer == nil {
        panic("nil producer")
    }

    w := &LazyFlushable{
        Flushable: WrapWithDrop(devnull, drop),
        producer:  producer,
    }
    return w
}

// InitUnderlyingDb is UnderlyingDb getter. Makes underlying in lazy case.
func (w *LazyFlushable) InitUnderlyingDb() kvdb.KeyValueStore {
    w.lock.Lock()
    defer w.lock.Unlock()

    return w.initUnderlyingDb()
}

func (w *LazyFlushable) initUnderlyingDb() kvdb.KeyValueStore {
    if w.underlying == devnull && w.producer != nil {
        w.underlying = w.producer()
        w.producer = nil // need once
    }

    return w.underlying
}

// Flush current cache into parent DB.
// Real db won't be produced until first .Flush() is called.
func (w *LazyFlushable) Flush() error {
    w.lock.Lock()
    defer w.lock.Unlock()

    w.underlying = w.initUnderlyingDb()

    return w.flush()
}