antispam.go

// Copyright 2024 The Tessera authors. All Rights Reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package tessera

import (
	"context"
	"fmt"
	"sync"

	lru "github.com/hashicorp/golang-lru/v2"
)

// newInMemoryDedup wraps an Add function to prevent duplicate entries being written to the underlying
// storage by keeping an in-memory cache of recently seen entries.
// Where an existing entry has already been `Add`ed, the previous `IndexFuture` will be returned.
// When no entry is found in the cache, the delegate method will be called to store the entry, and
// the result will be registered in the cache.
//
// Internally this uses a cache with a max size configured by the size parameter.
// If the entry being `Add`ed is not found in the cache, then it calls the delegate.
//
// This object can be used in isolation, or in conjunction with a persistent dedup implementation.
// When using this with a persistent dedup, the persistent layer should be the delegate of this
// InMemoryDedup. This allows recent duplicates to be deduplicated in memory, reducing the need to
// make calls to a persistent storage.
func newInMemoryDedup(size uint) func(AddFn) AddFn {
	return func(af AddFn) AddFn {
		c, err := lru.New[string, func() IndexFuture](int(size))
		if err != nil {
			panic(fmt.Errorf("lru.New(%d): %v", size, err))
		}
		dedup := &inMemoryDedup{
			delegate: af,
			cache:    c,
		}
		return dedup.add
	}
}

type inMemoryDedup struct {
	delegate func(ctx context.Context, e *Entry) IndexFuture
	cache    *lru.Cache[string, func() IndexFuture]
}

// Add adds the entry to the underlying delegate only if e hasn't been recently seen. In either case,
// an IndexFuture will be returned that the client can use to get the sequence number of this entry.
func (d *inMemoryDedup) add(ctx context.Context, e *Entry) IndexFuture {
	ctx, span := tracer.Start(ctx, "tessera.Appender.inmemoryDedup.Add")
	defer span.End()

	id := string(e.Identity())

	f := sync.OnceValue(func() IndexFuture {
		// However many calls with the same entry come in and are deduplicated, we should only call delegate
		// once for each unique entry:
		df := d.delegate(ctx, e)

		return func() (Index, error) {
			idx, err := df()
			// If things went wrong we shouldn't cache the error, but rather let the request be retried as the error
			// may be transient (including ErrPushback).
			if err != nil {
				d.cache.Remove(id)
			}
			return idx, err
		}
	})

	// if we've seen this entry before, discard our f and replace
	// with the one we created last time, otherwise store f against id.
	if prev, ok, _ := d.cache.PeekOrAdd(id, f); ok {
		f = func() IndexFuture {
			return func() (Index, error) {
				i, err := prev()()
				i.IsDup = true
				return i, err
			}
		}
	}

	return f()
}