Add a write-cache to the database interface options

database/interface_cache.go

@@ -0,0 +1,171 @@
+package database
+
+import (
+	"context"
+	"errors"
+	"time"
+
+	"github.com/safing/portbase/database/record"
+	"github.com/safing/portbase/log"
+)
+
+// DelayedCacheWriter must be run by the caller of an interface that uses delayed cache writing.
+func (i *Interface) DelayedCacheWriter(ctx context.Context) error {
+	// Check if the DelayedCacheWriter should be run at all.
+	if i.options.CacheSize <= 0 || len(i.options.DelayCachedWrites) == 0 {
+		return errors.New("delayed cache writer is not applicable to this database interface")
+	}
+
+	// Check if backend support the Batcher interface.
+	batchPut := i.PutMany(i.options.DelayCachedWrites)
+	// End batchPut immediately and check for an error.
+	err := batchPut(nil)
+	if err != nil {
+		return err
+	}
+
+	for {
+		// Wait for trigger for writing the cache.
+		select {
+		case <-ctx.Done():
+			// Module is shutting down, flush write cache to database.
+			i.flushWriteCache()
+			return nil
+
+		case <-i.triggerCacheWrite:
+		case <-time.After(5 * time.Second):
+		}
+
+		i.flushWriteCache()
+	}
+}
+
+func (i *Interface) flushWriteCache() {
+	i.writeCacheLock.Lock()
+	defer i.writeCacheLock.Unlock()
+
+	// Check if there is anything to do.
+	if len(i.writeCache) == 0 {
+		return
+	}
+
+	// Write the full cache in a batch operation.
+	batchPut := i.PutMany(i.options.DelayCachedWrites)
+	for _, r := range i.writeCache {
+		err := batchPut(r)
+		if err != nil {
+			log.Warningf("database: failed to write write-cached entry to %q database: %s", i.options.DelayCachedWrites, err)
+		}
+	}
+	// Finish batch.
+	err := batchPut(nil)
+	if err != nil {
+		log.Warningf("database: failed to finish flushing write cache to %q database: %s", i.options.DelayCachedWrites, err)
+	}
+
+	// Optimized map clearing following the Go1.11 recommendation.
+	for key := range i.writeCache {
+		delete(i.writeCache, key)
+	}
+}
+
+// cacheEvictHandler is run by the cache for every entry that gets evicted
+// from the cache.
+func (i *Interface) cacheEvictHandler(keyData, _ interface{}) {
+	// Transform the key into a string.
+	key, ok := keyData.(string)
+	if !ok {
+		return
+	}
+
+	// Check if the evicted record is one that is to be written.
+	i.writeCacheLock.Lock()
+	r, ok := i.writeCache[key]
+	if ok {
+		delete(i.writeCache, key)
+	}
+	i.writeCacheLock.Unlock()
+	if !ok {
+		return
+	}
+
+	// Write record to database in order to mitigate race conditions where the record would appear
+	// as non-existent for a short duration.
+	db, err := getController(r.DatabaseName())
+	if err != nil {
+		log.Warningf("database: failed to write evicted cache entry %q: database %q does not exist", key, r.DatabaseName())
+		return
+	}
+	err = db.Put(r)
+	if err != nil {
+		log.Warningf("database: failed to write evicted cache entry %q to database: %s", key, err)
+	}
+
+	// Finally, trigger writing the full write cache because a to-be-written
+	// entry was just evicted from the cache, and this makes it likely that more
+	// to-be-written entries will be evicted shortly.
+	select {
+	case i.triggerCacheWrite <- struct{}{}:
+	default:
+	}
+}
+
+func (i *Interface) checkCache(key string) record.Record {
+	// Check if cache is in use.
+	if i.cache == nil {
+		return nil
+	}
+
+	// Check if record exists in cache.
+	cacheVal, err := i.cache.Get(key)
+	if err == nil {
+		r, ok := cacheVal.(record.Record)
+		if ok {
+			return r
+		}
+	}
+	return nil
+}
+
+func (i *Interface) updateCache(r record.Record, write bool) (written bool) {
+	// Check if cache is in use.
+	if i.cache == nil {
+		return false
+	}
+
+	// Check if record should be deleted
+	if r.Meta().IsDeleted() {
+		// Remove entry from cache.
+		i.cache.Remove(r.Key())
+		// Let write through to database storage.
+		return false
+	}
+
+	// Update cache with record.
+	ttl := r.Meta().GetRelativeExpiry()
+	if ttl >= 0 {
+		_ = i.cache.SetWithExpire(
+			r.Key(),
+			r,
+			time.Duration(ttl)*time.Second,
+		)
+	} else {
+		_ = i.cache.Set(
+			r.Key(),
+			r,
+		)
+	}
+
+	// Add record to write cache instead if:
+	// 1. The record is being written.
+	// 2. Write delaying is active.
+	// 3. Write delaying is active for the database of this record.
+	if write && len(i.options.DelayCachedWrites) > 0 && r.DatabaseName() == i.options.DelayCachedWrites {
+		i.writeCacheLock.Lock()
+		defer i.writeCacheLock.Unlock()
+		i.writeCache[r.Key()] = r
+		return true
+	}
+
+	return false
+}