Diff features by fetching from GitHub Actions

This substantially speeds up feature diffing, where the enumeration has
already been done in GitHub Actions. Where possible, it trys to fetch
from GitHub Actions, or falls back to using a worktree to enumerate
directly.

See mdn#13695 for
background.

scripts/diff-features.js

@@ -20,8 +20,8 @@ function main({ ref1, ref2, format }) {
     refB = `${ref1}`;
   }
 
-  const aSide = new Set(enumerateFeatures(refA));
-  const bSide = new Set(enumerateFeatures(refB));
+  let aSide = enumerate(refA);
+  let bSide = enumerate(refB);
 
   const results = {
     added: [...bSide].filter(feature => !aSide.has(feature)),
@@ -35,6 +35,57 @@ function main({ ref1, ref2, format }) {
   }
 }
 
+function enumerate(ref) {
+  try {
+    return new Set(getEnumerationFromGithub(ref));
+  } catch {
+    console.error('Fetching artifact from GitHub failed. Using fallback.');
+    return new Set(enumerateFeatures(ref));
+  }
+}
+
+function getEnumerationFromGithub(ref) {
+  const ENUMERATE_WORKFLOW = '15595228';
+  const ENUMERATE_WORKFLOW_ARTIFACT = 'enumerate-features';
+  const ENUMERATE_WORKFLOW_FILE = 'features.json';
+
+  const unlinkFile = () => {
+    try {
+      fs.unlinkSync(ENUMERATE_WORKFLOW_FILE);
+    } catch (err) {
+      if (err.code == 'ENOENT') {
+        return;
+      } else {
+        throw err;
+      }
+    }
+  };
+
+  const hash = execSync(`git rev-parse ${ref}`, {
+    encoding: 'utf-8',
+  }).trim();
+  const workflowRun = execSync(
+    `gh api /repos/:owner/:repo/actions/workflows/${ENUMERATE_WORKFLOW}/runs --jq '.workflow_runs[] | select(.head_sha=="${hash}") | .id'`,
+    {
+      encoding: 'utf-8',
+    },
+  ).trim();
+
+  if (!workflowRun) throw Error('No workflow run found for commit.');
+
+  try {
+    unlinkFile();
+    execSync(
+      `gh run download ${workflowRun} -n ${ENUMERATE_WORKFLOW_ARTIFACT}`,
+    );
+    return JSON.parse(
+      fs.readFileSync(ENUMERATE_WORKFLOW_FILE, { encoding: 'utf-8' }),
+    );
+  } finally {
+    unlinkFile();
+  }
+}
+
 function enumerateFeatures(ref = 'HEAD') {
   // Get the short hash for this ref.
   // Most of the time, you check out named references (a branch or a tag).