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/code/git.go

@@ -10,23 +10,11 @@ import (
 
 	repo_model "code.gitea.io/gitea/models/repo"
 	"code.gitea.io/gitea/modules/git"
+	"code.gitea.io/gitea/modules/indexer/code/internal"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
 )
 
-type fileUpdate struct {
-	Filename string
-	BlobSha  string
-	Size     int64
-	Sized    bool
-}
-
-// repoChanges changes (file additions/updates/removals) to a repo
-type repoChanges struct {
-	Updates          []fileUpdate
-	RemovedFilenames []string
-}
-
 func getDefaultBranchSha(ctx context.Context, repo *repo_model.Repository) (string, error) {
 	stdout, _, err := git.NewCommand(ctx, "show-ref", "-s").AddDynamicArguments(git.BranchPrefix + repo.DefaultBranch).RunStdString(&git.RunOpts{Dir: repo.RepoPath()})
 	if err != nil {
@@ -36,7 +24,7 @@ func getDefaultBranchSha(ctx context.Context, repo *repo_model.Repository) (stri
 }
 
 // getRepoChanges returns changes to repo since last indexer update
-func getRepoChanges(ctx context.Context, repo *repo_model.Repository, revision string) (*repoChanges, error) {
+func getRepoChanges(ctx context.Context, repo *repo_model.Repository, revision string) (*internal.RepoChanges, error) {
 	status, err := repo_model.GetIndexerStatus(ctx, repo, repo_model.RepoIndexerTypeCode)
 	if err != nil {
 		return nil, err
@@ -67,16 +55,16 @@ func isIndexable(entry *git.TreeEntry) bool {
 }
 
 // parseGitLsTreeOutput parses the output of a `git ls-tree -r --full-name` command
-func parseGitLsTreeOutput(stdout []byte) ([]fileUpdate, error) {
+func parseGitLsTreeOutput(stdout []byte) ([]internal.FileUpdate, error) {
 	entries, err := git.ParseTreeEntries(stdout)
 	if err != nil {
 		return nil, err
 	}
 	idxCount := 0
-	updates := make([]fileUpdate, len(entries))
+	updates := make([]internal.FileUpdate, len(entries))
 	for _, entry := range entries {
 		if isIndexable(entry) {
-			updates[idxCount] = fileUpdate{
+			updates[idxCount] = internal.FileUpdate{
 				Filename: entry.Name(),
 				BlobSha:  entry.ID.String(),
 				Size:     entry.Size(),
@@ -89,8 +77,8 @@ func parseGitLsTreeOutput(stdout []byte) ([]fileUpdate, error) {
 }
 
 // genesisChanges get changes to add repo to the indexer for the first time
-func genesisChanges(ctx context.Context, repo *repo_model.Repository, revision string) (*repoChanges, error) {
-	var changes repoChanges
+func genesisChanges(ctx context.Context, repo *repo_model.Repository, revision string) (*internal.RepoChanges, error) {
+	var changes internal.RepoChanges
 	stdout, _, runErr := git.NewCommand(ctx, "ls-tree", "--full-tree", "-l", "-r").AddDynamicArguments(revision).RunStdBytes(&git.RunOpts{Dir: repo.RepoPath()})
 	if runErr != nil {
 		return nil, runErr
@@ -102,20 +90,20 @@ func genesisChanges(ctx context.Context, repo *repo_model.Repository, revision s
 }
 
 // nonGenesisChanges get changes since the previous indexer update
-func nonGenesisChanges(ctx context.Context, repo *repo_model.Repository, revision string) (*repoChanges, error) {
+func nonGenesisChanges(ctx context.Context, repo *repo_model.Repository, revision string) (*internal.RepoChanges, error) {
 	diffCmd := git.NewCommand(ctx, "diff", "--name-status").AddDynamicArguments(repo.CodeIndexerStatus.CommitSha, revision)
 	stdout, _, runErr := diffCmd.RunStdString(&git.RunOpts{Dir: repo.RepoPath()})
 	if runErr != nil {
 		// previous commit sha may have been removed by a force push, so
 		// try rebuilding from scratch
 		log.Warn("git diff: %v", runErr)
-		if err := indexer.Delete(repo.ID); err != nil {
+		if err := (*globalIndexer.Load()).Delete(ctx, repo.ID); err != nil {
 			return nil, err
 		}
 		return genesisChanges(ctx, repo, revision)
 	}
 
-	var changes repoChanges
+	var changes internal.RepoChanges
 	var err error
 	updatedFilenames := make([]string, 0, 10)
 	for _, line := range strings.Split(stdout, "\n") {