Add GitHub Action for backporting PRs to branches (#40955)

This adds an action that can be triggered with a special comment on a PR: `/backport to <some branch>`.
The action then creates a new PR to the target branch with the commits from the source PR applied.

.github/workflows/backport.yml

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+name: Backport PR to branch
+on:
+  issue_comment:
+    types: [created]
+
+jobs:
+  backport:
+    if: github.event.issue.pull_request != '' && contains(github.event.comment.body, '/backport to')
+    runs-on: ubuntu-20.04
+    steps:
+    - name: Checkout repo
+      uses: actions/checkout@v2
+    - name: Run backport
+      uses: ./eng/actions/backport
+      with:
+        auth_token: ${{ secrets.GITHUB_TOKEN }}
+        pr_description_template: |
+          Backport of #%source_pr_number% to %target_branch%
+
+          /cc %cc_users%
+
+          ## Customer Impact
+
+          ## Testing
+
+          ## Risk

eng/actions/backport/action.yml

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+name: 'PR Backporter'
+description: 'Backports a pull request to a branch using the "/backport to <branch>" comment'
+inputs:
+  auth_token:
+    description: 'The token used to authenticate to GitHub.'
+  pr_title_template:
+      description: 'The template used for the PR title. Special placeholder tokens that will be replaced with a value: %target_branch%, %source_pr_title%, %source_pr_number%, %cc_users%.'
+      default: '[%target_branch%] %source_pr_title%'
+  pr_description_template:
+    description: 'The template used for the PR description. Special placeholder tokens that will be replaced with a value: %target_branch%, %source_pr_title%, %source_pr_number%, %cc_users%.'
+    default: |
+      Backport of #%source_pr_number% to %target_branch%
+
+      /cc %cc_users%
+
+runs:
+  using: 'node12'
+  main: 'index.js'

eng/actions/backport/index.js

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+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+function BackportException(message, postToGitHub = true) {
+  this.message = message;
+  this.postToGitHub = postToGitHub;
+}
+
+async function run() {
+  const util = require("util");
+  const jsExec = util.promisify(require("child_process").exec);
+
+  console.log("Installing npm dependencies");
+  const { stdout, stderr } = await jsExec("npm install @actions/core @actions/github @actions/exec");
+  console.log("npm-install stderr:\n\n" + stderr);
+  console.log("npm-install stdout:\n\n" + stdout);
+  console.log("Finished installing npm dependencies");
+
+  const core = require("@actions/core");
+  const github = require("@actions/github");
+  const exec = require("@actions/exec");
+
+  if (github.context.eventName !== "issue_comment") throw "Error: This action only works on issue_comment events.";
+
+  const run_id = process.env.GITHUB_RUN_ID;
+  const repo_owner = github.context.payload.repository.owner.login;
+  const repo_name = github.context.payload.repository.name;
+  const pr_number = github.context.payload.issue.number;
+  const pr_source_ref = process.env.GITHUB_REF;
+  const comment_user = github.context.payload.comment.user.login;
+
+  let octokit = github.getOctokit(core.getInput("auth_token"));
+  let target_branch = "";
+
+  try {
+    // verify the comment user is a repo collaborator
+    try {
+      await octokit.repos.checkCollaborator({
+        owner: repo_owner,
+        repo: repo_name,
+        username: comment_user
+      });
+      console.log(`Verified ${comment_user} is a repo collaborator.`);
+    } catch {
+      throw new BackportException(`Error: @${comment_user} is not a repo collaborator, backporting is not allowed.`);
+    }
+
+    // extract the target branch name from the trigger phrase containing these characters: a-z, A-Z, digits, forward slash, dot, hyphen, underscore
+    console.log(`Extracting target branch`);
+    const regex = /\/backport to ([a-zA-Z\d\/\.\-\_]+)/;
+    target_branch = regex.exec(github.context.payload.comment.body)[1];
+    if (target_branch == null) throw new BackportException("Error: No backport branch found in the trigger phrase.");
+    try { await exec.exec(`git ls-remote --exit-code --heads origin ${target_branch}`) } catch { throw new BackportException(`Error: The specified backport target branch ${target_branch} wasn't found in the repo.`); }
+    console.log(`Backport target branch: ${target_branch}`);
+
+    // Post backport started comment to pull request
+    const backport_start_body = `Started backporting to ${target_branch}: https://github.com/${repo_owner}/${repo_name}/actions/runs/${run_id}`;
+    await octokit.issues.createComment({
+      owner: repo_owner,
+      repo: repo_name,
+      issue_number: pr_number,
+      body: backport_start_body
+    });
+
+    console.log("Applying backport patch");
+
+    await exec.exec(`git -c protocol.version=2 fetch --no-tags --progress --no-recurse-submodules origin ${target_branch} ${pr_source_ref}`);
+    await exec.exec(`git checkout ${target_branch}`);
+    await exec.exec(`git clean -xdff`);
+
+    // configure git
+    await exec.exec(`git config user.name "github-actions"`);
+    await exec.exec(`git config user.email "github-actions@github.com"`);
+
+    // create temporary backport branch
+    const temp_branch = `backport/pr-${pr_number}-to-${target_branch}`;
+    await exec.exec(`git checkout -b ${temp_branch}`);
+
+    // skip opening PR if the branch already exists on the origin remote since that means it was opened
+    // by an earlier backport and force pushing to the branch updates the existing PR
+    let should_open_pull_request = true;
+    try {
+      await exec.exec(`git ls-remote --exit-code --heads origin ${temp_branch}`);
+      should_open_pull_request = false;
+    } catch { }
+
+    // download and apply patch
+    await exec.exec(`curl -sSL "${github.context.payload.issue.pull_request.patch_url}" --output changes.patch`);
+
+    const git_am_command = "git am --3way --ignore-whitespace --keep-non-patch changes.patch";
+    let git_am_output = `$ ${git_am_command}\n\n`;
+    let git_am_failed = false;
+    try {
+      await exec.exec(git_am_command, [], {
+        listeners: {
+          stdout: function stdout(data) { git_am_output += data; },
+          stderr: function stderr(data) { git_am_output += data; }
+        }
+      });
+    } catch (error) {
+      git_am_output += error;
+      git_am_failed = true;
+    }
+
+    if (git_am_failed) {
+      const git_am_failed_body = `@${github.context.payload.comment.user.login} backporting to ${target_branch} failed, the patch most likely resulted in conflicts:\n\n\`\`\`shell\n${git_am_output}\n\`\`\`\n\nPlease backport manually!`;
+      await octokit.issues.createComment({
+        owner: repo_owner,
+        repo: repo_name,
+        issue_number: pr_number,
+        body: git_am_failed_body
+      });
+      throw new BackportException("Error: git am failed, most likely due to a merge conflict.", false);
+    }
+    else {
+      // push the temp branch to the repository
+      await exec.exec(`git push --force --set-upstream origin HEAD:${temp_branch}`);
+    }
+
+    if (!should_open_pull_request) {
+      console.log("Backport temp branch already exists, skipping opening a PR.");
+      return;
+    }
+
+    // prepate the GitHub PR details
+    let backport_pr_title = core.getInput("pr_title_template");
+    let backport_pr_description = core.getInput("pr_description_template");
+
+    // get users to cc (append PR author if different from user who issued the backport command)
+    let cc_users = `@${comment_user}`;
+    if (comment_user != github.context.payload.issue.user.login) cc_users += ` @${github.context.payload.issue.user.login}`;
+
+    // replace the special placeholder tokens with values
+    backport_pr_title = backport_pr_title
+      .replace(/%target_branch%/g, target_branch)
+      .replace(/%source_pr_title%/g, github.context.payload.issue.title)
+      .replace(/%source_pr_number%/g, github.context.payload.issue.number)
+      .replace(/%cc_users%/g, cc_users);
+
+    backport_pr_description = backport_pr_description
+      .replace(/%target_branch%/g, target_branch)
+      .replace(/%source_pr_title%/g, github.context.payload.issue.title)
+      .replace(/%source_pr_number%/g, github.context.payload.issue.number)
+      .replace(/%cc_users%/g, cc_users);
+
+    // open the GitHub PR
+    await octokit.pulls.create({
+      owner: repo_owner,
+      repo: repo_name,
+      title: backport_pr_title,
+      body: backport_pr_description,
+      head: temp_branch,
+      base: target_branch
+    });
+
+    console.log("Successfully opened the GitHub PR.");
+  } catch (error) {
+
+    core.setFailed(error);
+
+    if (error.postToGitHub === undefined || error.postToGitHub == true) {
+      // post failure to GitHub comment
+      const unknown_error_body = `@${comment_user} an error occurred while backporting to ${target_branch}, please check the run log for details!\n\n${error.message}`;
+      await octokit.issues.createComment({
+        owner: repo_owner,
+        repo: repo_name,
+        issue_number: pr_number,
+        body: unknown_error_body
+      });
+    }
+  }
+}
+
+run();