[GITEA] Avoid WHERE IN for comment migration query

- Rewrite `UpdateCommentsMigrationsByType` to not use `WHERE IN` as
that's a performance diaster for MariaDB, it now use batching to query
the the relevant comment IDs via JOINs (which is not possible in a
UPDATE query for SQLite) and then update them in a seperate query.
- Add unit test.
- Resolves https://codeberg.org/forgejo/forgejo/issues/1856

(cherry picked from commit 8098ca9d2e)

Conflicts:
	models/issues/comment.go
	https://codeberg.org/forgejo/forgejo/pulls/2075

models/issues/comment_test.go

@@ -12,6 +12,7 @@ import (
 	repo_model "code.gitea.io/gitea/models/repo"
 	"code.gitea.io/gitea/models/unittest"
 	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/structs"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -97,3 +98,29 @@ func TestMigrate_InsertIssueComments(t *testing.T) {
 
 	unittest.CheckConsistencyFor(t, &issues_model.Issue{})
 }
+
+func TestUpdateCommentsMigrationsByType(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	issue := unittest.AssertExistsAndLoadBean(t, &issues_model.Issue{ID: 1})
+	repo := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: issue.RepoID})
+	comment := unittest.AssertExistsAndLoadBean(t, &issues_model.Comment{ID: 1, IssueID: issue.ID})
+
+	// Set repository to migrated from Gitea.
+	repo.OriginalServiceType = structs.GiteaService
+	repo_model.UpdateRepositoryCols(db.DefaultContext, repo, "original_service_type")
+
+	// Set comment to have an original author.
+	comment.OriginalAuthor = "Example User"
+	comment.OriginalAuthorID = 1
+	comment.PosterID = 0
+	_, err := db.GetEngine(db.DefaultContext).ID(comment.ID).Cols("original_author", "original_author_id", "poster_id").Update(comment)
+	assert.NoError(t, err)
+
+	assert.NoError(t, issues_model.UpdateCommentsMigrationsByType(db.DefaultContext, structs.GiteaService, "1", 513))
+
+	comment = unittest.AssertExistsAndLoadBean(t, &issues_model.Comment{ID: 1, IssueID: issue.ID})
+	assert.Empty(t, comment.OriginalAuthor)
+	assert.Empty(t, comment.OriginalAuthorID)
+	assert.EqualValues(t, 513, comment.PosterID)
+}