Fix check resource to allow empty results in check

Signed-off-by: Benjamin Isinger <Benjamin.Isinger@ibm.com>

internal/dtr/check.go

@@ -21,7 +21,6 @@
 package dtr
 
 import (
-	"fmt"
 	"io"
 )
 
@@ -42,7 +41,8 @@ func Check(in io.Reader) (CheckResult, error) {
 	}
 
 	if len(result) == 0 {
-		return nil, fmt.Errorf("run command returned no output")
+		// Only return the incoming version to signal concourse that there is nothing new
+		result = append(result, config.Version)
 	}
 
 	return result, nil

internal/dtr/check_test.go

@@ -29,7 +29,7 @@ import (
 
 var _ = Describe("Check", func() {
 	Context("invalid configuration", func() {
-		It("should fail if not run command is configured", func() {
+		It("should fail if no run command is configured", func() {
 			_, err := Check(feed(Config{}))
 			Expect(err).To(HaveOccurred())
 		})
@@ -46,14 +46,17 @@ var _ = Describe("Check", func() {
 			Expect(err).To(HaveOccurred())
 		})
 
-		It("should fail when no line is return by run", func() {
-			_, err := Check(feed(Config{
+		It("should return provided version when no line is return by run", func() {
+			versionIn := Version{"ref": "barfoo"}
+			result, err := Check(feed(Config{
 				Source: Source{
 					Check: Custom{Run: "true"},
 				},
+				Version: versionIn,
 			}))
 
-			Expect(err).To(HaveOccurred())
+			Expect(err).ToNot(HaveOccurred())
+			Expect(result).To(Equal(CheckResult{versionIn}))
 		})
 
 		It("should return two versions when two lines are returned", func() {

internal/dtr/common.go

@@ -104,9 +104,15 @@ func execute(entry Custom) ([]string, error) {
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 
-	before := command(ctx, entry.Env, entry.Before, os.Stderr)
-	if err := before.Run(); err != nil {
-		return nil, fmt.Errorf("failure while running before command: %w", err)
+	if entry.Before != "" {
+		before := command(ctx, entry.Env, entry.Before, os.Stderr)
+		if err := before.Run(); err != nil {
+			return nil, fmt.Errorf("failure while running before command: %w", err)
+		}
+	}
+
+	if entry.Run == "" {
+		return nil, fmt.Errorf("run command not specified. Bailing out")
 	}
 
 	var outStream bytes.Buffer