From b5212fa61a9f0de4145191accacd5c51af08792d Mon Sep 17 00:00:00 2001
From: Chinthaka Godawita <chin.godawita@me.com>
Date: Mon, 27 Sep 2021 19:50:33 +1000
Subject: [PATCH] feat: add support for `PR_FILTER=auto_merge`

---
 README.md               |  1 +
 src/autoupdater.ts      | 20 +++++++++++++++-
 src/config-loader.ts    |  2 +-
 test/autoupdate.test.ts | 53 +++++++++++++++++++++++++++++++++++++++++
 4 files changed, 74 insertions(+), 2 deletions(-)

diff --git a/README.md b/README.md
index 2f639467..bc7dc164 100644
--- a/README.md
+++ b/README.md
@@ -58,6 +58,7 @@ All configuration values, except `GITHUB_TOKEN`, are optional.
   - `"all"` (default): No filter, _autoupdate_ will monitor and update all pull requests.
   - `"labelled"`: Only monitor PRs with a particular label (or set of labels). Requires the `PR_LABELS` option to be defined to. If `PR_LABELS` is not defined, _autoupdate_ will not monitor any pull requests.
   - `"protected"`: Only monitor PRs that are raised against [protected branches](https://help.github.com/en/github/administering-a-repository/about-protected-branches).
+  - `"auto_merge"`: Only monitor PRs that have ['auto merge'](https://docs.github.com/en/github/collaborating-with-pull-requests/incorporating-changes-from-a-pull-request/automatically-merging-a-pull-request) enabled
 
 - `PR_LABELS`: Controls which labels _autoupdate_ will look for when monitoring PRs. Only used if `PR_FILTER="labelled"`. This can be either a single label or a comma-separated list of labels.
 
diff --git a/src/autoupdater.ts b/src/autoupdater.ts
index 53c5108a..fa4770a7 100644
--- a/src/autoupdater.ts
+++ b/src/autoupdater.ts
@@ -341,7 +341,7 @@ export class AutoUpdater {
       return false;
     }
 
-    if (this.config.pullRequestFilter() === 'protected') {
+    if (prFilter === 'protected') {
       ghCore.info('Checking if this PR is against a protected branch.');
       const { data: branch } = await this.octokit.repos.getBranch({
         owner: pull.head.repo.owner.login,
@@ -362,6 +362,24 @@ export class AutoUpdater {
       return false;
     }
 
+    if (prFilter === 'auto_merge') {
+      ghCore.info('Checking if this PR has auto_merge enabled.');
+
+      if (pull.auto_merge === null) {
+        ghCore.info(
+          'Pull request does not have auto_merge enabled, skipping update.',
+        );
+
+        return false;
+      }
+
+      ghCore.info(
+        'Pull request has auto_merge enabled and is behind base branch.',
+      );
+
+      return true;
+    }
+
     ghCore.info('All checks pass and PR branch is behind base branch.');
     return true;
   }
diff --git a/src/config-loader.ts b/src/config-loader.ts
index be5ed4ff..d787bd6c 100644
--- a/src/config-loader.ts
+++ b/src/config-loader.ts
@@ -15,7 +15,7 @@ export class ConfigLoader {
   }
 
   pullRequestFilter(): string {
-    // one of 'all', 'protected' or 'labelled'.
+    // one of 'all', 'protected', 'labelled' or 'auto_merge'.
     return this.getValue('PR_FILTER', false, 'all');
   }
 
diff --git a/test/autoupdate.test.ts b/test/autoupdate.test.ts
index bc68072b..dcefb054 100644
--- a/test/autoupdate.test.ts
+++ b/test/autoupdate.test.ts
@@ -434,6 +434,59 @@ describe('test `prNeedsUpdate`', () => {
     expect(config.excludedLabels).toHaveBeenCalled();
   });
 
+  test('pull request is against branch with auto_merge enabled', async () => {
+    (config.pullRequestFilter as jest.Mock).mockReturnValue('auto_merge');
+    (config.excludedLabels as jest.Mock).mockReturnValue([]);
+
+    const comparePr = nock('https://api.github.com:443')
+      .get(`/repos/${owner}/${repo}/compare/${head}...${base}`)
+      .reply(200, {
+        behind_by: 1,
+      });
+
+    const updater = new AutoUpdater(config, emptyEvent);
+
+    const pull = {
+      ...validPull,
+      auto_merge: {
+        enabled_by: {
+          login: 'chinthakagodawita',
+        },
+        merge_method: 'squash',
+        commit_title: 'some-commit-title',
+        commit_message: 'fixing a thing',
+      },
+    } as unknown as PullRequestResponse['data'];
+    const needsUpdate = await updater.prNeedsUpdate(pull);
+
+    expect(needsUpdate).toEqual(true);
+    expect(comparePr.isDone()).toEqual(true);
+    expect(config.pullRequestFilter).toHaveBeenCalled();
+  });
+
+  test('pull request is against branch with auto_merge disabled', async () => {
+    (config.pullRequestFilter as jest.Mock).mockReturnValue('auto_merge');
+    (config.excludedLabels as jest.Mock).mockReturnValue([]);
+
+    const comparePr = nock('https://api.github.com:443')
+      .get(`/repos/${owner}/${repo}/compare/${head}...${base}`)
+      .reply(200, {
+        behind_by: 1,
+      });
+
+    const updater = new AutoUpdater(config, emptyEvent);
+
+    const pull = {
+      ...validPull,
+      auto_merge: null,
+    } as unknown as PullRequestResponse['data'];
+    const needsUpdate = await updater.prNeedsUpdate(pull);
+
+    expect(needsUpdate).toEqual(false);
+    expect(comparePr.isDone()).toEqual(true);
+    expect(config.pullRequestFilter).toHaveBeenCalled();
+  });
+
   test('no filters configured', async () => {
     (config.pullRequestFilter as jest.Mock).mockReturnValue('all');
     (config.excludedLabels as jest.Mock).mockReturnValue([]);