plugins/jira: fix disabled project filtering

Fix behavior of `filterDisabledJiraProjects` when multiple disabled
projects are listed and add corresponding unit test.

prow/plugins/jira/jira.go

@@ -325,17 +325,16 @@ func filterOutDisabledJiraProjects(candidateNames []string, cfg *plugins.Jira) [
 		return candidateNames
 	}
 
-	var result []string
+	candidateSet := sets.NewString(candidateNames...)
 	for _, excludedProject := range cfg.DisabledJiraProjects {
 		for _, candidate := range candidateNames {
 			if strings.HasPrefix(strings.ToLower(candidate), strings.ToLower(excludedProject)) {
-				continue
+				candidateSet.Delete(candidate)
 			}
-			result = append(result, candidate)
 		}
 	}
 
-	return result
+	return candidateSet.UnsortedList()
 }
 
 // projectCachingJiraClient caches 404 for projects and uses them to introduce

prow/plugins/jira/jira_test.go

@@ -540,3 +540,42 @@ func TestProjectCachingJiraClient(t *testing.T) {
 		})
 	}
 }
+
+func TestFilterOutDisabledJiraProjects(t *testing.T) {
+	t.Parallel()
+	testCases := []struct {
+		name           string
+		candidates     []string
+		jiraConfig     *plugins.Jira
+		expectedOutput []string
+	}{{
+		name:           "empty jira config",
+		candidates:     []string{"ABC-123", "DEF-567"},
+		jiraConfig:     nil,
+		expectedOutput: []string{"ABC-123", "DEF-567"},
+	}, {
+		name:           "upper case disabled list",
+		candidates:     []string{"ABC-123", "DEF-567"},
+		jiraConfig:     &plugins.Jira{DisabledJiraProjects: []string{"ABC"}},
+		expectedOutput: []string{"DEF-567"},
+	}, {
+		name:           "lower case disabled list",
+		candidates:     []string{"ABC-123", "DEF-567"},
+		jiraConfig:     &plugins.Jira{DisabledJiraProjects: []string{"abc"}},
+		expectedOutput: []string{"DEF-567"},
+	}, {
+		name:           "multiple disabled projects",
+		candidates:     []string{"ABC-123", "DEF-567"},
+		jiraConfig:     &plugins.Jira{DisabledJiraProjects: []string{"abc", "def"}},
+		expectedOutput: []string{},
+	}}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			output := filterOutDisabledJiraProjects(tc.candidates, tc.jiraConfig)
+			if diff := cmp.Diff(tc.expectedOutput, output); diff != "" {
+				t.Fatalf("actual output differes from expected output: %s", diff)
+			}
+		})
+	}
+}