[GITEA] enable system users for comment.LoadPoster

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 97667e06b3)
(cherry picked from commit 8ef73a09c9)
(cherry picked from commit fa8a00d264)
(cherry picked from commit 2ada2074b5)
(cherry picked from commit f9a59b940a)
(cherry picked from commit cd82834043)

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

@@ -189,6 +189,43 @@ 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"