lib/versioner: Reduce surface area (#6186)

* lib/versioner: Reduce surface area

This is a refactor while I was anyway rooting around in the versioner.
Instead of exporting every possible implementation and the factory and
letting the caller do whatever, this now encapsulates all that and
exposes a New() that takes a config.VersioningConfiguration.

Given that and that we don't know (from the outside) how a versioner
works or what state it keeps, we now just construct it once per folder
and keep it around. Previously it was recreated for each restore
request.

* unparam

* wip

lib/versioner/versioner.go

@@ -12,6 +12,7 @@ import (
 	"fmt"
 	"time"
 
+	"github.com/syncthing/syncthing/lib/config"
 	"github.com/syncthing/syncthing/lib/fs"
 )
 
@@ -27,10 +28,22 @@ type FileVersion struct {
 	Size        int64     `json:"size"`
 }
 
-var Factories = map[string]func(folderID string, filesystem fs.Filesystem, params map[string]string) Versioner{}
+type factory func(filesystem fs.Filesystem, params map[string]string) Versioner
+
+var factories = make(map[string]factory)
+
 var ErrRestorationNotSupported = fmt.Errorf("version restoration not supported with the current versioner")
 
 const (
 	TimeFormat = "20060102-150405"
-	TimeGlob   = "[0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]-[0-9][0-9][0-9][0-9][0-9][0-9]" // glob pattern matching TimeFormat
+	timeGlob   = "[0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]-[0-9][0-9][0-9][0-9][0-9][0-9]" // glob pattern matching TimeFormat
 )
+
+func New(fs fs.Filesystem, cfg config.VersioningConfiguration) (Versioner, error) {
+	fac, ok := factories[cfg.Type]
+	if !ok {
+		return nil, fmt.Errorf("requested versioning type %q does not exist", cfg.Type)
+	}
+
+	return fac(fs, cfg.Params), nil
+}