move executable.go into its own package,

- port over cnb library from node-engine rewrite
- update tests a bit to remove os.Args bug
[#169918669]

Co-authored-by: Daniel Thornton <[email protected]>

build.go

@@ -0,0 +1,134 @@
+package packit
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/BurntSushi/toml"
+	"github.com/cloudfoundry/packit/internal"
+)
+
+type BuildFunc func(BuildContext) (BuildResult, error)
+
+type BuildContext struct {
+	CNBPath    string
+	Stack      string
+	WorkingDir string
+	Plan       BuildpackPlan
+	Layers     Layers
+}
+
+type BuildResult struct {
+	Plan      BuildpackPlan
+	Layers    []Layer
+	Processes []Process
+}
+
+type Process struct {
+	Type    string   `toml:"type"`
+	Command string   `toml:"command"`
+	Args    []string `toml:"args"`
+	Direct  bool     `toml:"direct"`
+}
+
+type BuildpackPlanEntry struct {
+	Name     string                 `toml:"name"`
+	Version  string                 `toml:"version"`
+	Metadata map[string]interface{} `toml:"metadata"`
+}
+
+type BuildpackPlan struct {
+	Entries []BuildpackPlanEntry `toml:"entries"`
+}
+
+func Build(f BuildFunc, options ...Option) {
+	config := Config{
+		exitHandler: internal.NewExitHandler(),
+		args:        os.Args,
+		tomlWriter:  internal.NewTOMLWriter(),
+		envWriter:   internal.NewEnvironmentWriter(),
+	}
+
+	for _, option := range options {
+		config = option(config)
+	}
+
+	var (
+		layersPath = config.args[1]
+		planPath   = config.args[3]
+	)
+
+	pwd, err := os.Getwd()
+	if err != nil {
+		config.exitHandler.Error(err)
+		return
+	}
+
+	var plan BuildpackPlan
+	_, err = toml.DecodeFile(planPath, &plan)
+	if err != nil {
+		config.exitHandler.Error(err)
+		return
+	}
+
+	result, err := f(BuildContext{
+		CNBPath:    filepath.Clean(strings.TrimSuffix(config.args[0], filepath.Join("bin", "build"))),
+		Stack:      os.Getenv("CNB_STACK_ID"),
+		WorkingDir: pwd,
+		Plan:       plan,
+		Layers: Layers{
+			Path: layersPath,
+		},
+	})
+	if err != nil {
+		config.exitHandler.Error(err)
+		return
+	}
+
+	err = config.tomlWriter.Write(planPath, result.Plan)
+	if err != nil {
+		config.exitHandler.Error(err)
+		return
+	}
+
+	for _, layer := range result.Layers {
+		err = config.tomlWriter.Write(filepath.Join(layersPath, fmt.Sprintf("%s.toml", layer.Name)), layer)
+		if err != nil {
+			config.exitHandler.Error(err)
+			return
+		}
+
+		err = config.envWriter.Write(filepath.Join(layer.Path, "env"), layer.SharedEnv)
+		if err != nil {
+			config.exitHandler.Error(err)
+			return
+		}
+
+		err = config.envWriter.Write(filepath.Join(layer.Path, "env.launch"), layer.LaunchEnv)
+		if err != nil {
+			config.exitHandler.Error(err)
+			return
+		}
+
+		err = config.envWriter.Write(filepath.Join(layer.Path, "env.build"), layer.BuildEnv)
+		if err != nil {
+			config.exitHandler.Error(err)
+			return
+		}
+	}
+
+	if len(result.Processes) > 0 {
+		var launch struct {
+			Processes []Process `toml:"processes"`
+		}
+		launch.Processes = result.Processes
+
+		err = config.tomlWriter.Write(filepath.Join(layersPath, "launch.toml"), launch)
+		if err != nil {
+			config.exitHandler.Error(err)
+			return
+		}
+	}
+}