Merge pull request #47 from paketo-buildpacks/additional-help-msg

Enable ArtifactResolver to provide additional help instructions if it cannot find an artifact

resolvers.go

@@ -111,6 +111,9 @@ type ArtifactResolver struct {
 
 	// InterestingFileDetector is used to determine if a file is a candidate for artifact resolution.
 	InterestingFileDetector InterestingFileDetector
+
+	// AdditionalHelpMessage can be used to supply context specific instructions if no matching artifact is found
+	AdditionalHelpMessage string
 }
 
 // Pattern returns the glob that ArtifactResolver will use for resolution.
@@ -153,7 +156,11 @@ func (a *ArtifactResolver) Resolve(applicationPath string) (string, error) {
 	}
 
 	sort.Strings(artifacts)
-	return "", fmt.Errorf("unable to find single built artifact in %s, candidates: %s", pattern, candidates)
+	helpMsg := fmt.Sprintf("unable to find single built artifact in %s, candidates: %s", pattern, candidates)
+	if len(a.AdditionalHelpMessage) > 0 {
+		helpMsg = fmt.Sprintf("%s. %s", helpMsg, a.AdditionalHelpMessage)
+	}
+	return "", fmt.Errorf(helpMsg)
 }
 
 // ResolveArguments resolves the arguments that should be passed to a build system.

resolvers_test.go

@@ -123,6 +123,19 @@ func testResolvers(t *testing.T, context spec.G, it spec.S) {
 				filepath.Join(path, "test-file-1"), filepath.Join(path, "test-file-2"))))
 		})
 
+		it("fails with multiple candidates and provides extra help", func() {
+			Expect(ioutil.WriteFile(filepath.Join(path, "test-file-1"), []byte{}, 0644)).To(Succeed())
+			Expect(ioutil.WriteFile(filepath.Join(path, "test-file-2"), []byte{}, 0644)).To(Succeed())
+			detector.On("Interesting", mock.Anything).Return(true, nil)
+
+			resolver.AdditionalHelpMessage = "Some more help."
+
+			_, err := resolver.Resolve(path)
+
+			Expect(err).To(MatchError(fmt.Sprintf("unable to find single built artifact in test-*, candidates: [%s %s]. Some more help.",
+				filepath.Join(path, "test-file-1"), filepath.Join(path, "test-file-2"))))
+		})
+
 		context("$TEST_ARTIFACT_CONFIGURATION_KEY", func() {
 			it.Before(func() {
 				Expect(os.Setenv("TEST_ARTIFACT_CONFIGURATION_KEY", "another-file")).To(Succeed())