Support GitLab merge conflict rebasing (#335)

Closes #132 

* Add gitlab deleteBranch

* Add GitLab rebasing logic to branch worker

* Add GitLab getBranch

* Record user email during initRepo

* Add canRebase logic

* lint fix

* Add test

lib/api/gitlab.js

@@ -10,8 +10,10 @@ module.exports = {
   findFilePaths,
   // Branch
   branchExists,
+  getBranch,
   getBranchPr,
   getBranchStatus,
+  deleteBranch,
   // issue
   addAssignees,
   addReviewers,
@@ -81,6 +83,8 @@ async function initRepo(repoName, token, endpoint, repoLogger) {
     const res = await glGot(`projects/${config.repoName}`);
     config.defaultBranch = res.body.default_branch;
     logger.debug(`${repoName} default branch = ${config.defaultBranch}`);
+    // Discover our user email
+    config.email = (await glGot(`user`)).body.email;
   } catch (err) {
     logger.error(`GitLab init error: ${JSON.stringify(err)}`);
     throw err;
@@ -122,6 +126,19 @@ async function branchExists(branchName) {
   }
 }
 
+// Returns branch object
+async function getBranch(branchName) {
+  logger.debug(`getBranch(${branchName})`);
+  const url = `projects/${config.repoName}/repository/branches/${branchName}`;
+  try {
+    return (await glGot(url)).body;
+  } catch (err) {
+    logger.warn(`Failed to getBranch ${branchName}`);
+    logger.debug(JSON.stringify(err));
+    return null;
+  }
+}
+
 // Returns the Pull Request for a branch. Null if not exists.
 async function getBranchPr(branchName) {
   logger.debug(`getBranchPr(${branchName})`);
@@ -170,6 +187,12 @@ async function getBranchStatus(branchName) {
   return status;
 }
 
+async function deleteBranch(branchName) {
+  await glGot.delete(
+    `projects/${config.repoName}/repository/branches/${branchName}`
+  );
+}
+
 // Issue
 
 async function addAssignees(prNo, assignees) {
@@ -259,8 +282,12 @@ async function getPr(prNo) {
     logger.debug('pr cannot be merged');
     pr.isUnmergeable = true;
   }
-  // We can't rebase through GitLab API
-  pr.canRebase = false;
+  // Check if the most recent branch commit is by us
+  // If not then we don't allow it to be rebased, in case someone's changes would be lost
+  const branch = await getBranch(pr.source_branch);
+  if (branch && branch.commit.author_email === config.email) {
+    pr.canRebase = true;
+  }
   return pr;
 }
 

lib/workers/branch.js

@@ -41,10 +41,15 @@ async function getParentBranch(branchName, config) {
   if (pr.isUnmergeable) {
     logger.debug('PR is unmergeable');
     if (pr.canRebase) {
-      // Only supported by GitHub
-      // Setting parentBranch back to undefined means that we'll use the default branch
-      logger.debug(`Branch ${branchName} is not mergeable and needs rebasing`);
-      return undefined;
+      if (config.platform === 'github') {
+        // Setting parentBranch back to undefined means that we'll use the default branch
+        logger.info(`Branch is not mergeable and needs rebasing`);
+        return undefined;
+      } else if (config.platform === 'gitlab') {
+        logger.info(`Deleting unmergeable branch in order to recreate/rebase`);
+        await config.api.deleteBranch(branchName);
+        return undefined;
+      }
     }
     // Don't do anything different, but warn
     logger.warn(`Branch ${branchName} is not mergeable but can't be rebased`);

test/workers/branch.spec.js

@@ -22,8 +22,10 @@ describe('workers/branch', () => {
     const branchName = 'foo';
     beforeEach(() => {
       config = {
+        platform: 'github',
         api: {
           branchExists: jest.fn(() => true),
+          deleteBranch: jest.fn(),
           getBranchPr: jest.fn(),
           getBranchStatus: jest.fn(),
           isBranchStale: jest.fn(() => false),
@@ -68,6 +70,17 @@ describe('workers/branch', () => {
         undefined
       );
     });
+    it('returns undefined if unmergeable and can rebase (gitlab)', async () => {
+      config.platform = 'gitlab';
+      config.api.getBranchPr.mockReturnValue({
+        isUnmergeable: true,
+        canRebase: true,
+      });
+      expect(await branchWorker.getParentBranch(branchName, config)).toBe(
+        undefined
+      );
+      expect(config.api.deleteBranch.mock.calls.length).toBe(1);
+    });
     it('returns branchName if automerge branch-push and not stale', async () => {
       config.automergeEnabled = true;
       config.automergeType = 'branch-push';