Rewrite per RFC 0001 (#60)

Signed-off-by: Arjun Sreedharan <[email protected]>

README.md

@@ -1,13 +1,14 @@
 # Dep Cloud Native Buildpack
 
-The Go Dep CNB builds a Go application binary, using the
-[`dep`](https://golang.github.io/dep/docs/introduction.html) tool to package
-dependencies.
+The Dep CNB provides the
+[`dep`](https://golang.github.io/dep/docs/introduction.html) executable. The
+buildpack installs dep onto the `$PATH` which makes it available for subsequent
+buildpacks and/or the final container image.
 
 ## Integration
 
-The Dep CNB provides dep as a dependency. Downstream
-buildpacks can require the node dependency by generating a [Build Plan
+The Dep CNB provides `dep` as a dependency. Downstream
+buildpacks can require the dep dependency by generating a [Build Plan
 TOML](https://github.com/buildpacks/spec/blob/master/buildpack.md#build-plan-toml)
 file that looks like the following:
 
@@ -26,10 +27,16 @@ file that looks like the following:
   [requires.metadata]
 
     # Setting the build flag to true will ensure that the Dep
-    # depdendency is available on the $PATH for subsequent buildpacks during
+    # dependency is available on the $PATH for subsequent buildpacks during
     # their build phase. If you are writing a buildpack that needs to run Dep
     # during its build process, this flag should be set to true.
     build = true
+
+    # Setting the launch flag to true will ensure that the Dep
+    # dependency is available on the $PATH for the running application. If you are
+    # writing an application that needs to run Dep at runtime, this flag should
+    # be set to true.
+    launch = true
 ```
 
 ## Usage
@@ -42,20 +49,4 @@ This builds the buildpack's Go source using GOOS=linux by default. You can suppl
 
 ## `buildpack.yml` Configuration
 
-The `dep` requires a `buildpack.yml` file in the root of the application directory, and must contain the `import-path` directive.
-
-```yaml
-go:
-  # this sets the go import-path (required)
-  import-path: hubgit.net/user/app
-
-  # this allows you to override the location of the main package of the app
-  targets: ["./cmd/web"]
-
-  # this allows you to set Go ldflags for compilation
-  ldflags:
-    main.version: v1.2.3
-    main.sha: 1234567
-```
-
-See `integration/testdata/` subfolders for examples.
+The dep buildpack does not support configurations via `buildpack.yml`.

build.go

@@ -0,0 +1,105 @@
+package dep
+
+import (
+	"path/filepath"
+	"time"
+
+	"github.com/paketo-buildpacks/packit"
+	"github.com/paketo-buildpacks/packit/chronos"
+	"github.com/paketo-buildpacks/packit/postal"
+)
+
+//go:generate faux --interface EntryResolver --output fakes/entry_resolver.go
+type EntryResolver interface {
+	Resolve([]packit.BuildpackPlanEntry) packit.BuildpackPlanEntry
+}
+
+//go:generate faux --interface DependencyManager --output fakes/dependency_manager.go
+type DependencyManager interface {
+	Resolve(path, id, version, stack string) (postal.Dependency, error)
+	Install(dependency postal.Dependency, cnbPath, layerPath string) error
+}
+
+//go:generate faux --interface BuildPlanRefinery --output fakes/build_plan_refinery.go
+type BuildPlanRefinery interface {
+	BillOfMaterials(postal.Dependency) packit.BuildpackPlanEntry
+}
+
+func Build(
+	entries EntryResolver,
+	dependencies DependencyManager,
+	planRefinery BuildPlanRefinery,
+	clock chronos.Clock,
+	logger LogEmitter,
+) packit.BuildFunc {
+	return func(context packit.BuildContext) (packit.BuildResult, error) {
+		logger.Title("%s %s", context.BuildpackInfo.Name, context.BuildpackInfo.Version)
+
+		entry := entries.Resolve(context.Plan.Entries)
+
+		dependency, err := dependencies.Resolve(
+			filepath.Join(context.CNBPath, "buildpack.toml"),
+			entry.Name,
+			entry.Version,
+			context.Stack)
+		if err != nil {
+			return packit.BuildResult{}, err
+		}
+
+		bom := planRefinery.BillOfMaterials(dependency)
+
+		depLayer, err := context.Layers.Get(Dep)
+		if err != nil {
+			return packit.BuildResult{}, err
+		}
+
+		depLayer.Launch = entry.Metadata["launch"] == true
+		depLayer.Build = entry.Metadata["build"] == true
+		depLayer.Cache = entry.Metadata["build"] == true
+
+		cachedSHA, ok := depLayer.Metadata[DependencyCacheKey].(string)
+		if ok && cachedSHA == dependency.SHA256 {
+			logger.Process("Reusing cached layer %s", depLayer.Path)
+			logger.Break()
+
+			return packit.BuildResult{
+				Plan: packit.BuildpackPlan{
+					Entries: []packit.BuildpackPlanEntry{bom},
+				},
+				Layers: []packit.Layer{depLayer},
+			}, nil
+		}
+
+		logger.Process("Executing build process")
+
+		err = depLayer.Reset()
+		if err != nil {
+			return packit.BuildResult{}, err
+		}
+
+		logger.Subprocess("Installing Dep")
+
+		duration, err := clock.Measure(func() error {
+			return dependencies.Install(dependency, context.CNBPath, depLayer.Path)
+		})
+		if err != nil {
+			return packit.BuildResult{}, err
+		}
+
+		logger.Action("Completed in %s", duration.Round(time.Millisecond))
+
+		depLayer.Metadata = map[string]interface{}{
+			DependencyCacheKey: dependency.SHA256,
+			"built_at":         clock.Now().Format(time.RFC3339Nano),
+		}
+
+		return packit.BuildResult{
+			Plan: packit.BuildpackPlan{
+				Entries: []packit.BuildpackPlanEntry{bom},
+			},
+			Layers: []packit.Layer{
+				depLayer,
+			},
+		}, nil
+	}
+}