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/issues/db/db.go

@@ -0,0 +1,54 @@
+// Copyright 2019 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package db
+
+import (
+	"context"
+
+	issues_model "code.gitea.io/gitea/models/issues"
+	indexer_internal "code.gitea.io/gitea/modules/indexer/internal"
+	inner_db "code.gitea.io/gitea/modules/indexer/internal/db"
+	"code.gitea.io/gitea/modules/indexer/issues/internal"
+)
+
+var _ internal.Indexer = &Indexer{}
+
+// Indexer implements Indexer interface to use database's like search
+type Indexer struct {
+	indexer_internal.Indexer
+}
+
+func NewIndexer() *Indexer {
+	return &Indexer{
+		Indexer: &inner_db.Indexer{},
+	}
+}
+
+// Index dummy function
+func (i *Indexer) Index(_ context.Context, _ []*internal.IndexerData) error {
+	return nil
+}
+
+// Delete dummy function
+func (i *Indexer) Delete(_ context.Context, _ ...int64) error {
+	return nil
+}
+
+// Search searches for issues
+func (i *Indexer) Search(ctx context.Context, kw string, repoIDs []int64, limit, start int) (*internal.SearchResult, error) {
+	total, ids, err := issues_model.SearchIssueIDsByKeyword(ctx, kw, repoIDs, limit, start)
+	if err != nil {
+		return nil, err
+	}
+	result := internal.SearchResult{
+		Total: total,
+		Hits:  make([]internal.Match, 0, limit),
+	}
+	for _, id := range ids {
+		result.Hits = append(result.Hits, internal.Match{
+			ID: id,
+		})
+	}
+	return &result, nil
+}