Fix: Set correct branch when it's not specified in the config

packages/netlify-cms-backend-github/src/API.ts

@@ -42,6 +42,7 @@ type GitHubCompareCommit = Octokit.ReposCompareCommitsResponseCommitsItem;
 type GitHubAuthor = Octokit.GitCreateCommitResponseAuthor;
 type GitHubCommitter = Octokit.GitCreateCommitResponseCommitter;
 type GitHubPull = Octokit.PullsListResponseItem;
+type GithubRepo = Octokit.ReposGetResponse;
 
 export const API_NAME = 'GitHub';
 
@@ -1188,6 +1189,16 @@ export default class API {
     return result;
   }
 
+  async newGetDefaultBranch() {
+    try {
+      const result: GithubRepo = await this.request(`${this.originRepoURL}`);
+      return result.default_branch;
+    } catch (e) {
+      console.error('Problem fetching repo data from Github');
+      return null;
+    }
+  }
+
   async backupBranch(branchName: string) {
     try {
       const existingBranch = await this.getBranch(branchName);

packages/netlify-cms-backend-github/src/__tests__/API.spec.js

@@ -347,6 +347,21 @@ describe('github API', () => {
     });
   });
 
+  describe('newGetDefaultBranch', () => {
+    it('should return a non-empty value for the default branch', async () => {
+      const defaultBranch = 'main';
+      const repo = `owner/my-repo`;
+      const api = new API({ branch: defaultBranch, repo });
+      const responses = {
+        '/repos/owner/my-repo': () => ({ default_branch: defaultBranch }),
+      };
+      mockAPI(api, responses);
+
+      await expect(api.newGetDefaultBranch()).resolves.toBe(defaultBranch);
+      expect(api.request).toHaveBeenCalledTimes(1);
+      expect(api.request.mock.calls[0]).toEqual([`/repos/${repo}`]);
+    });
+  });
   describe('migratePullRequest', () => {
     it('should migrate to pull request labels when no version', async () => {
       const api = new API({ branch: 'master', repo: 'owner/repo' });

packages/netlify-cms-backend-github/src/implementation.tsx

@@ -116,7 +116,7 @@ export default class GitHub implements Implementation {
       this.repo = this.originRepo = config.backend.repo || '';
     }
     this.alwaysForkEnabled = config.backend.always_fork || false;
-    this.branch = config.backend.branch?.trim() || 'master';
+    this.branch = config.backend.branch?.trim() || '';
     this.apiRoot = config.backend.api_root || 'https://api.github.com';
     this.token = '';
     this.squashMerges = config.backend.squash_merges || false;
@@ -180,6 +180,20 @@ export default class GitHub implements Implementation {
       : this.authenticate(user);
   }
 
+  async setDefaultBranch() {
+    try {
+      const masterBranch = await this.api!.getBranch('master');
+      if (!masterBranch) {
+        // Get default branch of the repo
+        const defaultBranch = await this.api!.newGetDefaultBranch();
+        this.branch = defaultBranch || '';
+      } else {
+        this.branch = 'master';
+      }
+    } catch (e) {
+      console.warn(e);
+    }
+  }
   async pollUntilForkExists({ repo, token }: { repo: string; token: string }) {
     const pollDelay = 250; // milliseconds
     let repoExists = false;
@@ -331,6 +345,13 @@ export default class GitHub implements Implementation {
     if (!isCollab) {
       throw new Error('Your GitHub user account does not have access to this repo.');
     }
+    // In the constructor, `this.branch` is set by reading the config file
+    // If it is an empty string at this point,
+    // it means there is no `branch` property set in the config
+
+    if (this.branch === '') {
+      await this.setDefaultBranch();
+    }
 
     // Authorized user
     return { ...user, token: state.token as string, useOpenAuthoring: this.useOpenAuthoring };