datasources: Add support for specifying multiple datasources

If multiple sources are specified, the first available source is used.

datasource/configdrive.go

@@ -14,6 +14,15 @@ func NewConfigDrive(root string) *configDrive {
 	return &configDrive{path.Join(root, "openstack")}
 }
 
+func (cd *configDrive) IsAvailable() bool {
+	_, err := os.Stat(cd.root)
+	return !os.IsNotExist(err)
+}
+
+func (cd *configDrive) AvailabilityChanges() bool {
+	return true
+}
+
 func (cd *configDrive) ConfigRoot() string {
 	return cd.root
 }

datasource/datasource.go

@@ -1,6 +1,8 @@
 package datasource
 
 type Datasource interface {
+	IsAvailable() bool
+	AvailabilityChanges() bool
 	ConfigRoot() string
 	FetchMetadata() ([]byte, error)
 	FetchUserdata() ([]byte, error)

datasource/file.go

@@ -2,6 +2,7 @@ package datasource
 
 import (
 	"io/ioutil"
+	"os"
 )
 
 type localFile struct {
@@ -12,6 +13,15 @@ func NewLocalFile(path string) *localFile {
 	return &localFile{path}
 }
 
+func (f *localFile) IsAvailable() bool {
+	_, err := os.Stat(f.path)
+	return !os.IsNotExist(err)
+}
+
+func (f *localFile) AvailabilityChanges() bool {
+	return true
+}
+
 func (f *localFile) ConfigRoot() string {
 	return ""
 }

datasource/metadata_service.go

@@ -39,6 +39,16 @@ func NewMetadataService() *metadataService {
 	return &metadataService{}
 }
 
+func (ms *metadataService) IsAvailable() bool {
+	client := pkg.NewHttpClient()
+	_, err := client.Get(BaseUrl)
+	return (err == nil)
+}
+
+func (ms *metadataService) AvailabilityChanges() bool {
+	return true
+}
+
 func (ms *metadataService) ConfigRoot() string {
 	return ""
 }

datasource/proc_cmdline.go

@@ -22,6 +22,21 @@ func NewProcCmdline() *procCmdline {
 	return &procCmdline{Location: ProcCmdlineLocation}
 }
 
+func (c *procCmdline) IsAvailable() bool {
+	contents, err := ioutil.ReadFile(c.Location)
+	if err != nil {
+		return false
+	}
+
+	cmdline := strings.TrimSpace(string(contents))
+	_, err = findCloudConfigURL(cmdline)
+	return (err == nil)
+}
+
+func (c *procCmdline) AvailabilityChanges() bool {
+	return false
+}
+
 func (c *procCmdline) ConfigRoot() string {
 	return ""
 }

datasource/url.go

@@ -10,6 +10,16 @@ func NewRemoteFile(url string) *remoteFile {
 	return &remoteFile{url}
 }
 
+func (f *remoteFile) IsAvailable() bool {
+	client := pkg.NewHttpClient()
+	_, err := client.Get(f.url)
+	return (err == nil)
+}
+
+func (f *remoteFile) AvailabilityChanges() bool {
+	return true
+}
+
 func (f *remoteFile) ConfigRoot() string {
 	return ""
 }