feat(backend-common): exposed onAuth callback

Exposed the `onAuth` callback from isomorphic git to allow callers to specify a callback to resolve credentials based on the URL of the remote instead of providing static credentials. For example, the [DefaultAzureDevOpsCredentialsProvider](https://github.com/backstage/backstage/blob/master/packages/integration/src/azure/DefaultAzureDevOpsCredentialsProvider.ts) can be used to resolve Azure DevOps credentials based on the request URL.

Signed-off-by: Sander Aernouts <sander.aernouts@gmail.com>

.changeset/tiny-cobras-look.md

@@ -0,0 +1,5 @@
+---
+'@backstage/backend-common': patch
+---
+
+Expose an `onAuth` handler for `git` actions to provide custom credentials

packages/backend-common/api-report.md

@@ -7,6 +7,7 @@
 /// <reference types="webpack-env" />
 
 import { AppConfig } from '@backstage/config';
+import { AuthCallback } from 'isomorphic-git';
 import { AwsCredentialsManager } from '@backstage/integration-aws-node';
 import { AwsS3Integration } from '@backstage/integration';
 import { AzureDevOpsCredentialsProvider } from '@backstage/integration';
@@ -66,6 +67,12 @@ import { V1PodTemplateSpec } from '@kubernetes/client-node';
 import * as winston from 'winston';
 import { Writable } from 'stream';
 
+// @public
+export type AuthCallbackOptions = {
+  onAuth: AuthCallback;
+  logger?: LoggerService;
+};
+
 // @public
 export class AwsS3UrlReader implements UrlReader {
   constructor(
@@ -379,12 +386,7 @@ export class Git {
   deleteRemote(options: { dir: string; remote: string }): Promise<void>;
   fetch(options: { dir: string; remote?: string }): Promise<void>;
   // (undocumented)
-  static fromAuth: (options: {
-    username?: string;
-    password?: string;
-    token?: string;
-    logger?: LoggerService;
-  }) => Git;
+  static fromAuth: (options: StaticAuthOptions | AuthCallbackOptions) => Git;
   // (undocumented)
   init(options: { dir: string; defaultBranch?: string }): Promise<void>;
   log(options: { dir: string; ref?: string }): Promise<ReadCommitResult[]>;
@@ -749,6 +751,14 @@ export function setRootLogger(newLogger: winston.Logger): void;
 // @public @deprecated
 export const SingleHostDiscovery: typeof HostDiscovery;
 
+// @public
+export type StaticAuthOptions = {
+  username?: string;
+  password?: string;
+  token?: string;
+  logger?: LoggerService;
+};
+
 // @public
 export type StatusCheck = () => Promise<any>;
 

packages/backend-common/src/scm/git.test.ts

@@ -190,7 +190,7 @@ describe('Git', () => {
       });
     });
 
-    it('should pass a function that returns the authorization as the onAuth handler', async () => {
+    it('should pass a function that returns the authorization as the onAuth handler when username and password are specified', async () => {
       const url = 'http://github.com/some/repo';
       const dir = '/some/mock/dir';
       const auth = {
@@ -208,6 +208,25 @@ describe('Git', () => {
       expect(onAuth()).toEqual(auth);
     });
 
+    it('should pass the provided callback as the onAuth handler when on auth is specified', async () => {
+      const url = 'http://github.com/some/repo';
+      const dir = '/some/mock/dir';
+      const auth = {
+        username: 'from',
+        password: 'callback',
+      };
+
+      const git = Git.fromAuth({ onAuth: () => auth });
+
+      await git.clone({ url, dir });
+
+      const { onAuth } = (
+        isomorphic.clone as unknown as jest.Mock<(typeof isomorphic)['clone']>
+      ).mock.calls[0][0]!;
+
+      expect(onAuth()).toEqual(auth);
+    });
+
     it('should propagate the data from the error handler', async () => {
       const url = 'http://github.com/some/repo';
       const dir = '/some/mock/dir';

packages/backend-common/src/scm/git.ts

@@ -18,11 +18,40 @@ import git, {
   ProgressCallback,
   MergeResult,
   ReadCommitResult,
+  AuthCallback,
 } from 'isomorphic-git';
 import http from 'isomorphic-git/http/node';
 import fs from 'fs-extra';
 import { LoggerService } from '@backstage/backend-plugin-api';
 
+function isAuthCallbackOptions(
+  options: StaticAuthOptions | AuthCallbackOptions,
+): options is AuthCallbackOptions {
+  return 'onAuth' in options;
+}
+
+/**
+ * Configure static credential for authentication
+ *
+ * @public
+ */
+export type StaticAuthOptions = {
+  username?: string;
+  password?: string;
+  token?: string;
+  logger?: LoggerService;
+};
+
+/**
+ * Configure an authentication callback that can provide credentials on demand
+ *
+ * @public
+ */
+export type AuthCallbackOptions = {
+  onAuth: AuthCallback;
+  logger?: LoggerService;
+};
+
 /*
 provider          username         password
 Azure             'notempty'       token
@@ -42,6 +71,7 @@ instead of Basic Auth (e.g., Bitbucket Server).
  *
  * @public
  */
+
 export class Git {
   private readonly headers: {
     [x: string]: string;
@@ -49,12 +79,13 @@ export class Git {
 
   private constructor(
     private readonly config: {
-      username?: string;
-      password?: string;
+      onAuth: AuthCallback;
       token?: string;
       logger?: LoggerService;
     },
   ) {
+    this.onAuth = config.onAuth;
+
     this.headers = {
       'user-agent': 'git/@isomorphic-git',
       ...(config.token ? { Authorization: `Bearer ${config.token}` } : {}),
@@ -283,10 +314,7 @@ export class Git {
     });
   }
 
-  private onAuth = () => ({
-    username: this.config.username,
-    password: this.config.password,
-  });
+  private onAuth: AuthCallback;
 
   private onProgressHandler = (): ProgressCallback => {
     let currentPhase = '';
@@ -303,13 +331,13 @@ export class Git {
     };
   };
 
-  static fromAuth = (options: {
-    username?: string;
-    password?: string;
-    token?: string;
-    logger?: LoggerService;
-  }) => {
+  static fromAuth = (options: StaticAuthOptions | AuthCallbackOptions) => {
+    if (isAuthCallbackOptions(options)) {
+      const { onAuth, logger } = options;
+      return new Git({ onAuth, logger });
+    }
+
     const { username, password, token, logger } = options;
-    return new Git({ username, password, token, logger });
+    return new Git({ onAuth: () => ({ username, password }), token, logger });
   };
 }

packages/backend-common/src/scm/index.ts

@@ -15,3 +15,4 @@
  */
 
 export { Git } from './git';
+export type { StaticAuthOptions, AuthCallbackOptions } from './git';