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>

routers/web/explore/code.go

@@ -79,13 +79,13 @@ func Code(ctx *context.Context) {
 	if (len(repoIDs) > 0) || isAdmin {
 		total, searchResults, searchResultLanguages, err = code_indexer.PerformSearch(ctx, repoIDs, language, keyword, page, setting.UI.RepoSearchPagingNum, isMatch)
 		if err != nil {
-			if code_indexer.IsAvailable() {
+			if code_indexer.IsAvailable(ctx) {
 				ctx.ServerError("SearchResults", err)
 				return
 			}
 			ctx.Data["CodeIndexerUnavailable"] = true
 		} else {
-			ctx.Data["CodeIndexerUnavailable"] = !code_indexer.IsAvailable()
+			ctx.Data["CodeIndexerUnavailable"] = !code_indexer.IsAvailable(ctx)
 		}
 
 		loadRepoIDs := make([]int64, 0, len(searchResults))

routers/web/repo/issue.go

@@ -191,7 +191,7 @@ func issues(ctx *context.Context, milestoneID, projectID int64, isPullOption uti
 	if len(keyword) > 0 {
 		issueIDs, err = issue_indexer.SearchIssuesByKeyword(ctx, []int64{repo.ID}, keyword)
 		if err != nil {
-			if issue_indexer.IsAvailable() {
+			if issue_indexer.IsAvailable(ctx) {
 				ctx.ServerError("issueIndexer.Search", err)
 				return
 			}

routers/web/repo/search.go

@@ -45,13 +45,13 @@ func Search(ctx *context.Context) {
 	total, searchResults, searchResultLanguages, err := code_indexer.PerformSearch(ctx, []int64{ctx.Repo.Repository.ID},
 		language, keyword, page, setting.UI.RepoSearchPagingNum, isMatch)
 	if err != nil {
-		if code_indexer.IsAvailable() {
+		if code_indexer.IsAvailable(ctx) {
 			ctx.ServerError("SearchResults", err)
 			return
 		}
 		ctx.Data["CodeIndexerUnavailable"] = true
 	} else {
-		ctx.Data["CodeIndexerUnavailable"] = !code_indexer.IsAvailable()
+		ctx.Data["CodeIndexerUnavailable"] = !code_indexer.IsAvailable(ctx)
 	}
 
 	ctx.Data["SourcePath"] = ctx.Repo.Repository.Link()

routers/web/user/code.go

@@ -71,13 +71,13 @@ func CodeSearch(ctx *context.Context) {
 	if len(repoIDs) > 0 {
 		total, searchResults, searchResultLanguages, err = code_indexer.PerformSearch(ctx, repoIDs, language, keyword, page, setting.UI.RepoSearchPagingNum, isMatch)
 		if err != nil {
-			if code_indexer.IsAvailable() {
+			if code_indexer.IsAvailable(ctx) {
 				ctx.ServerError("SearchResults", err)
 				return
 			}
 			ctx.Data["CodeIndexerUnavailable"] = true
 		} else {
-			ctx.Data["CodeIndexerUnavailable"] = !code_indexer.IsAvailable()
+			ctx.Data["CodeIndexerUnavailable"] = !code_indexer.IsAvailable(ctx)
 		}
 
 		loadRepoIDs := make([]int64, 0, len(searchResults))