feat: mergify

mergify rule that will merge automatically for a single approver and "build" job passes.

.gitignore

@@ -58,4 +58,6 @@ tsconfig.json
 !# synthesized by projen, but committed to git
 !/.github/workflows/release.yml
 !# synthesized by projen, but committed to git
-!/.github/workflows/build.yml
+!/.github/workflows/build.yml
+!# synthesized by projen, but committed to git
+!/.mergify.yml

.mergify.yml

@@ -0,0 +1,14 @@
+# Generated by projen. To modify, edit .projenrc.js and run "npx projen".
+
+pull_request_rules:
+  - name: Automatic merge on approval and successful build
+    conditions:
+      - "#approved-reviews-by>=1"
+      - status-success=build
+    actions:
+      merge:
+        method: squash
+        commit_message: title+body
+        strict: smart
+        strict_method: merge
+      delete_head_branch: {}

lib/jsii-project.ts

@@ -5,6 +5,7 @@ import { GithubWorkflow } from './github-workflow';
 import { Project } from './project';
 import { PROJEN_VERSION } from './common';
 import { Jest } from './jest';
+import { Mergify } from './mergify';
 
 export interface JsiiProjectOptions extends CommonOptions {
   /**
@@ -43,6 +44,12 @@ export interface JsiiProjectOptions extends CommonOptions {
    * @default true
    */
   readonly jest?: boolean;
+
+  /**
+   * Add mergify configuration
+   * @default true
+   */
+  readonly mergify?: boolean;
 }
 
 export enum Stability {
@@ -163,12 +170,38 @@ export class JsiiProject extends NodeProject {
     this.npmignore.comment('include .jsii manifest');
     this.npmignore.include('.jsii');
 
-    new JsiiBuildWorkflow(this, options.workflowOptions);
+    const buildWorkflow = new JsiiBuildWorkflow(this, options.workflowOptions);
 
     const jest = options.jest ?? true;
     if (jest) {
       new Jest(this);
     }
+
+    const mergify = options.mergify ?? true;
+    if (mergify) {
+      const m = new Mergify(this);
+      m.addRule({
+        name: 'Automatic merge on approval and successful build',
+        conditions: [
+          '#approved-reviews-by>=1',
+          `status-success=${buildWorkflow.jobName}`,
+        ],
+        actions: {
+          merge: {
+            // squash all commits into a single commit when merging
+            method: 'squash',
+
+            // use PR title+body as the commit message
+            commit_message: 'title+body',
+
+            // update PR branch so it's up-to-date before merging
+            strict: 'smart',
+            strict_method: 'merge',
+          },
+          delete_head_branch: { },
+        },
+      });
+    }
   }
 }
 
@@ -340,13 +373,18 @@ class JsiiReleaseWorkflow extends GithubWorkflow {
 }
 
 export class JsiiBuildWorkflow extends GithubWorkflow {
+
+  public readonly jobName: string;
+
   constructor(project: Project, options: WorkflowOptions = { }) {
     super(project, 'build', { name: 'Build' });
 
+    this.jobName = 'build';
+
     this.on({ pull_request: { } });
 
     this.addJobs({
-      build: {
+      [this.jobName]: {
         'runs-on': 'ubuntu-latest',
         container: {
           image: 'jsii/superchain',

lib/json.ts

@@ -6,7 +6,7 @@ export interface JsonFileOptions extends FileBaseOptions {
 }
 
 export class JsonFile extends FileBase {
-  private readonly obj: object;
+  protected readonly obj: object;
 
   constructor(project: Project, filePath: string, options: JsonFileOptions) {
     super(project, filePath, options);

lib/mergify.ts

@@ -0,0 +1,32 @@
+import { Construct } from 'constructs';
+import { Project } from './project';
+import { YamlFile } from './yaml';
+
+export interface MergifyRule {
+  readonly name: string;
+  readonly conditions: string[];
+  readonly actions: { [action: string]: any };
+}
+
+export interface MergifyOptions {
+  readonly rules?: MergifyRule[];
+}
+
+export class Mergify extends Construct {
+  private readonly rules = new Array<MergifyRule>();
+
+  constructor(project: Project) {
+    super(project, 'mergify');
+
+    new YamlFile(project, '.mergify.yml', {
+      committed: true, // must be committed for mergify to be able to find it dah!
+      obj: {
+        pull_request_rules: this.rules,
+      },
+    });
+  }
+
+  public addRule(rule: MergifyRule) {
+    this.rules.push(rule);
+  }
+}

lib/yaml.ts

@@ -0,0 +1,24 @@
+import { JsonFile, JsonFileOptions } from './json';
+import * as YAML from 'yaml';
+import { Project } from './project';
+import { GENERATION_DISCLAIMER } from './common';
+
+export interface YamlFileOptions extends JsonFileOptions {
+
+}
+
+export class YamlFile extends JsonFile {
+  constructor(project: Project, filePath: string, options: YamlFileOptions) {
+    super(project, filePath, options);
+  }
+
+  protected get data() {
+    // sanitize object references by serializaing and deserializing to JSON
+    const sanitized = JSON.parse(JSON.stringify(this.obj));
+    return [
+      `# ${GENERATION_DISCLAIMER}`,
+      '',
+      YAML.stringify(sanitized, { indent: 2 }),
+    ].join('\n')
+  }
+}