ci: rebase PRs on target branch (#29215)

PR Close #29215

.circleci/config.yml

@@ -63,26 +63,19 @@ var_6: &job_defaults
   docker:
   - image: *default_docker_image
 
-# After checkout, rebase on top of master.
-# Similar to travis behavior, but not quite the same.
-# See https://discuss.circleci.com/t/1662
+# After checkout, rebase on top of target branch.
 var_7: &post_checkout
   run:
-    name: Post checkout step
+    name: Rebase PR on target branch
     command: >
       if [[ -n "${CIRCLE_PR_NUMBER}" ]]; then
-        # Fetch the head and merge commits for this PR.
-        git fetch origin +refs/pull/$CIRCLE_PR_NUMBER/head:pr/$CIRCLE_PR_NUMBER/head
-        git fetch origin +refs/pull/$CIRCLE_PR_NUMBER/merge:pr/$CIRCLE_PR_NUMBER/merge || (echo "Could not fetch merge result with master for this PR. Please check and fix any merge conflicts." ; exit 1)
-        # Checkout the merged PR for testing as CircleCI will just use the PR head otherwise.
-        git checkout -qf pr/$CIRCLE_PR_NUMBER/merge
-        # Reset the merge commit into its PR head.
-        git reset pr/$CIRCLE_PR_NUMBER/head
-        # Commit the merge changes into the head of the PR.
-        # This way we keep the last commit message.
+        # User is required for rebase.
         git config user.name "angular-ci"
         git config user.email "angular-ci"
-        git commit . --amend --no-edit
+        # Rebase PR on top of target branch.
+        node tools/rebase-pr.js angular/angular ${CIRCLE_PR_NUMBER}
+      else
+        echo "This build is not over a PR, nothing to do."
       fi
 
 var_8: &yarn_install

tools/rebase-pr.js

@@ -0,0 +1,113 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+/**
+ * **Usage:**
+ * ```
+ * node rebase-pr <github-repository> <pull-request-number>
+ * ```
+ * **Example:**
+ * ```
+ * node rebase-pr angular/angular 123
+ * ```
+ *
+ * Rebases the current branch on top of the GitHub PR target branch.
+ *
+ * **Context:**
+ * Since a GitHub PR is not necessarily up to date with its target branch, it is useful to rebase
+ * prior to testing it on CI to ensure more up to date test results.
+ *
+ * **Implementation details:**
+ * This script obtains the base for a GitHub PR via the
+ * [GitHub PR API](https://developer.github.com/v3/pulls/#get-a-single-pull-request), then
+ * fetches that branch, and rebases the current branch on top of it.
+ *
+ * **NOTE:**
+ * This script cannot use external dependencies or be compiled because it needs to run before the
+ * environment is setup.
+ * Use only features supported by the NodeJS versions used in the environment.
+ */
+
+// This script uses `console` to print messages to the user.
+// tslint:disable:no-console
+
+// Imports
+const util = require('util');
+const https = require('https');
+const child_process = require('child_process');
+const exec = util.promisify(child_process.exec);
+
+// CLI validation
+if (process.argv.length != 4) {
+  console.error(`This script requires the GitHub repository and PR number as arguments.`);
+  console.error(`Example: node tools/rebase-pr.js angular/angular 123`);
+  process.exitCode = 1;
+  return;
+}
+
+// Run
+_main(...process.argv.slice(2)).catch(err => {
+  console.log('Failed to rebase on top of target branch.\n');
+  console.error(err);
+  process.exitCode = 1;
+});
+
+// Helpers
+async function _main(repository, prNumber) {
+  console.log(`Determining target branch for PR ${prNumber} on ${repository}.`);
+  const targetBranch = await determineTargetBranch(repository, prNumber);
+  console.log(`Target branch is ${targetBranch}.`);
+  await exec(`git fetch origin ${targetBranch}`);
+  console.log(`Rebasing current branch on ${targetBranch}.`);
+  await exec(`git rebase origin/${targetBranch}`);
+  console.log('Rebase successful.');
+}
+
+function determineTargetBranch(repository, prNumber) {
+  const pullsUrl = `https://api.github.com/repos/${repository}/pulls/${prNumber}`;
+  // GitHub requires a user agent: https://developer.github.com/v3/#user-agent-required
+  const options = {headers: {'User-Agent': repository}};
+
+  return new Promise((resolve, reject) => {
+    https
+        .get(
+            pullsUrl, options,
+            (res) => {
+              const {statusCode} = res;
+              const contentType = res.headers['content-type'];
+              let rawData = '';
+
+              res.on('data', (chunk) => { rawData += chunk; });
+              res.on('end', () => {
+
+                let error;
+                if (statusCode !== 200) {
+                  error = new Error(
+                      `Request Failed.\nStatus Code: ${statusCode}.\nResponse: ${rawData}`);
+                } else if (!/^application\/json/.test(contentType)) {
+                  error = new Error(
+                      'Invalid content-type.\n' +
+                      `Expected application/json but received ${contentType}`);
+                }
+
+                if (error) {
+                  reject(error);
+                  return;
+                }
+
+                try {
+                  const parsedData = JSON.parse(rawData);
+                  resolve(parsedData['base']['ref']);
+                } catch (e) {
+                  reject(e);
+                }
+              });
+            })
+        .on('error', (e) => { reject(e); });
+  });
+}