// Copyright 2023 The Gitea Authors. All rights reserved.// SPDX-License-Identifier: MIT // Package queue implements a specialized concurrent queue system for Gitea.//// Terminology:////  1. Item://     - An item can be a simple value, such as an integer, or a more complex structure that has multiple fields.//     Usually a item serves as a task or a message. Sets of items will be sent to a queue handler to be processed.//     - It's represented as a JSON-marshaled binary slice in the queue//     - Since the item is marshaled by JSON, and JSON doesn't have stable key-order/type support,//     so the decoded handler item may not be the same as the original "pushed" one if you use map/any types,////  2. Batch://     - A collection of items that are grouped together for processing. Each worker receives a batch of items.////  3. Worker://     - Individual unit of execution designed to process items from the queue. It's a goroutine that calls the Handler.//     - Workers will get new items through a channel (WorkerPoolQueue is responsible for the distribution).//     - Workers operate in parallel. The default value of max workers is determined by the setting system.////  4. Handler (represented by HandlerFuncT type)://     - It's the function responsible for processing items. Each active worker will call it.//     - If an item or some items are not successfully processed, the handler could return them as "unhandled items".//     In such scenarios, the queue system ensures these unhandled items are returned to the base queue after a brief delay.//     This mechanism is particularly beneficial in cases where the processing entity (like a document indexer) is//     temporarily unavailable. It ensures that no item is skipped or lost due to transient failures in the processing//     mechanism.////  5. Base queue://     - Represents the underlying storage mechanism for the queue. There are several implementations://     - Channel: Uses Go's native channel constructs to manage the queue, suitable for in-memory queuing.//     - LevelDB: Especially useful in persistent queues for single instances.//     - Redis: Suitable for clusters, where we may have multiple nodes.//     - Dummy: This is special, it's not a real queue, it's a immediate no-op queue, which is useful for tests.//     - They all have the same abstraction, the same interface, and they are tested by the same testing code.//// 6. WorkerPoolQueue://   - It's responsible to glue all together, using the "base queue" to provide "worker pool" functionality. It creates//     new workers if needed and can flush the queue, running all the items synchronously till it finishes.//   - Its "Push" function doesn't block forever, it will return an error if the queue is full after the timeout.//// 7. Manager://   - The purpose of it is to serve as a centralized manager for multiple WorkerPoolQueue instances. Whenever we want//     to create a new queue, flush, or get a specific queue, we could use it.//// A queue can be "simple" or "unique". A unique queue will try to avoid duplicate items.// Unique queue's "Has" function can be used to check whether an item is already in the queue,// although it's not 100% reliable due to the lack of proper transaction support.// Simple queue's "Has" function always returns "has=false".//// A WorkerPoolQueue is a generic struct; this means it will work with any type but just for that type.// If you want another kind of items to run, you would have to call the manager to create a new WorkerPoolQueue for you// with a different handler that works with this new type of item. As an example of this:////	 func Init() error {//		 itemQueue = queue.CreateSimpleQueue(graceful.GetManager().ShutdownContext(), "queue-name", handler)//		 ...//	 }//	 func handler(items ...*mypkg.QueueItem) []*mypkg.QueueItem { ... }package queue import "code.gitea.io/gitea/modules/util" type HandlerFuncT[T any] func(...T) (unhandled []T) var ErrAlreadyInQueue = util.NewAlreadyExistErrorf("already in queue")