Fail to build when buildpack.yml is present (#285)

* fail detection when bp-yml is present

per [Go Build RFC 0003](https://github.com/paketo-buildpacks/rfcs/blob/d4fde0656a823330f78a77e69a81d2d6f7768daf/text/go/go-build/0003-buildpack-yml-to-env-var.md)

* add integration test asserting build failure

* use fs.Exists()

build_configuration_parser.go

@@ -7,6 +7,7 @@ import (
 	"strings"
 
 	"github.com/mattn/go-shellwords"
+	"github.com/paketo-buildpacks/packit/v2/fs"
 )
 
 //go:generate faux --interface TargetManager --output fakes/target_manager.go
@@ -32,13 +33,19 @@ func NewBuildConfigurationParser(targetManager TargetManager) BuildConfiguration
 }
 
 func (p BuildConfigurationParser) Parse(buildpackVersion, workingDir string) (BuildConfiguration, error) {
-	var buildConfiguration BuildConfiguration
+	bpYML, err := fs.Exists(filepath.Join(workingDir, "buildpack.yml"))
+	if err != nil {
+		return BuildConfiguration{}, fmt.Errorf("failed to check for buildpack.yml: %w", err)
+	}
+	if bpYML {
+		return BuildConfiguration{}, fmt.Errorf("working directory contains deprecated 'buildpack.yml'; use environment variables for configuration")
+	}
 
+	var buildConfiguration BuildConfiguration
 	if val, ok := os.LookupEnv("BP_GO_TARGETS"); ok {
 		buildConfiguration.Targets = filepath.SplitList(val)
 	}
 
-	var err error
 	if len(buildConfiguration.Targets) > 0 {
 		buildConfiguration.Targets, err = p.targetManager.CleanAndValidate(buildConfiguration.Targets, workingDir)
 		if err != nil {

build_configuration_parser_test.go

@@ -3,6 +3,7 @@ package gobuild_test
 import (
 	"errors"
 	"os"
+	"path/filepath"
 	"testing"
 
 	gobuild "github.com/paketo-buildpacks/go-build"
@@ -195,18 +196,36 @@ func testBuildConfigurationParser(t *testing.T, context spec.G, it spec.S) {
 	})
 
 	context("failure cases", func() {
+		context("when the working directory contains a buildpack.yml", func() {
+			it.Before(func() {
+				Expect(os.WriteFile(filepath.Join(workingDir, "buildpack.yml"), nil, os.ModePerm)).To(Succeed())
+			})
+			it("returns an error", func() {
+				_, err := parser.Parse("1.2.3", workingDir)
+				Expect(err).To(MatchError("working directory contains deprecated 'buildpack.yml'; use environment variables for configuration"))
+			})
+			context("and it's not readable", func() {
+				it.Before(func() {
+					Expect(os.Chmod(workingDir, 0000)).To(Succeed())
+				})
+
+				it.After(func() {
+					Expect(os.Chmod(workingDir, os.ModePerm)).To(Succeed())
+				})
+				it("returns the error from stat", func() {
+					_, err := parser.Parse("1.2.3", workingDir)
+					Expect(err).To(MatchError(ContainSubstring("permission denied")))
+				})
+			})
+		})
 		context("go targets fail to be cleaned an validated", func() {
 			it.Before(func() {
 				os.Setenv("BP_GO_TARGETS", "./some/target")
-
 				targetManager.CleanAndValidateCall.Returns.Error = errors.New("failed to clean and validate targets")
-
 			})
-
 			it.After(func() {
 				os.Unsetenv("BP_GO_TARGETS")
 			})
-
 			it("returns an error", func() {
 				_, err := parser.Parse("1.2.3", workingDir)
 				Expect(err).To(MatchError("failed to clean and validate targets"))

integration/build_failure_test.go

@@ -79,4 +79,45 @@ func testBuildFailure(t *testing.T, context spec.G, it spec.S) {
 			))
 		})
 	})
+
+	context("when building an app that has a buildpack.yml", func() {
+		var (
+			name   string
+			source string
+		)
+
+		it.Before(func() {
+			var err error
+			name, err = occam.RandomName()
+			Expect(err).NotTo(HaveOccurred())
+		})
+
+		it.After(func() {
+			Expect(docker.Volume.Remove.Execute(occam.CacheVolumeNames(name))).To(Succeed())
+			Expect(os.RemoveAll(source)).To(Succeed())
+		})
+
+		it("fails the build", func() {
+			var err error
+			source, err = occam.Source(filepath.Join("testdata", "default"))
+			Expect(err).NotTo(HaveOccurred())
+
+			Expect(os.WriteFile(filepath.Join(source, "buildpack.yml"), nil, os.ModePerm)).To(Succeed())
+
+			_, logs, err := pack.Build.
+				WithPullPolicy("never").
+				WithBuildpacks(
+					settings.Buildpacks.GoDist.Online,
+					settings.Buildpacks.GoBuild.Online,
+				).
+				Execute(name, source)
+			Expect(err).To(HaveOccurred(), logs.String)
+
+			Expect(logs).To(ContainSubstring("working directory contains deprecated 'buildpack.yml'; use environment variables for configuration"))
+			Expect(logs).NotTo(ContainLines(
+				MatchRegexp(fmt.Sprintf(`%s \d+\.\d+\.\d+`, settings.Buildpack.Name)),
+				"  Executing build process",
+			))
+		})
+	})
 }