Rewrite queue (#24505)

# ⚠️ Breaking

Many deprecated queue config options are removed (actually, they should
have been removed in 1.18/1.19).

If you see the fatal message when starting Gitea: "Please update your
app.ini to remove deprecated config options", please follow the error
messages to remove these options from your app.ini.

Example:

```
2023/05/06 19:39:22 [E] Removed queue option: `[indexer].ISSUE_INDEXER_QUEUE_TYPE`. Use new options in `[queue.issue_indexer]`
2023/05/06 19:39:22 [E] Removed queue option: `[indexer].UPDATE_BUFFER_LEN`. Use new options in `[queue.issue_indexer]`
2023/05/06 19:39:22 [F] Please update your app.ini to remove deprecated config options
```

Many options in `[queue]` are are dropped, including:
`WRAP_IF_NECESSARY`, `MAX_ATTEMPTS`, `TIMEOUT`, `WORKERS`,
`BLOCK_TIMEOUT`, `BOOST_TIMEOUT`, `BOOST_WORKERS`, they can be removed
from app.ini.

# The problem

The old queue package has some legacy problems:

* complexity: I doubt few people could tell how it works.
* maintainability: Too many channels and mutex/cond are mixed together,
too many different structs/interfaces depends each other.
* stability: due to the complexity & maintainability, sometimes there
are strange bugs and difficult to debug, and some code doesn't have test
(indeed some code is difficult to test because a lot of things are mixed
together).
* general applicability: although it is called "queue", its behavior is
not a well-known queue.
* scalability: it doesn't seem easy to make it work with a cluster
without breaking its behaviors.

It came from some very old code to "avoid breaking", however, its
technical debt is too heavy now. It's a good time to introduce a better
"queue" package.

# The new queue package

It keeps using old config and concept as much as possible.

* It only contains two major kinds of concepts:
    * The "base queue": channel, levelqueue, redis
* They have the same abstraction, the same interface, and they are
tested by the same testing code.
* The "WokerPoolQueue", it uses the "base queue" to provide "worker
pool" function, calls the "handler" to process the data in the base
queue.
* The new code doesn't do "PushBack"
* Think about a queue with many workers, the "PushBack" can't guarantee
the order for re-queued unhandled items, so in new code it just does
"normal push"
* The new code doesn't do "pause/resume"
* The "pause/resume" was designed to handle some handler's failure: eg:
document indexer (elasticsearch) is down
* If a queue is paused for long time, either the producers blocks or the
new items are dropped.
* The new code doesn't do such "pause/resume" trick, it's not a common
queue's behavior and it doesn't help much.
* If there are unhandled items, the "push" function just blocks for a
few seconds and then re-queue them and retry.
* The new code doesn't do "worker booster"
* Gitea's queue's handlers are light functions, the cost is only the
go-routine, so it doesn't make sense to "boost" them.
* The new code only use "max worker number" to limit the concurrent
workers.
* The new "Push" never blocks forever
* Instead of creating more and more blocking goroutines, return an error
is more friendly to the server and to the end user.

There are more details in code comments: eg: the "Flush" problem, the
strange "code.index" hanging problem, the "immediate" queue problem.

Almost ready for review.

TODO:

* [x] add some necessary comments during review
* [x] add some more tests if necessary
* [x] update documents and config options
* [x] test max worker / active worker
* [x] re-run the CI tasks to see whether any test is flaky
* [x] improve the `handleOldLengthConfiguration` to provide more
friendly messages
* [x] fine tune default config values (eg: length?)

## Code coverage:

![image](https://user-images.githubusercontent.com/2114189/236620635-55576955-f95d-4810-b12f-879026a3afdf.png)

11 files changed, 65 insertions, 162 deletions

diff --git a/modules/indexer/code/bleve.go b/modules/indexer/code/bleve.go
index e9085f4107..5936613e3a 100644
--- a/modules/indexer/code/bleve.go
+++ b/modules/indexer/code/bleve.go
@@ -273,10 +273,6 @@ func (b *BleveIndexer) Close() {
 	log.Info("PID: %d Repository Indexer closed", os.Getpid())
 }
 
-// SetAvailabilityChangeCallback does nothing
-func (b *BleveIndexer) SetAvailabilityChangeCallback(callback func(bool)) {
-}
-
 // Ping does nothing
 func (b *BleveIndexer) Ping() bool {
 	return true
diff --git a/modules/indexer/code/elastic_search.go b/modules/indexer/code/elastic_search.go
index 68c8096758..6097538009 100644
--- a/modules/indexer/code/elastic_search.go
+++ b/modules/indexer/code/elastic_search.go
@@ -42,12 +42,11 @@ var _ Indexer = &ElasticSearchIndexer{}
 
 // ElasticSearchIndexer implements Indexer interface
 type ElasticSearchIndexer struct {
-	client               *elastic.Client
-	indexerAliasName     string
-	available            bool
-	availabilityCallback func(bool)
-	stopTimer            chan struct{}
-	lock                 sync.RWMutex
+	client           *elastic.Client
+	indexerAliasName string
+	available        bool
+	stopTimer        chan struct{}
+	lock             sync.RWMutex
 }
 
 type elasticLogger struct {
@@ -198,13 +197,6 @@ func (b *ElasticSearchIndexer) init() (bool, error) {
 	return exists, nil
 }
 
-// SetAvailabilityChangeCallback sets callback that will be triggered when availability changes
-func (b *ElasticSearchIndexer) SetAvailabilityChangeCallback(callback func(bool)) {
-	b.lock.Lock()
-	defer b.lock.Unlock()
-	b.availabilityCallback = callback
-}
-
 // Ping checks if elastic is available
 func (b *ElasticSearchIndexer) Ping() bool {
 	b.lock.RLock()
@@ -529,8 +521,4 @@ func (b *ElasticSearchIndexer) setAvailability(available bool) {
 	}
 
 	b.available = available
-	if b.availabilityCallback != nil {
-		// Call the callback from within the lock to ensure that the ordering remains correct
-		b.availabilityCallback(b.available)
-	}
 }
diff --git a/modules/indexer/code/indexer.go b/modules/indexer/code/indexer.go
index 2c493ccf94..a5e40b52c1 100644
--- a/modules/indexer/code/indexer.go
+++ b/modules/indexer/code/indexer.go
@@ -44,7 +44,6 @@ type SearchResultLanguages struct {
 // Indexer defines an interface to index and search code contents
 type Indexer interface {
 	Ping() bool
-	SetAvailabilityChangeCallback(callback func(bool))
 	Index(ctx context.Context, repo *repo_model.Repository, sha string, changes *repoChanges) error
 	Delete(repoID int64) error
 	Search(ctx context.Context, repoIDs []int64, language, keyword string, page, pageSize int, isMatch bool) (int64, []*SearchResult, []*SearchResultLanguages, error)
@@ -81,7 +80,7 @@ type IndexerData struct {
 	RepoID int64
 }
 
-var indexerQueue queue.UniqueQueue
+var indexerQueue *queue.WorkerPoolQueue[*IndexerData]
 
 func index(ctx context.Context, indexer Indexer, repoID int64) error {
 	repo, err := repo_model.GetRepositoryByID(ctx, repoID)
@@ -137,37 +136,45 @@ func Init() {
 	// Create the Queue
 	switch setting.Indexer.RepoType {
 	case "bleve", "elasticsearch":
-		handler := func(data ...queue.Data) []queue.Data {
+		handler := func(items ...*IndexerData) (unhandled []*IndexerData) {
 			idx, err := indexer.get()
 			if idx == nil || err != nil {
 				log.Error("Codes indexer handler: unable to get indexer!")
-				return data
+				return items
 			}
 
-			unhandled := make([]queue.Data, 0, len(data))
-			for _, datum := range data {
-				indexerData, ok := datum.(*IndexerData)
-				if !ok {
-					log.Error("Unable to process provided datum: %v - not possible to cast to IndexerData", datum)
-					continue
-				}
+			for _, indexerData := range items {
 				log.Trace("IndexerData Process Repo: %d", indexerData.RepoID)
 
+				// FIXME: it seems there is a bug in `CatFileBatch` or `nio.Pipe`, which will cause the process to hang forever in rare cases
+				/*
+					sync.(*Cond).Wait(cond.go:70)
+					github.com/djherbis/nio/v3.(*PipeReader).Read(sync.go:106)
+					bufio.(*Reader).fill(bufio.go:106)
+					bufio.(*Reader).ReadSlice(bufio.go:372)
+					bufio.(*Reader).collectFragments(bufio.go:447)
+					bufio.(*Reader).ReadString(bufio.go:494)
+					code.gitea.io/gitea/modules/git.ReadBatchLine(batch_reader.go:149)
+					code.gitea.io/gitea/modules/indexer/code.(*BleveIndexer).addUpdate(bleve.go:214)
+					code.gitea.io/gitea/modules/indexer/code.(*BleveIndexer).Index(bleve.go:296)
+					code.gitea.io/gitea/modules/indexer/code.(*wrappedIndexer).Index(wrapped.go:74)
+					code.gitea.io/gitea/modules/indexer/code.index(indexer.go:105)
+				*/
 				if err := index(ctx, indexer, indexerData.RepoID); err != nil {
-					if !setting.IsInTesting {
-						log.Error("indexer index error for repo %v: %v", indexerData.RepoID, err)
-					}
-					if indexer.Ping() {
+					if !idx.Ping() {
+						log.Error("Code indexer handler: indexer is unavailable.")
+						unhandled = append(unhandled, indexerData)
 						continue
 					}
-					// Add back to queue
-					unhandled = append(unhandled, datum)
+					if !setting.IsInTesting {
+						log.Error("Codes indexer handler: index error for repo %v: %v", indexerData.RepoID, err)
+					}
 				}
 			}
 			return unhandled
 		}
 
-		indexerQueue = queue.CreateUniqueQueue("code_indexer", handler, &IndexerData{})
+		indexerQueue = queue.CreateUniqueQueue("code_indexer", handler)
 		if indexerQueue == nil {
 			log.Fatal("Unable to create codes indexer queue")
 		}
@@ -224,18 +231,6 @@ func Init() {
 
 		indexer.set(rIndexer)
 
-		if queue, ok := indexerQueue.(queue.Pausable); ok {
-			rIndexer.SetAvailabilityChangeCallback(func(available bool) {
-				if !available {
-					log.Info("Code index queue paused")
-					queue.Pause()
-				} else {
-					log.Info("Code index queue resumed")
-					queue.Resume()
-				}
-			})
-		}
-
 		// Start processing the queue
 		go graceful.GetManager().RunWithShutdownFns(indexerQueue.Run)
 
diff --git a/modules/indexer/code/wrapped.go b/modules/indexer/code/wrapped.go
index 33ba57a094..7eed3e8557 100644
--- a/modules/indexer/code/wrapped.go
+++ b/modules/indexer/code/wrapped.go
@@ -56,16 +56,6 @@ func (w *wrappedIndexer) get() (Indexer, error) {
 	return w.internal, nil
 }
 
-// SetAvailabilityChangeCallback sets callback that will be triggered when availability changes
-func (w *wrappedIndexer) SetAvailabilityChangeCallback(callback func(bool)) {
-	indexer, err := w.get()
-	if err != nil {
-		log.Error("Failed to get indexer: %v", err)
-		return
-	}
-	indexer.SetAvailabilityChangeCallback(callback)
-}
-
 // Ping checks if elastic is available
 func (w *wrappedIndexer) Ping() bool {
 	indexer, err := w.get()
diff --git a/modules/indexer/issues/bleve.go b/modules/indexer/issues/bleve.go
index e3ef9af5b9..60d9ef7617 100644
--- a/modules/indexer/issues/bleve.go
+++ b/modules/indexer/issues/bleve.go
@@ -187,10 +187,6 @@ func (b *BleveIndexer) Init() (bool, error) {
 	return false, err
 }
 
-// SetAvailabilityChangeCallback does nothing
-func (b *BleveIndexer) SetAvailabilityChangeCallback(callback func(bool)) {
-}
-
 // Ping does nothing
 func (b *BleveIndexer) Ping() bool {
 	return true
diff --git a/modules/indexer/issues/db.go b/modules/indexer/issues/db.go
index d28b536e02..04c101c356 100644
--- a/modules/indexer/issues/db.go
+++ b/modules/indexer/issues/db.go
@@ -18,10 +18,6 @@ func (i *DBIndexer) Init() (bool, error) {
 	return false, nil
 }
 
-// SetAvailabilityChangeCallback dummy function
-func (i *DBIndexer) SetAvailabilityChangeCallback(callback func(bool)) {
-}
-
 // Ping checks if database is available
 func (i *DBIndexer) Ping() bool {
 	return db.GetEngine(db.DefaultContext).Ping() != nil
diff --git a/modules/indexer/issues/elastic_search.go b/modules/indexer/issues/elastic_search.go
index ee8e3df62f..fd1dd4b452 100644
--- a/modules/indexer/issues/elastic_search.go
+++ b/modules/indexer/issues/elastic_search.go
@@ -22,12 +22,11 @@ var _ Indexer = &ElasticSearchIndexer{}
 
 // ElasticSearchIndexer implements Indexer interface
 type ElasticSearchIndexer struct {
-	client               *elastic.Client
-	indexerName          string
-	available            bool
-	availabilityCallback func(bool)
-	stopTimer            chan struct{}
-	lock                 sync.RWMutex
+	client      *elastic.Client
+	indexerName string
+	available   bool
+	stopTimer   chan struct{}
+	lock        sync.RWMutex
 }
 
 type elasticLogger struct {
@@ -138,13 +137,6 @@ func (b *ElasticSearchIndexer) Init() (bool, error) {
 	return true, nil
 }
 
-// SetAvailabilityChangeCallback sets callback that will be triggered when availability changes
-func (b *ElasticSearchIndexer) SetAvailabilityChangeCallback(callback func(bool)) {
-	b.lock.Lock()
-	defer b.lock.Unlock()
-	b.availabilityCallback = callback
-}
-
 // Ping checks if elastic is available
 func (b *ElasticSearchIndexer) Ping() bool {
 	b.lock.RLock()
@@ -305,8 +297,4 @@ func (b *ElasticSearchIndexer) setAvailability(available bool) {
 	}
 
 	b.available = available
-	if b.availabilityCallback != nil {
-		// Call the callback from within the lock to ensure that the ordering remains correct
-		b.availabilityCallback(b.available)
-	}
 }
diff --git a/modules/indexer/issues/indexer.go b/modules/indexer/issues/indexer.go
index 47a8b10794..e88b1b2bef 100644
--- a/modules/indexer/issues/indexer.go
+++ b/modules/indexer/issues/indexer.go
@@ -49,7 +49,6 @@ type SearchResult struct {
 type Indexer interface {
 	Init() (bool, error)
 	Ping() bool
-	SetAvailabilityChangeCallback(callback func(bool))
 	Index(issue []*IndexerData) error
 	Delete(ids ...int64) error
 	Search(ctx context.Context, kw string, repoIDs []int64, limit, start int) (*SearchResult, error)
@@ -94,7 +93,7 @@ func (h *indexerHolder) get() Indexer {
 
 var (
 	// issueIndexerQueue queue of issue ids to be updated
-	issueIndexerQueue queue.Queue
+	issueIndexerQueue *queue.WorkerPoolQueue[*IndexerData]
 	holder            = newIndexerHolder()
 )
 
@@ -108,62 +107,44 @@ func InitIssueIndexer(syncReindex bool) {
 	// Create the Queue
 	switch setting.Indexer.IssueType {
 	case "bleve", "elasticsearch", "meilisearch":
-		handler := func(data ...queue.Data) []queue.Data {
+		handler := func(items ...*IndexerData) (unhandled []*IndexerData) {
 			indexer := holder.get()
 			if indexer == nil {
-				log.Error("Issue indexer handler: unable to get indexer!")
-				return data
+				log.Error("Issue indexer handler: unable to get indexer.")
+				return items
 			}
-
-			iData := make([]*IndexerData, 0, len(data))
-			unhandled := make([]queue.Data, 0, len(data))
-			for _, datum := range data {
-				indexerData, ok := datum.(*IndexerData)
-				if !ok {
-					log.Error("Unable to process provided datum: %v - not possible to cast to IndexerData", datum)
-					continue
-				}
+			toIndex := make([]*IndexerData, 0, len(items))
+			for _, indexerData := range items {
 				log.Trace("IndexerData Process: %d %v %t", indexerData.ID, indexerData.IDs, indexerData.IsDelete)
 				if indexerData.IsDelete {
 					if err := indexer.Delete(indexerData.IDs...); err != nil {
-						log.Error("Error whilst deleting from index: %v Error: %v", indexerData.IDs, err)
-						if indexer.Ping() {
-							continue
+						log.Error("Issue indexer handler: failed to from index: %v Error: %v", indexerData.IDs, err)
+						if !indexer.Ping() {
+							log.Error("Issue indexer handler: indexer is unavailable when deleting")
+							unhandled = append(unhandled, indexerData)
 						}
-						// Add back to queue
-						unhandled = append(unhandled, datum)
 					}
 					continue
 				}
-				iData = append(iData, indexerData)
+				toIndex = append(toIndex, indexerData)
 			}
-			if len(unhandled) > 0 {
-				for _, indexerData := range iData {
-					unhandled = append(unhandled, indexerData)
-				}
-				return unhandled
-			}
-			if err := indexer.Index(iData); err != nil {
-				log.Error("Error whilst indexing: %v Error: %v", iData, err)
-				if indexer.Ping() {
-					return nil
-				}
-				// Add back to queue
-				for _, indexerData := range iData {
-					unhandled = append(unhandled, indexerData)
+			if err := indexer.Index(toIndex); err != nil {
+				log.Error("Error whilst indexing: %v Error: %v", toIndex, err)
+				if !indexer.Ping() {
+					log.Error("Issue indexer handler: indexer is unavailable when indexing")
+					unhandled = append(unhandled, toIndex...)
 				}
-				return unhandled
 			}
-			return nil
+			return unhandled
 		}
 
-		issueIndexerQueue = queue.CreateQueue("issue_indexer", handler, &IndexerData{})
+		issueIndexerQueue = queue.CreateSimpleQueue("issue_indexer", handler)
 
 		if issueIndexerQueue == nil {
 			log.Fatal("Unable to create issue indexer queue")
 		}
 	default:
-		issueIndexerQueue = &queue.DummyQueue{}
+		issueIndexerQueue = queue.CreateSimpleQueue[*IndexerData]("issue_indexer", nil)
 	}
 
 	// Create the Indexer
@@ -240,18 +221,6 @@ func InitIssueIndexer(syncReindex bool) {
 			log.Fatal("Unknown issue indexer type: %s", setting.Indexer.IssueType)
 		}
 
-		if queue, ok := issueIndexerQueue.(queue.Pausable); ok {
-			holder.get().SetAvailabilityChangeCallback(func(available bool) {
-				if !available {
-					log.Info("Issue index queue paused")
-					queue.Pause()
-				} else {
-					log.Info("Issue index queue resumed")
-					queue.Resume()
-				}
-			})
-		}
-
 		// Start processing the queue
 		go graceful.GetManager().RunWithShutdownFns(issueIndexerQueue.Run)
 
@@ -285,9 +254,7 @@ func InitIssueIndexer(syncReindex bool) {
 			case <-graceful.GetManager().IsShutdown():
 				log.Warn("Shutdown occurred before issue index initialisation was complete")
 			case <-time.After(timeout):
-				if shutdownable, ok := issueIndexerQueue.(queue.Shutdownable); ok {
-					shutdownable.Terminate()
-				}
+				issueIndexerQueue.ShutdownWait(5 * time.Second)
 				log.Fatal("Issue Indexer Initialization timed-out after: %v", timeout)
 			}
 		}()
diff --git a/modules/indexer/issues/meilisearch.go b/modules/indexer/issues/meilisearch.go
index 319dc3e30b..990bc57a05 100644
--- a/modules/indexer/issues/meilisearch.go
+++ b/modules/indexer/issues/meilisearch.go
@@ -17,12 +17,11 @@ var _ Indexer = &MeilisearchIndexer{}
 
 // MeilisearchIndexer implements Indexer interface
 type MeilisearchIndexer struct {
-	client               *meilisearch.Client
-	indexerName          string
-	available            bool
-	availabilityCallback func(bool)
-	stopTimer            chan struct{}
-	lock                 sync.RWMutex
+	client      *meilisearch.Client
+	indexerName string
+	available   bool
+	stopTimer   chan struct{}
+	lock        sync.RWMutex
 }
 
 // MeilisearchIndexer creates a new meilisearch indexer
@@ -73,13 +72,6 @@ func (b *MeilisearchIndexer) Init() (bool, error) {
 	return false, b.checkError(err)
 }
 
-// SetAvailabilityChangeCallback sets callback that will be triggered when availability changes
-func (b *MeilisearchIndexer) SetAvailabilityChangeCallback(callback func(bool)) {
-	b.lock.Lock()
-	defer b.lock.Unlock()
-	b.availabilityCallback = callback
-}
-
 // Ping checks if meilisearch is available
 func (b *MeilisearchIndexer) Ping() bool {
 	b.lock.RLock()
@@ -178,8 +170,4 @@ func (b *MeilisearchIndexer) setAvailability(available bool) {
 	}
 
 	b.available = available
-	if b.availabilityCallback != nil {
-		// Call the callback from within the lock to ensure that the ordering remains correct
-		b.availabilityCallback(b.available)
-	}
 }
diff --git a/modules/indexer/stats/indexer_test.go b/modules/indexer/stats/indexer_test.go
index 8d9b4e36d9..be9c6659f1 100644
--- a/modules/indexer/stats/indexer_test.go
+++ b/modules/indexer/stats/indexer_test.go
@@ -41,7 +41,7 @@ func TestRepoStatsIndex(t *testing.T) {
 	err = UpdateRepoIndexer(repo)
 	assert.NoError(t, err)
 
-	queue.GetManager().FlushAll(context.Background(), 5*time.Second)
+	assert.NoError(t, queue.GetManager().FlushAll(context.Background(), 5*time.Second))
 
 	status, err := repo_model.GetIndexerStatus(db.DefaultContext, repo, repo_model.RepoIndexerTypeStats)
 	assert.NoError(t, err)
diff --git a/modules/indexer/stats/queue.go b/modules/indexer/stats/queue.go
index a57338e07d..46438925e4 100644
--- a/modules/indexer/stats/queue.go
+++ b/modules/indexer/stats/queue.go
@@ -14,12 +14,11 @@ import (
 )
 
 // statsQueue represents a queue to handle repository stats updates
-var statsQueue queue.UniqueQueue
+var statsQueue *queue.WorkerPoolQueue[int64]
 
 // handle passed PR IDs and test the PRs
-func handle(data ...queue.Data) []queue.Data {
-	for _, datum := range data {
-		opts := datum.(int64)
+func handler(items ...int64) []int64 {
+	for _, opts := range items {
 		if err := indexer.Index(opts); err != nil {
 			if !setting.IsInTesting {
 				log.Error("stats queue indexer.Index(%d) failed: %v", opts, err)
@@ -30,7 +29,7 @@ func handle(data ...queue.Data) []queue.Data {
 }
 
 func initStatsQueue() error {
-	statsQueue = queue.CreateUniqueQueue("repo_stats_update", handle, int64(0))
+	statsQueue = queue.CreateUniqueQueue("repo_stats_update", handler)
 	if statsQueue == nil {
 		return fmt.Errorf("Unable to create repo_stats_update Queue")
 	}