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

Label removal events on GitLab merge requests were incorrectly triggering
pipeline runs. The hasOnlyLabelsChanged check used an OR condition that
matched both additions and removals. Changed to compare current vs previous
label count so only label additions are processed.

Signed-off-by: Zaki Shaikh <zashaikh@redhat.com>

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: zakisk

pkg/provider/gitlab/detect.go

@@ -90,7 +90,7 @@ func (v *Provider) Detect(req *http.Request, payload string, logger *zap.Sugared
 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 &&
@@ -108,3 +108,21 @@ func hasOnlyLabelsChanged(gitEvent *gitlab.MergeEvent) bool {
 
 	return onlyUpdatedAtOrLabels
 }
+
+func isNewLabelAdded(previous, current []*gitlab.EventLabel) bool {
+	newLabels := 0
+	for _, label := range current {
+		found := false
+		for _, previousLabel := range previous {
+			if label.Title == previousLabel.Title {
+				found = true
+				break
+			}
+		}
+		if !found {
+			newLabels++
+		}
+	}
+
+	return newLabels > 0
+}

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 +179,37 @@ 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 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"),