Add output variable support (#77)

* add digest as output variable for kaniko plugin

.drone.yml

@@ -130,12 +130,11 @@ steps:
       - pull_request
 ---
 kind: pipeline
-type: docker
+type: vm
 name: arm
 
-platform:
-  os: linux
-  arch: arm64
+pool:
+  use: ubuntu_arm64
 
 steps:
 - name: build

cmd/kaniko-docker/main.go

@@ -191,6 +191,11 @@ func main() {
 			Usage:  "build only used stages",
 			EnvVar: "PLUGIN_SKIP_UNUSED_STAGES",
 		},
+		cli.StringFlag{
+			Name:   "output-file",
+			Usage:  "Output file location that will be generated by the plugin. This file will include information of the output that are exported by the plugin.",
+			EnvVar: "DRONE_OUTPUT",
+		},
 	}
 
 	if err := app.Run(os.Args); err != nil {
@@ -248,6 +253,9 @@ func run(c *cli.Context) error {
 			ArtifactFile: c.String("artifact-file"),
 			RegistryType: artifact.Docker,
 		},
+		Output: kaniko.Output{
+			OutputFile: c.String("output-file"),
+		},
 	}
 	return plugin.Exec()
 }

kaniko.go

@@ -8,6 +8,7 @@ import (
 	"strings"
 
 	"github.com/drone/drone-kaniko/pkg/artifact"
+	"github.com/drone/drone-kaniko/pkg/output"
 	"github.com/drone/drone-kaniko/pkg/tagger"
 	"golang.org/x/mod/semver"
 )
@@ -49,10 +50,16 @@ type (
 		ArtifactFile string                    // Artifact file location
 	}
 
+	// Output defines content of output file
+	Output struct {
+		OutputFile string // File where plugin output are saved
+	}
+
 	// Plugin defines the Docker plugin parameters.
 	Plugin struct {
 		Build    Build    // Docker build configuration
 		Artifact Artifact // Artifact file content
+		Output   Output   // Output file content
 	}
 )
 
@@ -223,19 +230,29 @@ func (p Plugin) Exec() error {
 	}
 
 	if p.Build.DigestFile != "" && p.Artifact.ArtifactFile != "" {
-		content, err := ioutil.ReadFile(p.Build.DigestFile)
-		if err != nil {
-			fmt.Fprintf(os.Stderr, "failed to read digest file contents at path: %s with error: %s\n", p.Build.DigestFile, err)
-		}
-		err = artifact.WritePluginArtifactFile(p.Artifact.RegistryType, p.Artifact.ArtifactFile, p.Artifact.Registry, p.Artifact.Repo, string(content), p.Artifact.Tags)
+		err = artifact.WritePluginArtifactFile(p.Artifact.RegistryType, p.Artifact.ArtifactFile, p.Artifact.Registry, p.Artifact.Repo, getDigest(p.Build.DigestFile), p.Artifact.Tags)
 		if err != nil {
 			fmt.Fprintf(os.Stderr, "failed to write plugin artifact file at path: %s with error: %s\n", p.Artifact.ArtifactFile, err)
 		}
 	}
 
+	if p.Output.OutputFile != "" {
+		if err = output.WritePluginOutputFile(p.Output.OutputFile, getDigest(p.Build.DigestFile)); err != nil {
+			fmt.Fprintf(os.Stderr, "failed to write plugin output file at path: %s with error: %s\n", p.Output.OutputFile, err)
+		}
+	}
+
 	return nil
 }
 
+func getDigest(digestFile string) string {
+	content, err := ioutil.ReadFile(digestFile)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "failed to read digest file contents at path: %s with error: %s\n", digestFile, err)
+	}
+	return string(content)
+}
+
 // trace writes each command to stdout with the command wrapped in an xml
 // tag so that it can be extracted and displayed in the logs.
 func trace(cmd *exec.Cmd) {

pkg/output/output.go

@@ -0,0 +1,12 @@
+package output
+
+import (
+	"github.com/joho/godotenv"
+)
+
+func WritePluginOutputFile(outputFilePath, digest string) error {
+	output := map[string]string{
+		"digest": digest,
+	}
+	return godotenv.Write(output, outputFilePath)
+}