tools: add GitHub Action linter for pr-url

PR-URL: #37221
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

.github/workflows/linters.yml

@@ -98,3 +98,12 @@ jobs:
       - uses: mszostok/codeowners-validator@v0.4.0
         with:
           checks: "files,duppatterns"
+  lint-pr-url:
+    if: ${{ github.event.pull_request }}
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          fetch-depth: 2
+      # GH Actions squashes all PR commits, HEAD^ refers to the base branch.
+      - run: git diff HEAD^ HEAD -G"pr-url:" -- "*.md" | ./tools/lint-pr-url.mjs ${{ github.event.pull_request.html_url }}

tools/lint-pr-url.mjs

@@ -0,0 +1,37 @@
+#!/usr/bin/env node
+
+// Usage:
+// git diff upstream/master...HEAD -G"pr-url:" -- "*.md" | \
+// ./tools/lint-pr-url.mjs <expected-pr-url>
+
+import process from 'node:process';
+import readline from 'node:readline';
+
+const [, , expectedPrUrl] = process.argv;
+
+const fileDelimiter = /^\+\+\+ b\/(.+\.md)$/;
+const changeDelimiter = /^@@ -\d+,\d+ \+(\d+),\d+ @@/;
+const prUrlDefinition = /^\+\s+pr-url: (.+)$/;
+
+const validatePrUrl = (url) => url == null || url === expectedPrUrl;
+
+let currentFile;
+let currentLine;
+
+const diff = readline.createInterface({ input: process.stdin });
+for await (const line of diff) {
+  if (fileDelimiter.test(line)) {
+    currentFile = line.match(fileDelimiter)[1];
+    console.log(`Parsing changes in ${currentFile}.`);
+  } else if (changeDelimiter.test(line)) {
+    currentLine = Number(line.match(changeDelimiter)[1]);
+  } else if (!validatePrUrl(line.match(prUrlDefinition)?.[1])) {
+    console.warn(
+      `::warning file=${currentFile},line=${currentLine++},col=${line.length}` +
+      '::pr-url doesn\'t match the actual PR URL.'
+    );
+  } else if (line[0] !== '-') {
+    // Increment line counter if line is not being deleted.
+    currentLine++;
+  }
+}