Make buildpack API generic

Instead of using `interface{}` for things like metadata, we can use generic types. The buildpack API doesn't care what's in those values, so it can just be an `any` type. This will allow buildpack authors to set the types they want to use and even use custom types for their metadata. Then when accessing the metadata in their buildpacks, no casting is required. This reduces the amount of code and makes the code less error prone.

Signed-off-by: Daniel Mikusa <[email protected]>

build.go

@@ -22,6 +22,7 @@ import (
 	"os"
 	"path/filepath"
 	"reflect"
+	"slices"
 	"strings"
 
 	"github.com/BurntSushi/toml"
@@ -32,25 +33,25 @@ import (
 )
 
 // BuildContext contains the inputs to build.
-type BuildContext struct {
+type BuildContext[PL any, PM any, LM any, BM any] struct {
 	// ApplicationPath is the location of the application source code as provided by
 	// the lifecycle.
 	ApplicationPath string
 
 	// Buildpack is metadata about the buildpack, from buildpack.toml.
-	Buildpack Buildpack
+	Buildpack Buildpack[BM]
 
 	// Layers is the layers available to the buildpack.
-	Layers Layers
+	Layers Layers[LM]
 
 	// Logger is the way to write messages to the end user
 	Logger log.Logger
 
 	// PersistentMetadata is metadata that is persisted even across cache cleaning.
-	PersistentMetadata map[string]interface{}
+	PersistentMetadata map[string]PM
 
 	// Plan is the buildpack plan provided to the buildpack.
-	Plan BuildpackPlan
+	Plan BuildpackPlan[PL]
 
 	// Platform is the contents of the platform.
 	Platform Platform
@@ -66,15 +67,15 @@ type BuildContext struct {
 }
 
 // BuildResult contains the results of detection.
-type BuildResult struct {
+type BuildResult[PM any, LM any] struct {
 	// Labels are the image labels contributed by the buildpack.
 	Labels []Label
 
 	// Layers is the collection of LayerCreators contributed by the buildpack.
-	Layers []Layer
+	Layers []Layer[LM]
 
 	// PersistentMetadata is metadata that is persisted even across cache cleaning.
-	PersistentMetadata map[string]interface{}
+	PersistentMetadata map[string]PM
 
 	// Processes are the process types contributed by the buildpack.
 	Processes []Process
@@ -97,13 +98,13 @@ const (
 )
 
 // NewBuildResult creates a new BuildResult instance, initializing empty fields.
-func NewBuildResult() BuildResult {
-	return BuildResult{
-		PersistentMetadata: make(map[string]interface{}),
+func NewBuildResult[PM any, LM any]() BuildResult[PM, LM] {
+	return BuildResult[PM, LM]{
+		PersistentMetadata: make(map[string]PM),
 	}
 }
 
-func (b BuildResult) String() string {
+func (b BuildResult[PM, LM]) String() string {
 	var l []string
 	for _, c := range b.Layers {
 		l = append(l, reflect.TypeOf(c).Name())
@@ -116,16 +117,20 @@ func (b BuildResult) String() string {
 }
 
 // BuildFunc takes a context and returns a result, performing buildpack build behaviors.
-type BuildFunc func(context BuildContext) (BuildResult, error)
+type BuildFunc[PL any, PM any, LM any, BM any] func(context BuildContext[PL, PM, LM, BM]) (BuildResult[PM, LM], error)
+
+func EmptyBuildFunc[BPL any, PM any, LM any, BM any](context BuildContext[BPL, PM, LM, BM]) (BuildResult[PM, LM], error) {
+	return BuildResult[PM, LM]{}, nil
+}
 
 // Build is called by the main function of a buildpack, for build.
-func Build(build BuildFunc, config Config) {
+func Build[PL any, PM any, LM any, BM any](build BuildFunc[PL, PM, LM, BM], config Config) {
 	var (
 		err  error
 		file string
 		ok   bool
 	)
-	ctx := BuildContext{Logger: config.logger}
+	ctx := BuildContext[PL, PM, LM, BM]{Logger: config.logger}
 
 	ctx.ApplicationPath, err = os.Getwd()
 	if err != nil {
@@ -181,7 +186,7 @@ func Build(build BuildFunc, config Config) {
 		config.exitHandler.Error(fmt.Errorf("expected CNB_LAYERS_DIR to be set"))
 		return
 	}
-	ctx.Layers = Layers{layersDir}
+	ctx.Layers = Layers[LM]{layersDir}
 
 	ctx.Platform.Path, ok = os.LookupEnv(EnvPlatformDirectory)
 	if !ok {
@@ -216,7 +221,7 @@ func Build(build BuildFunc, config Config) {
 	}
 	config.logger.Debugf("Platform Environment: %s", ctx.Platform.Environment)
 
-	var store Store
+	var store Store[PM]
 	file = filepath.Join(ctx.Layers.Path, "store.toml")
 	if _, err = toml.DecodeFile(file, &store); err != nil && !os.IsNotExist(err) {
 		config.exitHandler.Error(fmt.Errorf("unable to decode persistent metadata %s\n%w", file, err))
@@ -297,7 +302,7 @@ func Build(build BuildFunc, config Config) {
 	}
 
 	for _, e := range existing {
-		if strings.HasSuffix(e, "store.toml") || contains(contributed, e) {
+		if strings.HasSuffix(e, "store.toml") || slices.Contains(contributed, e) {
 			continue
 		}
 
@@ -345,7 +350,7 @@ func Build(build BuildFunc, config Config) {
 	}
 
 	if len(result.PersistentMetadata) > 0 {
-		store = Store{
+		store = Store[PM]{
 			Metadata: result.PersistentMetadata,
 		}
 		file = filepath.Join(ctx.Layers.Path, "store.toml")
@@ -357,16 +362,6 @@ func Build(build BuildFunc, config Config) {
 	}
 }
 
-func contains(candidates []string, s string) bool {
-	for _, c := range candidates {
-		if s == c {
-			return true
-		}
-	}
-
-	return false
-}
-
 func validateSBOMFormats(layersPath string, acceptedSBOMFormats []string) error {
 	sbomFiles, err := filepath.Glob(filepath.Join(layersPath, "*.sbom.*"))
 	if err != nil {
@@ -383,7 +378,7 @@ func validateSBOMFormats(layersPath string, acceptedSBOMFormats []string) error
 			return fmt.Errorf("unable to parse SBOM %s\n%w", sbomFormat, err)
 		}
 
-		if !contains(acceptedSBOMFormats, sbomFormat.MediaType()) {
+		if !slices.Contains(acceptedSBOMFormats, sbomFormat.MediaType()) {
 			return fmt.Errorf("unable to find actual SBOM Type %s in list of supported SBOM types %s", sbomFormat.MediaType(), acceptedSBOMFormats)
 		}
 	}

build_plan.go

@@ -23,28 +23,28 @@ type BuildPlanProvide struct {
 }
 
 // BuildPlanRequire represents a dependency required by a buildpack.
-type BuildPlanRequire struct {
+type BuildPlanRequire[PL any] struct {
 	// Name is the name of the dependency.
 	Name string `toml:"name"`
 
 	// Metadata is the metadata for the dependency. Optional.
-	Metadata map[string]interface{} `toml:"metadata,omitempty"`
+	Metadata map[string]PL `toml:"metadata,omitempty"`
 }
 
 // BuildPlan represents the provisions and requirements of a buildpack during detection.
-type BuildPlan struct {
+type BuildPlan[PL any] struct {
 	// Provides is the dependencies provided by the buildpack.
 	Provides []BuildPlanProvide `toml:"provides,omitempty"`
 
 	// Requires is the dependencies required by the buildpack.
-	Requires []BuildPlanRequire `toml:"requires,omitempty"`
+	Requires []BuildPlanRequire[PL] `toml:"requires,omitempty"`
 }
 
 // BuildPlans represents a collection of build plans produced by a buildpack during detection.
-type BuildPlans struct {
+type BuildPlans[PL any] struct {
 	// BuildPlan is the first build plan.
-	BuildPlan
+	BuildPlan[PL]
 
 	// Or is the collection of other build plans.
-	Or []BuildPlan `toml:"or,omitempty"`
+	Or []BuildPlan[PL] `toml:"or,omitempty"`
 }