feat: Detect GitLab default branch for presets (#3015)

Usually master will be used, but just in case - let's find which branch is default first, before retrieving the file.

Closes #2991

lib/datasource/gitlab/index.js

@@ -5,6 +5,8 @@ module.exports = {
   getPkgReleases,
 };
 
+const GitLabApiUrl = 'https://gitlab.com/api/v4/projects';
+
 async function getPreset(pkgName, presetName = 'default') {
   if (presetName !== 'default') {
     throw new Error(
@@ -15,8 +17,13 @@ async function getPreset(pkgName, presetName = 'default') {
   let res;
   try {
     const urlEncodedPkgName = encodeURIComponent(pkgName);
-    const url = `https://gitlab.com/api/v4/projects/${urlEncodedPkgName}/repository/files/renovate.json?ref=master`;
+    const defautlBranchName = await getDefaultBranchName(urlEncodedPkgName);
-    res = Buffer.from((await glGot(url)).body.content, 'base64').toString();
+
+    const presetUrl = `${GitLabApiUrl}/${urlEncodedPkgName}/repository/files/renovate.json?ref=${defautlBranchName}`;
+    res = Buffer.from(
+      (await glGot(presetUrl)).body.content,
+      'base64'
+    ).toString();
   } catch (err) {
     logger.debug('Failed to retrieve renovate.json from repo');
     throw new Error('dep not found');
@@ -32,3 +39,18 @@ async function getPreset(pkgName, presetName = 'default') {
 /* eslint-disable no-unused-vars */
 function getPkgReleases(purl) {}
 /* eslint-enable no-unused-vars */
+
+async function getDefaultBranchName(urlEncodedPkgName) {
+  const branchesUrl = `${GitLabApiUrl}/${urlEncodedPkgName}/repository/branches`;
+  const res = await glGot(branchesUrl);
+  const branches = res.body;
+  let defautlBranchName = 'master';
+  for (const branch of branches) {
+    if (branch.default) {
+      defautlBranchName = branch.name;
+      break;
+    }
+  }
+
+  return defautlBranchName;
+}

test/datasource/gitlab.spec.js

@@ -31,6 +31,14 @@ describe('datasource/gitlab', () => {
       await expect(gitlab.getPreset('some/repo')).rejects.toThrow();
     });
     it('should return the preset', async () => {
+      glGot.mockImplementationOnce(() => ({
+        body: [
+          {
+            name: 'master',
+            default: true,
+          },
+        ],
+      }));
       glGot.mockImplementationOnce(() => ({
         body: {
           content: Buffer.from('{"foo":"bar"}').toString('base64'),