enable system users for comment.LoadPoster (#28014) (#28032)

Backport #28014 by @earl-warren

System users (Ghost, ActionsUser, etc) have a negative id and may be the
author of a comment, either because it was created by a now deleted user
or via an action using a transient token.

The GetPossibleUserByID function has special cases related to system
users and will not fail if given a negative id.

Refs: https://codeberg.org/forgejo/forgejo/issues/1425 

(cherry picked from commit 6a2d2fa243)

Co-authored-by: Earl Warren <109468362+earl-warren@users.noreply.github.com>

models/issues/comment.go

@@ -342,7 +342,7 @@ func (c *Comment) AfterLoad(session *xorm.Session) {
 
 // LoadPoster loads comment poster
 func (c *Comment) LoadPoster(ctx context.Context) (err error) {
-	if c.PosterID <= 0 || c.Poster != nil {
+	if c.Poster != nil {
 		return nil
 	}
 

tests/integration/api_comment_test.go

@@ -136,6 +136,43 @@ func TestAPIGetComment(t *testing.T) {
 	assert.Equal(t, expect.Created.Unix(), apiComment.Created.Unix())
 }
 
+func TestAPIGetSystemUserComment(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	issue := unittest.AssertExistsAndLoadBean(t, &issues_model.Issue{})
+	repo := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: issue.RepoID})
+	repoOwner := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: repo.OwnerID})
+
+	for _, systemUser := range []*user_model.User{
+		user_model.NewGhostUser(),
+		user_model.NewActionsUser(),
+	} {
+		body := fmt.Sprintf("Hello %s", systemUser.Name)
+		comment, err := issues_model.CreateComment(db.DefaultContext, &issues_model.CreateCommentOptions{
+			Type:    issues_model.CommentTypeComment,
+			Doer:    systemUser,
+			Repo:    repo,
+			Issue:   issue,
+			Content: body,
+		})
+		assert.NoError(t, err)
+
+		req := NewRequestf(t, "GET", "/api/v1/repos/%s/%s/issues/comments/%d", repoOwner.Name, repo.Name, comment.ID)
+		resp := MakeRequest(t, req, http.StatusOK)
+
+		var apiComment api.Comment
+		DecodeJSON(t, resp, &apiComment)
+
+		if assert.NotNil(t, apiComment.Poster) {
+			if assert.Equal(t, systemUser.ID, apiComment.Poster.ID) {
+				assert.NoError(t, comment.LoadPoster(db.DefaultContext))
+				assert.Equal(t, systemUser.Name, apiComment.Poster.UserName)
+			}
+		}
+		assert.Equal(t, body, apiComment.Body)
+	}
+}
+
 func TestAPIEditComment(t *testing.T) {
 	defer tests.PrepareTestEnv(t)()
 	const newCommentBody = "This is the new comment body"