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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.
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‎.github/workflows/backport.yml‎

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

‎eng/actions/backport/action.yml‎

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name: 'PR Backporter'
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description: 'Backports a pull request to a branch using the "/backport to <branch>" comment'
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inputs:
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auth_token:
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description: 'The token used to authenticate to GitHub.'
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pr_title_template:
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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%.'
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default: '[%target_branch%] %source_pr_title%'
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pr_description_template:
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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%.'
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default: |
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Backport of #%source_pr_number% to %target_branch%
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/cc %cc_users%
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runs:
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using: 'node12'
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main: 'index.js'

‎eng/actions/backport/index.js‎

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

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