Don't fetch Mirror when it's migrating (go-gitea#19588)

- When a repository is still being migrated, don't try to fetch the
Mirror from the database. Instead skip it. This allows to visit
repositories that are still being migrated and were configured to be
mirrored.
- Resolves go-gitea#19585
- Regression: go-gitea#19295

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/task.go

@@ -181,6 +181,14 @@ func GetMigratingTask(repoID int64) (*Task, error) {
 	return &task, nil
 }
 
+// HasMigratingTask returns if migrating task exist for repo.
+func HasMigratingTask(repoID int64) (bool, error) {
+	return db.GetEngine(db.DefaultContext).Exist(&Task{
+		RepoID: repoID,
+		Type:   structs.TaskTypeMigrateRepo,
+	})
+}
+
 // GetMigratingTaskByID returns the migrating task by repo's id
 func GetMigratingTaskByID(id, doerID int64) (*Task, *migration.MigrateOptions, error) {
 	task := Task{

modules/context/repo.go

@@ -370,15 +370,24 @@ func repoAssignment(ctx *Context, repo *repo_model.Repository) {
 	ctx.Data["Permission"] = &ctx.Repo.Permission
 
 	if repo.IsMirror {
-		var err error
-		ctx.Repo.Mirror, err = repo_model.GetMirrorByRepoID(repo.ID)
+
+		// Check if there's a migrating task.
+		// If it does exist, don't fetch the Mirror from the database as it doesn't exist yet.
+		hasTask, err := models.HasMigratingTask(repo.ID)
 		if err != nil {
 			ctx.ServerError("GetMirrorByRepoID", err)
 			return
 		}
-		ctx.Data["MirrorEnablePrune"] = ctx.Repo.Mirror.EnablePrune
-		ctx.Data["MirrorInterval"] = ctx.Repo.Mirror.Interval
-		ctx.Data["Mirror"] = ctx.Repo.Mirror
+		if !hasTask {
+			ctx.Repo.Mirror, err = repo_model.GetMirrorByRepoID(repo.ID)
+			if err != nil {
+				ctx.ServerError("GetMirrorByRepoID", err)
+				return
+			}
+			ctx.Data["MirrorEnablePrune"] = ctx.Repo.Mirror.EnablePrune
+			ctx.Data["MirrorInterval"] = ctx.Repo.Mirror.Interval
+			ctx.Data["Mirror"] = ctx.Repo.Mirror
+		}
 	}
 
 	pushMirrors, err := repo_model.GetPushMirrorsByRepoID(repo.ID)