Style and error handling fixes

* Better error messages
* Handling missing mappings file
* Don't export entire file schema

Signed-off-by: Emily Casey <[email protected]>

dependency_cache.go

@@ -49,30 +49,28 @@ type DependencyCache struct {
 	// UserAgent is the User-Agent string to use with requests.
 	UserAgent string
 
-	// DependencyMap optionally provides override URIs for BuildpackDependencies
+	// Mappings optionally provides URIs mapping for BuildpackDependencies
 	Mappings []DependencyMapping
 }
 
 // NewDependencyCache creates a new instance setting the default cache path (<BUILDPACK_PATH>/dependencies) and user
 // agent (<BUILDPACK_ID>/<BUILDPACK_VERSION>).
 // Mappings will be read from <PLATFORM_DIR>/dependencies/mappings.toml
 func NewDependencyCache(context libcnb.BuildContext) (DependencyCache, error) {
-	mappingsFile, err := ReadMappingsFile(DefaultMappingsFilePath(context.Platform.Path))
-	if err != nil {
-		return DependencyCache{}, err
-	}
-	var mappings []DependencyMapping
-	for _, bpm := range mappingsFile.BuildpackMappings {
-		if bpm.BuildpackID == context.Buildpack.Info.ID {
-			mappings = bpm.Mappings
-		}
-	}
-	return DependencyCache{
+	cache := DependencyCache{
 		CachePath:    filepath.Join(context.Buildpack.Path, "dependencies"),
 		DownloadPath: os.TempDir(),
 		UserAgent:    fmt.Sprintf("%s/%s", context.Buildpack.Info.ID, context.Buildpack.Info.Version),
-		Mappings:     mappings,
-	}, nil
+	}
+	mappings, err := ReadMappingsForBuildpack(
+		DefaultMappingsFilePath(context.Platform.Path),
+		context.Buildpack.Info.ID,
+	)
+	if err != nil {
+		return DependencyCache{}, fmt.Errorf("unable to read dependency mappings file\n%w", err)
+	}
+	cache.Mappings = mappings
+	return cache, nil
 }
 
 // RequestModifierFunc is a callback that enables modification of a download request before it is sent.  It is often

dependency_cache_test.go

@@ -37,56 +37,62 @@ import (
 func testDependencyCache(t *testing.T, context spec.G, it spec.S) {
 	var (
 		Expect = NewWithT(t).Expect
-
-		dependencyCache libpak.DependencyCache
 	)
 
 	context("NewDependencyCache", func() {
+		var ctx libcnb.BuildContext
+
 		it.Before(func() {
-			var err error
-			dependencyCache, err = libpak.NewDependencyCache(libcnb.BuildContext{
+			ctx = libcnb.BuildContext{
 				Buildpack: libcnb.Buildpack{
 					Info: libcnb.BuildpackInfo{
 						ID:      "some-buildpack-id",
 						Version: "some-buildpack-version",
 					},
 					Path: "some/path",
 				},
-				Platform: libcnb.Platform{
-					Path: filepath.Join("testdata", "platform"),
-				},
-			})
-			Expect(err).NotTo(HaveOccurred())
+			}
 		})
 
-		it("defaults CachePath to <BUILDPACK_PATH>/dependencies", func() {
+		it("set default CachePath and UserAgent", func() {
+			dependencyCache, err := libpak.NewDependencyCache(ctx)
+			Expect(err).NotTo(HaveOccurred())
 			Expect(dependencyCache.CachePath).To(Equal(filepath.Join("some/path/dependencies")))
-		})
-		it("sets the user agent to <BUILDPACK_ID>/<BUILDPACK_VERSION>", func() {
 			Expect(dependencyCache.UserAgent).To(Equal("some-buildpack-id/some-buildpack-version"))
+			Expect(dependencyCache.Mappings).To(BeEmpty())
 		})
-		it("reads mappings for buildpack from the platform dir", func() {
-			Expect(dependencyCache.Mappings).To(ConsistOf(
-				libpak.DependencyMapping{
-					ID:      "some-dependency-id",
-					Version: "some-dependency-version",
-					URI:     "some-uri",
-				},
-				libpak.DependencyMapping{
-					ID:      "other-dependency-id",
-					Version: "other-dependency-version",
-					URI:     "other-uri",
-				},
-			))
+
+		context("mappings file exists in the platform dir", func() {
+			it.Before(func() {
+				ctx.Platform.Path = filepath.Join("testdata", "platform")
+			})
+
+			it("reads mappings for buildpack", func() {
+				dependencyCache, err := libpak.NewDependencyCache(ctx)
+				Expect(err).NotTo(HaveOccurred())
+				Expect(dependencyCache.Mappings).To(ConsistOf(
+					libpak.DependencyMapping{
+						ID:      "some-dependency-id",
+						Version: "some-dependency-version",
+						URI:     "some-uri",
+					},
+					libpak.DependencyMapping{
+						ID:      "other-dependency-id",
+						Version: "other-dependency-version",
+						URI:     "other-uri",
+					},
+				))
+			})
 		})
 	})
 
 	context("artifacts", func() {
 		var (
-			cachePath    string
-			downloadPath string
-			dependency   libpak.BuildpackDependency
-			server       *ghttp.Server
+			cachePath       string
+			downloadPath    string
+			dependency      libpak.BuildpackDependency
+			dependencyCache libpak.DependencyCache
+			server          *ghttp.Server
 		)
 
 		it.Before(func() {

dependency_mapping.go

@@ -17,38 +17,43 @@
 package libpak
 
 import (
+	"fmt"
+	"os"
 	"path/filepath"
 
 	"github.com/BurntSushi/toml"
 )
 
-// MappingsFile defines dependency mappings for a set of buildpacks
-type MappingsFile struct {
-	BuildpackMappings []BuildpackMappings `toml:"buildpacks"`
-}
-
-type BuildpackMappings struct {
-	BuildpackID string              `toml:"id"`
-	Mappings    []DependencyMapping `toml:"mappings"`
-}
-
 // DependencyMapping defines a new URI for a dependency with the given ID and Version
 type DependencyMapping struct {
 	ID      string `toml:"id"`
 	Version string `toml:"version"`
 	URI     string `toml:"uri"`
 }
 
-// ReadMappingsFile read MappingsFile from path
-func ReadMappingsFile(path string) (MappingsFile, error) {
-	mappingsFile := MappingsFile{}
-	_, err := toml.DecodeFile(path, &mappingsFile)
-	if err != nil {
-		return MappingsFile{}, err
+// ReadMappingsForBuildpack reads the mappings for the buildpack with ID buildpackID from the file at path
+func ReadMappingsForBuildpack(path string, buildpackID string) ([]DependencyMapping, error) {
+	mappingsFile := struct {
+		Buildpacks []struct {
+			ID       string              `toml:"id"`
+			Mappings []DependencyMapping `toml:"mappings"`
+		} `toml:"buildpacks"`
+	}{}
+	if _, err := toml.DecodeFile(path, &mappingsFile); err != nil {
+		if os.IsNotExist(err) {
+			return nil, nil
+		}
+		return nil, fmt.Errorf("unable to decode dependency mappings file%s\n%w", path, err)
+	}
+	for _, bps := range mappingsFile.Buildpacks {
+		if bps.ID == buildpackID {
+			return bps.Mappings, nil
+		}
 	}
-	return mappingsFile, nil
+	return nil, nil
 }
 
+// DefaultMappingsFilePath returns default path for mappings file
 func DefaultMappingsFilePath(platformDir string) string {
 	return filepath.Join(platformDir, "dependencies", "mappings.toml")
 }

testdata/platform/dependencies/mappings.toml

@@ -1,16 +1,19 @@
 [[buildpacks]]
   id = "some-buildpack-id"
+
   [[buildpacks.mappings]]
    id = "some-dependency-id"
    version = "some-dependency-version"
    uri = "some-uri"
+
   [[buildpacks.mappings]]
    id = "other-dependency-id"
    version = "other-dependency-version"
    uri = "other-uri"
 
 [[buildpacks]]
   id = "exclude-buildpack-id"
+
   [[buildpacks.mappings]]
    id = "exclude-dependency-id"
    version = "exclude-dependency-version"