Refactor indexer (#25174)

Refactor `modules/indexer` to make it more maintainable. And it can be
easier to support more features. I'm trying to solve some of issue
searching, this is a precursor to making functional changes.

Current supported engines and the index versions:

| engines | issues | code |
| - | - | - |
| db | Just a wrapper for database queries, doesn't need version | - |
| bleve | The version of index is **2** | The version of index is **6**
|
| elasticsearch | The old index has no version, will be treated as
version **0** in this PR | The version of index is **1** |
| meilisearch | The old index has no version, will be treated as version
**0** in this PR | - |


## Changes

### Split

Splited it into mutiple packages

```text
indexer
├── internal
│   ├── bleve
│   ├── db
│   ├── elasticsearch
│   └── meilisearch
├── code
│   ├── bleve
│   ├── elasticsearch
│   └── internal
└── issues
    ├── bleve
    ├── db
    ├── elasticsearch
    ├── internal
    └── meilisearch
```

- `indexer/interanal`: Internal shared package for indexer.
- `indexer/interanal/[engine]`: Internal shared package for each engine
(bleve/db/elasticsearch/meilisearch).
- `indexer/code`: Implementations for code indexer.
- `indexer/code/internal`: Internal shared package for code indexer.
- `indexer/code/[engine]`: Implementation via each engine for code
indexer.
- `indexer/issues`: Implementations for issues indexer.

### Deduplication

- Combine `Init/Ping/Close` for code indexer and issues indexer.
- ~Combine `issues.indexerHolder` and `code.wrappedIndexer` to
`internal.IndexHolder`.~ Remove it, use dummy indexer instead when the
indexer is not ready.
- Duplicate two copies of creating ES clients.
- Duplicate two copies of `indexerID()`.


### Enhancement

- [x] Support index version for elasticsearch issues indexer, the old
index without version will be treated as version 0.
- [x] Fix spell of `elastic_search/ElasticSearch`, it should be
`Elasticsearch`.
- [x] Improve versioning of ES index. We don't need `Aliases`:
- Gitea does't need aliases for "Zero Downtime" because it never delete
old indexes.
- The old code of issues indexer uses the orignal name to create issue
index, so it's tricky to convert it to an alias.
- [x] Support index version for meilisearch issues indexer, the old
index without version will be treated as version 0.
- [x] Do "ping" only when `Ping` has been called, don't ping
periodically and cache the status.
- [x] Support the context parameter whenever possible.
- [x] Fix outdated example config.
- [x] Give up the requeue logic of issues indexer: When indexing fails,
call Ping to check if it was caused by the engine being unavailable, and
only requeue the task if the engine is unavailable.
- It is fragile and tricky, could cause data losing (It did happen when
I was doing some tests for this PR). And it works for ES only.
- Just always requeue the failed task, if it caused by bad data, it's a
bug of Gitea which should be fixed.

---------

Co-authored-by: Giteabot <teabot@gitea.io>

modules/indexer/internal/bleve/batch.go

@@ -0,0 +1,58 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package bleve
+
+import (
+	"github.com/blevesearch/bleve/v2"
+)
+
+// FlushingBatch is a batch of operations that automatically flushes to the
+// underlying index once it reaches a certain size.
+type FlushingBatch struct {
+	maxBatchSize int
+	batch        *bleve.Batch
+	index        bleve.Index
+}
+
+// NewFlushingBatch creates a new flushing batch for the specified index. Once
+// the number of operations in the batch reaches the specified limit, the batch
+// automatically flushes its operations to the index.
+func NewFlushingBatch(index bleve.Index, maxBatchSize int) *FlushingBatch {
+	return &FlushingBatch{
+		maxBatchSize: maxBatchSize,
+		batch:        index.NewBatch(),
+		index:        index,
+	}
+}
+
+// Index add a new index to batch
+func (b *FlushingBatch) Index(id string, data interface{}) error {
+	if err := b.batch.Index(id, data); err != nil {
+		return err
+	}
+	return b.flushIfFull()
+}
+
+// Delete add a delete index to batch
+func (b *FlushingBatch) Delete(id string) error {
+	b.batch.Delete(id)
+	return b.flushIfFull()
+}
+
+func (b *FlushingBatch) flushIfFull() error {
+	if b.batch.Size() < b.maxBatchSize {
+		return nil
+	}
+	return b.Flush()
+}
+
+// Flush submit the batch and create a new one
+func (b *FlushingBatch) Flush() error {
+	err := b.index.Batch(b.batch)
+	if err != nil {
+		return err
+	}
+	b.batch = b.index.NewBatch()
+	return nil
+}

modules/indexer/internal/bleve/indexer.go

@@ -0,0 +1,103 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package bleve
+
+import (
+	"context"
+	"fmt"
+
+	"code.gitea.io/gitea/modules/indexer/internal"
+	"code.gitea.io/gitea/modules/log"
+
+	"github.com/blevesearch/bleve/v2"
+	"github.com/blevesearch/bleve/v2/mapping"
+	"github.com/ethantkoenig/rupture"
+)
+
+var _ internal.Indexer = &Indexer{}
+
+// Indexer represents a basic bleve indexer implementation
+type Indexer struct {
+	Indexer bleve.Index
+
+	indexDir      string
+	version       int
+	mappingGetter MappingGetter
+}
+
+type MappingGetter func() (mapping.IndexMapping, error)
+
+func NewIndexer(indexDir string, version int, mappingGetter func() (mapping.IndexMapping, error)) *Indexer {
+	return &Indexer{
+		indexDir:      indexDir,
+		version:       version,
+		mappingGetter: mappingGetter,
+	}
+}
+
+// Init initializes the indexer
+func (i *Indexer) Init(_ context.Context) (bool, error) {
+	if i == nil {
+		return false, fmt.Errorf("cannot init nil indexer")
+	}
+
+	if i.Indexer != nil {
+		return false, fmt.Errorf("indexer is already initialized")
+	}
+
+	indexer, version, err := openIndexer(i.indexDir, i.version)
+	if err != nil {
+		return false, err
+	}
+	if indexer != nil {
+		i.Indexer = indexer
+		return true, nil
+	}
+
+	if version != 0 {
+		log.Warn("Found older bleve index with version %d, Gitea will remove it and rebuild", version)
+	}
+
+	indexMapping, err := i.mappingGetter()
+	if err != nil {
+		return false, err
+	}
+
+	indexer, err = bleve.New(i.indexDir, indexMapping)
+	if err != nil {
+		return false, err
+	}
+
+	if err = rupture.WriteIndexMetadata(i.indexDir, &rupture.IndexMetadata{
+		Version: i.version,
+	}); err != nil {
+		return false, err
+	}
+
+	i.Indexer = indexer
+
+	return false, nil
+}
+
+// Ping checks if the indexer is available
+func (i *Indexer) Ping(_ context.Context) error {
+	if i == nil {
+		return fmt.Errorf("cannot ping nil indexer")
+	}
+	if i.Indexer == nil {
+		return fmt.Errorf("indexer is not initialized")
+	}
+	return nil
+}
+
+func (i *Indexer) Close() {
+	if i == nil {
+		return
+	}
+
+	if err := i.Indexer.Close(); err != nil {
+		log.Error("Failed to close bleve indexer in %q: %v", i.indexDir, err)
+	}
+	i.Indexer = nil
+}

modules/indexer/internal/bleve/util.go

@@ -0,0 +1,49 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package bleve
+
+import (
+	"errors"
+	"os"
+
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/util"
+
+	"github.com/blevesearch/bleve/v2"
+	"github.com/blevesearch/bleve/v2/index/upsidedown"
+	"github.com/ethantkoenig/rupture"
+)
+
+// openIndexer open the index at the specified path, checking for metadata
+// updates and bleve version updates.  If index needs to be created (or
+// re-created), returns (nil, nil)
+func openIndexer(path string, latestVersion int) (bleve.Index, int, error) {
+	_, err := os.Stat(path)
+	if err != nil && os.IsNotExist(err) {
+		return nil, 0, nil
+	} else if err != nil {
+		return nil, 0, err
+	}
+
+	metadata, err := rupture.ReadIndexMetadata(path)
+	if err != nil {
+		return nil, 0, err
+	}
+	if metadata.Version < latestVersion {
+		// the indexer is using a previous version, so we should delete it and
+		// re-populate
+		return nil, metadata.Version, util.RemoveAll(path)
+	}
+
+	index, err := bleve.Open(path)
+	if err != nil {
+		if errors.Is(err, upsidedown.IncompatibleVersion) {
+			log.Warn("Indexer was built with a previous version of bleve, deleting and rebuilding")
+			return nil, 0, util.RemoveAll(path)
+		}
+		return nil, 0, err
+	}
+
+	return index, 0, nil
+}