spread/google: add support for service accounts

README.md

@@ -894,6 +894,26 @@ Service accounts are best as they can be further constrained and not be
 associated with your overall authenticated access. Do not ship your own
 credentials to remote systems.
 
+A service account can be attached to a created instances using the following
+configuration:
+
+```
+(...)
+
+backends:
+    google:
+        key: $(HOST:echo $GOOGLE_JSON_FILENAME)
+        ...
+        systems:
+            - system-with-service-account:
+                attach-service-account: true
+...
+```
+
+Service account can only be attached to an instance if the authentication key is
+of `service_acccount` type, and the IAM role associated with the account has the
+necessary permissions.
+
 Images are located by first attempting to match the provided value exactly
 against the image name, and then some processing is done to verify if an
 image with the individual tokens in its description exists. Images are

spread/google.go

@@ -16,7 +16,6 @@ import (
 	"golang.org/x/net/context"
 	"golang.org/x/oauth2"
 	"golang.org/x/oauth2/google"
-	"golang.org/x/oauth2/jwt"
 
 	"github.com/niemeyer/pretty"
 	"regexp"
@@ -44,6 +43,8 @@ type googleProvider struct {
 
 	client *http.Client
 
+	serviceAccount string
+
 	mu sync.Mutex
 
 	keyChecked bool
@@ -459,6 +460,20 @@ func (p *googleProvider) createMachine(ctx context.Context, system *System) (*go
 		},
 	}
 
+	if system.AttachServiceAccount {
+		if p.serviceAccount == "" {
+			return nil, &FatalError{fmt.Errorf("no service account to attach")}
+		}
+		// XXX the service account could be set from google key
+		// credentials, but the account used in the context of the
+		// request may not have the permissions to attach a service
+		// account to the instance
+		params["serviceAccounts"] = []googleParams{{
+			"email":  p.serviceAccount,
+			"scopes": []string{"https://www.googleapis.com/auth/cloud-platform"},
+		}}
+	}
+
 	if system.SecureBoot {
 		params["shieldedInstanceConfig"] = googleParams{
 			"enableSecureBoot":          true,
@@ -755,6 +770,28 @@ func (p *googleProvider) waitOperation(ctx context.Context, s *googleServer, ver
 	panic("unreachable")
 }
 
+func serviceAccountFromKey(raw []byte) (string, error) {
+	const serviceAccountKey = "service_account"
+
+	// taken from golang.org/x/oauth/google
+	var credentials struct {
+		Type        string `json:"type"`
+		ClientEmail string `json:"client_email"`
+	}
+
+	if err := json.Unmarshal(raw, &credentials); err != nil {
+		return "", err
+	}
+
+	if credentials.Type != serviceAccountKey {
+		// email is only relevant if dealing with service_account
+		// credentials type
+		return "", nil
+	}
+
+	return credentials.ClientEmail, nil
+}
+
 func (p *googleProvider) checkKey() error {
 	p.mu.Lock()
 	defer p.mu.Unlock()
@@ -771,15 +808,29 @@ func (p *googleProvider) checkKey() error {
 
 	if err == nil && p.client == nil {
 		ctx := context.Background()
-		if strings.HasPrefix(p.backend.Key, "{") {
-			var cfg *jwt.Config
-			cfg, err = google.JWTConfigFromJSON([]byte(p.backend.Key), googleScope)
+		var creds *google.Credentials
+		if p.backend.Key != "" {
+			var raw []byte
+			if strings.HasPrefix(p.backend.Key, "{") {
+				// already a raw JSON blob
+				raw = []byte(p.backend.Key)
+			} else {
+				raw, err = ioutil.ReadFile(p.backend.Key)
+			}
+
 			if err == nil {
-				p.client = oauth2.NewClient(ctx, cfg.TokenSource(ctx))
+				creds, err = google.CredentialsFromJSON(ctx, raw, googleScope)
 			}
+
+			// identify service account if possible
+			p.serviceAccount, err = serviceAccountFromKey(raw)
 		} else {
-			os.Setenv("GOOGLE_APPLICATION_CREDENTIALS", p.backend.Key)
-			p.client, err = google.DefaultClient(context.Background(), googleScope)
+			// none provided, let the google library find whatever
+			// is appropriate
+			creds, err = google.FindDefaultCredentials(ctx, googleScope)
+		}
+		if err == nil {
+			p.client = oauth2.NewClient(ctx, creds.TokenSource)
 		}
 	}
 	if err == nil {

spread/project.go

@@ -139,6 +139,9 @@ type System struct {
 
 	Priority OptionalInt
 	Manual   bool
+
+	// Only for Google
+	AttachServiceAccount bool `yaml:"attach-service-account"`
 }
 
 func (system *System) String() string { return system.Backend + ":" + system.Name }