Add logic for pushing to forks on "pull_request" events

Only works partly due to GitHub's token scopes. See #13

src/git.js

@@ -1,12 +1,33 @@
 const { log, run } = require("./utils/action");
 
 /**
- * Switches the Git branch
- * @param {string} branchName - Name of the branch which should be switched to
+ * Fetches and checks out the remote Git branch (if it exists, the fork repository will be used)
+ * @param {import('./github/context').GithubContext} context - Information about the GitHub
  */
-function checkOutBranch(branchName) {
-	log(`Checking out the "${branchName}" branch`);
-	run(`git checkout ${branchName}`);
+function checkOutRemoteBranch(context) {
+	if (context.repository.hasFork) {
+		// Fork: Add fork repo as remote
+		log(`Adding "${context.repository.forkName}" fork as remote with Git`);
+		run(
+			`git remote add fork https://${context.actor}:${context.token}@github.com/${context.repository.forkName}.git`,
+		);
+	} else {
+		// No fork: Update remote URL to include auth information (so auto-fixes can be pushed)
+		log(`Adding auth information to Git remote URL`);
+		run(
+			`git remote set-url origin https://${context.actor}:${context.token}@github.com/${context.repository.repoName}.git`,
+		);
+	}
+
+	const remote = context.repository.hasFork ? "fork" : "origin";
+
+	// Fetch remote branch
+	log(`Fetching remote branch "${context.branch}"`);
+	run(`git fetch --no-tags --depth=1 ${remote} ${context.branch}`);
+
+	// Switch to remote branch
+	log(`Switching to the "${context.branch}" branch`);
+	run(`git switch --force-create ${context.branch} --track ${remote}/${context.branch}`);
 }
 
 /**
@@ -18,14 +39,6 @@ function commitChanges(message) {
 	run(`git commit -am "${message}"`);
 }
 
-/**
- * Fetches all remote Git branches
- */
-function fetchBranches() {
-	log(`Fetching all remote Git branches`);
-	run("git fetch --no-tags --prune --depth=1 origin +refs/heads/*:refs/remotes/origin/*");
-}
-
 /**
  * Returns the SHA of the head commit
  * @returns {string} - Head SHA
@@ -47,17 +60,11 @@ function hasChanges() {
 }
 
 /**
- * Pushes all changes to the GitHub repository
- * @param {import('./github/context').GithubContext} context - Information about the GitHub
- * repository and action trigger event
+ * Pushes all changes to the remote repository
  */
-function pushChanges(context) {
-	const remote = `https://${context.actor}:${context.token}@github.com/${context.repository.repoName}.git`;
-	const localBranch = "HEAD";
-	const remoteBranch = context.branch;
-
-	log(`Pushing changes to ${remote}`);
-	run(`git push "${remote}" ${localBranch}:${remoteBranch} --follow-tags`);
+function pushChanges() {
+	log("Pushing changes with Git");
+	run("git push");
 }
 
 /**
@@ -72,9 +79,8 @@ function setUserInfo(name, email) {
 }
 
 module.exports = {
-	checkOutBranch,
+	checkOutRemoteBranch,
 	commitChanges,
-	fetchBranches,
 	getHeadSha,
 	hasChanges,
 	pushChanges,

src/index.js

@@ -29,11 +29,16 @@ async function runAction() {
 		git.setUserInfo(GIT_NAME, GIT_EMAIL);
 	}
 	if (context.eventName === "pull_request") {
-		// Fetch and check out PR branch. This is required because for "pull_request" events, the
-		// Checkout Action (https://github.com/actions/checkout) checks out the PR's test merge commit
-		// in detached head state
-		git.fetchBranches();
-		git.checkOutBranch(context.branch);
+		// Fetch and check out PR branch:
+		// - "push" event: Already on correct branch
+		// - "pull_request" event on origin, for code on origin: The Checkout Action
+		//   (https://github.com/actions/checkout) checks out the PR's test merge commit instead of the
+		//   PR branch. Git is therefore in detached head state. To be able to push changes, the branch
+		//   needs to be fetched and checked out first
+		// - "pull_request" event on origin, for code on fork: Same as above, but the repo/branch where
+		//   changes need to be pushed is not yet available. The fork needs to be added as a Git remote
+		//   first
+		git.checkOutRemoteBranch(context);
 	}
 
 	const checks = [];
@@ -71,7 +76,7 @@ async function runAction() {
 				// Commit and push auto-fix changes
 				if (git.hasChanges()) {
 					git.commitChanges(commitMsg.replace(/\${linter}/g, linter.name));
-					git.pushChanges(context);
+					git.pushChanges();
 				}
 			}