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/meilisearch/meilisearch.go

@@ -0,0 +1,98 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package meilisearch
+
+import (
+	"context"
+	"strconv"
+	"strings"
+
+	indexer_internal "code.gitea.io/gitea/modules/indexer/internal"
+	inner_meilisearch "code.gitea.io/gitea/modules/indexer/internal/meilisearch"
+	"code.gitea.io/gitea/modules/indexer/issues/internal"
+
+	"github.com/meilisearch/meilisearch-go"
+)
+
+const (
+	issueIndexerLatestVersion = 0
+)
+
+var _ internal.Indexer = &Indexer{}
+
+// Indexer implements Indexer interface
+type Indexer struct {
+	inner                    *inner_meilisearch.Indexer
+	indexer_internal.Indexer // do not composite inner_meilisearch.Indexer directly to avoid exposing too much
+}
+
+// NewIndexer creates a new meilisearch indexer
+func NewIndexer(url, apiKey, indexerName string) *Indexer {
+	inner := inner_meilisearch.NewIndexer(url, apiKey, indexerName, issueIndexerLatestVersion)
+	indexer := &Indexer{
+		inner:   inner,
+		Indexer: inner,
+	}
+	return indexer
+}
+
+// Index will save the index data
+func (b *Indexer) Index(_ context.Context, issues []*internal.IndexerData) error {
+	if len(issues) == 0 {
+		return nil
+	}
+	for _, issue := range issues {
+		_, err := b.inner.Client.Index(b.inner.VersionedIndexName()).AddDocuments(issue)
+		if err != nil {
+			return err
+		}
+	}
+	// TODO: bulk send index data
+	return nil
+}
+
+// Delete deletes indexes by ids
+func (b *Indexer) Delete(_ context.Context, ids ...int64) error {
+	if len(ids) == 0 {
+		return nil
+	}
+
+	for _, id := range ids {
+		_, err := b.inner.Client.Index(b.inner.VersionedIndexName()).DeleteDocument(strconv.FormatInt(id, 10))
+		if err != nil {
+			return err
+		}
+	}
+	// TODO: bulk send deletes
+	return nil
+}
+
+// Search searches for issues by given conditions.
+// Returns the matching issue IDs
+func (b *Indexer) Search(ctx context.Context, keyword string, repoIDs []int64, limit, start int) (*internal.SearchResult, error) {
+	repoFilters := make([]string, 0, len(repoIDs))
+	for _, repoID := range repoIDs {
+		repoFilters = append(repoFilters, "repo_id = "+strconv.FormatInt(repoID, 10))
+	}
+	filter := strings.Join(repoFilters, " OR ")
+	searchRes, err := b.inner.Client.Index(b.inner.VersionedIndexName()).Search(keyword, &meilisearch.SearchRequest{
+		Filter: filter,
+		Limit:  int64(limit),
+		Offset: int64(start),
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	hits := make([]internal.Match, 0, len(searchRes.Hits))
+	for _, hit := range searchRes.Hits {
+		hits = append(hits, internal.Match{
+			ID: int64(hit.(map[string]interface{})["id"].(float64)),
+		})
+	}
+	return &internal.SearchResult{
+		Total: searchRes.TotalHits,
+		Hits:  hits,
+	}, nil
+}