fix(gitlab): discard label removal events on merge requests

pkg/provider/gitlab/detect.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"net/http"
+	"slices"
 
 	"github.com/openshift-pipelines/pipelines-as-code/pkg/provider"
 	gitlab "gitlab.com/gitlab-org/api/client-go"
@@ -87,13 +88,16 @@ func (v *Provider) Detect(req *http.Request, payload string, logger *zap.Sugared
 
 // hasOnlyLabelsChanged checks if the only change in the merge request is to its labels.
 // This function ensures that other fields remain unchanged.
+// Labels changes are treated when they're done standalone and not done with other changes
+// for example, if a merge request is marked as draft and then the labels are changed, this is not a valid merge request update.
 func hasOnlyLabelsChanged(gitEvent *gitlab.MergeEvent) bool {
 	changes := gitEvent.Changes
 
-	labelsChanged := len(changes.Labels.Previous) > 0 || len(changes.Labels.Current) > 0
+	labelsChanged := isNewLabelAdded(changes.Labels.Previous, changes.Labels.Current)
 
 	// Only Labels can change — everything else must be zero or nil
 	onlyUpdatedAtOrLabels := labelsChanged &&
+		!changes.Draft.Previous && !changes.Draft.Current &&
 		changes.Assignees.Previous == nil && changes.Assignees.Current == nil &&
 		changes.Reviewers.Previous == nil && changes.Reviewers.Current == nil &&
 		changes.Description.Previous == "" && changes.Description.Current == "" &&
@@ -108,3 +112,11 @@ func hasOnlyLabelsChanged(gitEvent *gitlab.MergeEvent) bool {
 
 	return onlyUpdatedAtOrLabels
 }
+
+func isNewLabelAdded(previous, current []*gitlab.EventLabel) bool {
+	return slices.ContainsFunc(current, func(label *gitlab.EventLabel) bool {
+		return !slices.ContainsFunc(previous, func(prev *gitlab.EventLabel) bool {
+			return prev.Title == label.Title
+		})
+	})
+}

pkg/provider/gitlab/detect_test.go

@@ -13,6 +13,12 @@ import (
 
 const largeComment = "/Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s"
 
+func mrEventWithChanges(sample thelp.TEvent, changesJSON string) string {
+	base := sample.MREventAsJSON("update", "")
+	idx := strings.LastIndex(base, "}")
+	return base[:idx] + `,"changes": {` + changesJSON + `}}`
+}
+
 func TestProviderDetect(t *testing.T) {
 	sample := thelp.TEvent{
 		Username:          "foo",
@@ -173,6 +179,53 @@ func TestProviderDetect(t *testing.T) {
 			isGL:       true,
 			processReq: true,
 		},
+		{
+			name:       "good/mergeRequest update Event with label addition",
+			event:      mrEventWithChanges(sample, `"labels": {"previous": [], "current": [{"id": 1, "title": "bug"}]}`),
+			eventType:  gitlab.EventTypeMergeRequest,
+			isGL:       true,
+			processReq: true,
+		},
+		{
+			name:       "bad/mergeRequest update Event with label removal",
+			event:      mrEventWithChanges(sample, `"labels": {"previous": [{"id": 1, "title": "bug"}], "current": []}`),
+			eventType:  gitlab.EventTypeMergeRequest,
+			isGL:       true,
+			processReq: false,
+			wantReason: "this 'Merge Request' update event changes are not supported",
+		},
+		{
+			name:       "bad/mergeRequest update Event with label removal partial",
+			event:      mrEventWithChanges(sample, `"labels": {"previous": [{"id": 1, "title": "bug"}, {"id": 2, "title": "feature"}], "current": [{"id": 1, "title": "bug"}]}`),
+			eventType:  gitlab.EventTypeMergeRequest,
+			isGL:       true,
+			processReq: false,
+			wantReason: "this 'Merge Request' update event changes are not supported",
+		},
+		{
+			name:       "bad/mergeRequest update Event with label addition on draft to ready transition",
+			event:      mrEventWithChanges(sample, `"labels": {"previous": [], "current": [{"id": 1, "title": "bug"}]}, "draft": {"previous": true, "current": false}`),
+			eventType:  gitlab.EventTypeMergeRequest,
+			isGL:       true,
+			processReq: false,
+			wantReason: "this 'Merge Request' update event changes are not supported",
+		},
+		{
+			name:       "bad/mergeRequest update Event with label addition on ready to draft transition",
+			event:      mrEventWithChanges(sample, `"labels": {"previous": [], "current": [{"id": 1, "title": "bug"}]}, "draft": {"previous": false, "current": true}`),
+			eventType:  gitlab.EventTypeMergeRequest,
+			isGL:       true,
+			processReq: false,
+			wantReason: "this 'Merge Request' update event changes are not supported",
+		},
+		{
+			name:       "bad/mergeRequest update Event with label addition and description change",
+			event:      mrEventWithChanges(sample, `"labels": {"previous": [], "current": [{"id": 1, "title": "bug"}]}, "description": {"previous": "old", "current": "new"}`),
+			eventType:  gitlab.EventTypeMergeRequest,
+			isGL:       true,
+			processReq: false,
+			wantReason: "this 'Merge Request' update event changes are not supported",
+		},
 		{
 			name:       "good/commit comment /retest command",
 			event:      sample.CommitNoteEventAsJSON("/retest", "create", "null"),