Add comment on PR workflow

Co-authored-by: Colin Rotherham <[email protected]>

.github/workflows/diff-change-to-dist.yaml

@@ -11,8 +11,10 @@ jobs:
   generate-diff:
     name: Generate Diff
     runs-on: ubuntu-latest
-    # Abort if run from a fork, as token won't have write access to post comment
-    if: github.event.pull_request.head.repo.full_name == github.repository
+
+    # Skip when token write permissions are unavailable on forks
+    if: ${{ !github.event.pull_request.head.repo.fork }}
+
     steps:
       - uses: actions/checkout@v3
         with:
@@ -49,19 +51,12 @@ jobs:
         with:
           github-token: ${{secrets.GITHUB_TOKEN}}
           script: |
-            const fs = require('fs').promises
-            const diff = await fs.readFile(
-              process.env.GITHUB_WORKSPACE + '/dist.diff', 'utf8'
-            )
+            const { readFile } = require('node:fs/promises')
+            const diff = await readFile('${{ github.workspace }}/dist.diff', 'utf8')
 
-            const commentText = '## Changes to dist\n' +
+            const { commentOnPr } = await import('${{ github.workspace }}/.github/workflows/scripts/comments.mjs')
+            await commentOnPr({ github, context }, 'Diff release', '## Changes to dist\n' +
               '```diff\n' +
               diff +
               '\n```'
-
-            github.rest.issues.createComment({
-              issue_number: context.payload.pull_request.number,
-              owner: context.repo.owner,
-              repo: context.repo.repo,
-              body: commentText
-            })
+            )

.github/workflows/scripts/comments.mjs

@@ -0,0 +1,58 @@
+/**
+ * @param {object} githubContext - Github Action context
+ * @param {import('@octokit/rest').Octokit} githubContext.github
+ * @param {import('@actions/github').context} githubContext.context
+ * @param {string} markerText - A unique marker used to identify the correct comment
+ * @param {string} bodyText - The body of the comment
+ */
+export async function commentOnPr ({ github, context }, markerText, bodyText) {
+  if (!context.payload.pull_request) {
+    throw new Error('Workflow "comment on PR" requires a PR')
+  }
+
+  const marker = `<!-- ${markerText} -->\n`
+  const sha = `\nSHA: ${context.sha}`
+  const body = marker + bodyText + sha
+
+  let commentId
+
+  /**
+   * @satisfies {IssueCommentsListParams}
+   */
+  const issueParameters = {
+    issue_number: context.payload.pull_request.number,
+    owner: context.repo.owner,
+    repo: context.repo.repo
+  }
+
+  /**
+   * Finds the first issue comment for which the `matcher` function returns `true`
+   *
+   * {@link https://github.com/peter-evans/find-comment/blob/main/src/find.ts}
+   */
+  for await (const { data: comments } of github.paginate.iterator(
+    github.rest.issues.listComments,
+    issueParameters
+  )) {
+    const comment = comments.find((comment) => !!comment.body?.includes(marker))
+
+    if (comment) {
+      commentId = comment.id
+    }
+  }
+
+  /**
+   * Create GitHub issue comment
+   *
+   * Updates existing commentby ID if available
+   */
+  await (!commentId
+    ? github.rest.issues.createComment({ ...issueParameters, body })
+    : github.rest.issues.updateComment({ ...issueParameters, body, comment_id: commentId }))
+}
+
+/**
+ * @typedef {import('@octokit/plugin-rest-endpoint-methods').RestEndpointMethodTypes["issues"]} IssuesEndpoint
+ * @typedef {IssuesEndpoint["updateComment"]["parameters"]} IssueCommentUpdateParams
+ * @typedef {IssuesEndpoint["listComments"]["parameters"]} IssueCommentsListParams
+ */

.github/workflows/scripts/package.json

@@ -0,0 +1,14 @@
+{
+  "private": true,
+  "name": "govuk-frontend-workflow-scripts",
+  "description": "GOV.UK Frontend GitHub Actions workflow scripts",
+  "engines": {
+    "node": "^18.12.0",
+    "npm": "^8.1.0 || ^9.1.0"
+  },
+  "license": "MIT",
+  "optionalDependencies": {
+    "@actions/github": "^5.1.1",
+    "@octokit/rest": "19.0.11"
+  }
+}

.github/workflows/scripts/tsconfig.json

@@ -0,0 +1,7 @@
+{
+  "extends": "../../../tsconfig.base.json",
+  "include": ["**/*.mjs"],
+  "compilerOptions": {
+    "strict": true
+  }
+}