Add a warning if git used as an output 🚨

In tektoncd#1109 we will be removing support for git as an output. In the
current implementation, git as an output is just a volume that holds the
data from the git repo, and copies it between Tasks (when git as an
output is linked to git as an input). As discussed in tektoncd#1076, the model
we want for PipelineResources is for them to take the outside world and
represent it on disk when used as an input, and when used as an output,
to update the outside world. In order to do this, what we actually want for a
git output is for it to create a commit the repo it is referencing.
However up until this point folks have been using git resources in the
way that we want Volume Resources to behave tektoncd#1062, so we want to
transition folks to Volume Resources and away from using git outputs.

Fixes tektoncd#1283

docs/resources.md

@@ -217,6 +217,8 @@ the source code to be built by the pipeline. Adding the git resource as an input
 to a Task will clone this repository and allow the Task to perform the required
 actions on the contents of the repo.
 
+_Git resources as outputs will soon not be supported, see https://github.com/tektoncd/pipeline/pull/1109._
+
 To create a git resource using the `PipelineResource` CRD:
 
 ```yaml

pkg/reconciler/taskrun/resources/output_resource.go

@@ -81,6 +81,10 @@ func AddOutputResources(
 			return nil, xerrors.Errorf("failed to get output pipeline Resource for task %q resource %v", taskName, boundResource)
 		}
 
+		if resource.GetType() == v1alpha1.PipelineResourceTypeGit {
+			logger.Warn("The Task %s uses a git output: in the next release, support for git outputs will be removed (See #1109)")
+		}
+
 		var sourcePath string
 		if output.TargetPath == "" {
 			sourcePath = filepath.Join(outputDir, boundResource.Name)

pkg/reconciler/taskrun/resources/output_resource_test.go

@@ -19,10 +19,13 @@ package resources
 import (
 	"testing"
 
+	"go.uber.org/zap"
+
 	"github.com/google/go-cmp/cmp"
 	"github.com/tektoncd/pipeline/pkg/apis/pipeline/v1alpha1"
 	"github.com/tektoncd/pipeline/pkg/logging"
 	"github.com/tektoncd/pipeline/test/names"
+	"go.uber.org/zap/zaptest/observer"
 	corev1 "k8s.io/api/core/v1"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 	fakek8s "k8s.io/client-go/kubernetes/fake"
@@ -777,6 +780,57 @@ func TestValidOutputResources(t *testing.T) {
 	}
 }
 
+// TestGitOutputWarns ensures we warn if git is used as an output.
+// In #1109 we will remove support for the git resource as an output, until we add
+// a different version via #1280 where a git output would create a new commit
+func TestGitOutputWarns(t *testing.T) {
+	tr := &v1alpha1.TaskRun{
+		ObjectMeta: metav1.ObjectMeta{
+			Name: "git-output-run",
+		},
+		Spec: v1alpha1.TaskRunSpec{
+			Outputs: v1alpha1.TaskRunOutputs{
+				Resources: []v1alpha1.TaskResourceBinding{{
+					Name: "some-repo",
+					ResourceRef: v1alpha1.PipelineResourceRef{
+						Name: "source-git",
+					},
+				}},
+			},
+		},
+	}
+	task := &v1alpha1.Task{
+		ObjectMeta: metav1.ObjectMeta{
+			Name: "task",
+		},
+		Spec: v1alpha1.TaskSpec{
+			Outputs: &v1alpha1.Outputs{
+				Resources: []v1alpha1.TaskResource{{
+					ResourceDeclaration: v1alpha1.ResourceDeclaration{
+						Name: "some-repo",
+						Type: "git",
+					}}},
+			},
+		},
+	}
+	r := map[string]v1alpha1.PipelineResourceInterface{
+		"some-repo": &v1alpha1.GitResource{
+			Name: "source-git",
+		},
+	}
+
+	outputResourceSetup(t)
+	o, ol := observer.New(zap.WarnLevel)
+	z := zap.New(o).Sugar()
+	_, err := AddOutputResources(fakek8s.NewSimpleClientset(), task.Name, &task.Spec, tr, r, z)
+	if err != nil {
+		t.Fatalf("Did not expect error but got %v", err)
+	}
+	if len(ol.All()) != 1 {
+		t.Errorf("Expected to see a warning logged for a git output but saw none")
+	}
+}
+
 func TestValidOutputResourcesWithBucketStorage(t *testing.T) {
 	for _, c := range []struct {
 		name      string