Try to use OIDC token from metadata server as first option

src/app/credential-internal.ts

@@ -32,6 +32,7 @@ const GOOGLE_AUTH_TOKEN_PATH = '/o/oauth2/token';
 // NOTE: the Google Metadata Service uses HTTP over a vlan
 const GOOGLE_METADATA_SERVICE_HOST = 'metadata.google.internal';
 const GOOGLE_METADATA_SERVICE_TOKEN_PATH = '/computeMetadata/v1/instance/service-accounts/default/token';
+const GOOGLE_METADATA_SERVICE_IDENTITY_PATH = '/computeMetadata/v1/instance/service-accounts/default/identity';
 const GOOGLE_METADATA_SERVICE_PROJECT_ID_PATH = '/computeMetadata/v1/project/project-id';
 const GOOGLE_METADATA_SERVICE_ACCOUNT_ID_PATH = '/computeMetadata/v1/instance/service-accounts/default/email';
 
@@ -209,6 +210,15 @@ export class ComputeEngineCredential implements Credential {
     return requestAccessToken(this.httpClient, request);
   }
 
+  /**
+   * getIDToken returns a OIDC token from the compute metadata service that can be used to make authenticated calls to audience
+   * @param audience the URL the returned ID token will be used to call.
+  */
+  public getIDToken(audience: string): Promise<string> {
+    const request = this.buildRequest(`${GOOGLE_METADATA_SERVICE_IDENTITY_PATH}?audience=${audience}`);
+    return requestIDToken(this.httpClient, request);
+  }
+
   public getProjectId(): Promise<string> {
     if (this.projectId) {
       return Promise.resolve(this.projectId);
@@ -421,6 +431,17 @@ function requestAccessToken(client: HttpClient, request: HttpRequestConfig): Pro
   });
 }
 
+/**
+ * Obtain a new OIDC token by making a remote service call.
+ */
+ function requestIDToken(client: HttpClient, request: HttpRequestConfig): Promise<string> {
+  return client.send(request).then((resp) => {
+    return resp.data;
+  }).catch((err) => {
+    throw new FirebaseAppError(AppErrorCodes.INVALID_CREDENTIAL, getErrorMessage(err));
+  });
+}
+
 /**
  * Constructs a human-readable error message from the given Error.
  */

src/app/credential.ts

@@ -44,4 +44,13 @@ export interface Credential {
    * @returns A Google OAuth2 access token object.
    */
   getAccessToken(): Promise<GoogleOAuthAccessToken>;
+
+    /**
+   * Returns an encoded OIDC token used to authenticate calls to 
+   * private Cloud Functions or other compute services.
+   *
+   * @param audience The URL this token will be used to call.
+   * @returns A base64 encoded OIDC token.
+   */
+  getIDToken(audience: string): Promise<string>;
 }

src/functions/functions-api-client-internal.ts

@@ -62,7 +62,7 @@ export class FunctionsApiClient {
    * @param extensionId - Optional canonical ID of the extension.
    * @param opts - Optional options when enqueuing a new task.
    */
-  public enqueue(data: any, functionName: string, extensionId?: string, opts?: TaskOptions): Promise<void> {
+  public async enqueue(data: any, functionName: string, extensionId?: string, opts?: TaskOptions): Promise<void> {
     if (!validator.isNonEmptyString(functionName)) {
       throw new FirebaseFunctionsError(
         'invalid-argument', 'Function name must be a non empty string');
@@ -81,10 +81,10 @@ export class FunctionsApiClient {
     if (typeof extensionId !== 'undefined' && validator.isNonEmptyString(extensionId)) {
       resources.resourceId = `ext-${extensionId}-${resources.resourceId}`;
     }
-    
+
     return this.getUrl(resources, CLOUD_TASKS_API_URL_FORMAT)
       .then((serviceUrl) => {
-        return this.updateTaskPayload(task, resources)
+        return this.updateTaskPayload(task, resources, )
           .then((task) => {
             const request: HttpRequestConfig = {
               method: 'POST',
@@ -224,22 +224,19 @@ export class FunctionsApiClient {
     return task;
   }
 
-  private updateTaskPayload(task: Task, resources: utils.ParsedResource): Promise<Task> {
-    return Promise.resolve()
-      .then(() => {
-        if (validator.isNonEmptyString(task.httpRequest.url)) {
-          return task.httpRequest.url;
-        }
-        return this.getUrl(resources, FIREBASE_FUNCTION_URL_FORMAT);
-      })
-      .then((functionUrl) => {
-        return this.getServiceAccount()
-          .then((account) => {
-            task.httpRequest.oidcToken.serviceAccountEmail = account;
-            task.httpRequest.url = functionUrl;
-            return task;
-          })
-      });
+  private async updateTaskPayload(task: Task, resources: utils.ParsedResource): Promise<Task> {
+    const functionUrl = validator.isNonEmptyString(task.httpRequest.url)
+      ? task.httpRequest.url 
+      : await this.getUrl(resources, FIREBASE_FUNCTION_URL_FORMAT);
+    task.httpRequest.url = functionUrl;
+    try {
+      const idToken = await this.app.options.credential.getIDToken(functionUrl)
+      task.httpRequest.headers = {...task.httpRequest.headers, "Authorization": `Bearer ${idToken}`}
+    } catch (err: any) {
+      const serviceAccount = await this.getServiceAccount();
+      task.httpRequest.oidcToken.serviceAccountEmail = serviceAccount;
+    }
+    return task;
   }
 
   private toFirebaseError(err: HttpError): PrefixedFirebaseError {