metadata: Improve logging

Implementing a String() interface for each provider makes it
easier for users to prefix log strings with the provider.

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

pkg/metadata/main.go

@@ -29,6 +29,9 @@ const (
 
 // Provider is a generic interface for metadata/userdata providers.
 type Provider interface {
+	// String should return a unique name for the Provider
+	String() string
+
 	// Probe returns true if the provider was detected.
 	Probe() bool
 
@@ -58,6 +61,7 @@ func main() {
 	found := false
 	for _, p := range netProviders {
 		if p.Probe() {
+			log.Printf("%s: Probe succeeded", p)
 			userdata, err = p.Extract()
 			found = true
 			break
@@ -78,6 +82,7 @@ func main() {
 
 		for _, p := range cdromProviders {
 			if p.Probe() {
+				log.Printf("%s: Probe succeeded", p)
 				userdata, err = p.Extract()
 				found = true
 				break

pkg/metadata/provider_cdrom.go

@@ -3,7 +3,6 @@ package main
 import (
 	"fmt"
 	"io/ioutil"
-	"log"
 	"os"
 	"path"
 )
@@ -22,12 +21,13 @@ func NewCDROM() *ProviderCDROM {
 	return &ProviderCDROM{}
 }
 
+func (p *ProviderCDROM) String() string {
+	return "CDROM"
+}
+
 // Probe checks if the CD has the right file
 func (p *ProviderCDROM) Probe() bool {
 	_, err := os.Stat(path.Join(MountPoint, configFile))
-	if err != nil {
-		log.Printf("CDROM: Probe -> %s", err)
-	}
 	return (!os.IsNotExist(err))
 }
 

pkg/metadata/provider_gcp.go

@@ -24,11 +24,14 @@ func NewGCP() *ProviderGCP {
 	return &ProviderGCP{}
 }
 
+func (p *ProviderGCP) String() string {
+	return "GCP"
+}
+
 // Probe checks if we are running on GCP
 func (p *ProviderGCP) Probe() bool {
 	// Getting the hostname should always work...
 	_, err := gcpGet(instance + "hostname")
-	log.Printf("GCP: Probe -> %s", err)
 	return (err == nil)
 }