Fix preview links for long branch names (#4469)

* Fix preview links for long branch names

When a branch name produces a subdomain exceeding 63 characters (the
DNS label limit), Vercel truncates it with a hash suffix. Our workflow
constructed the URL independently and got it wrong in those cases.

Now the workflow checks the subdomain length first. Short branch names
use the constructed URL directly with no delay. Long branch names poll
for the Vercel bot comment and extract the actual preview URL from its
base64 payload, falling back to the constructed URL after 3 minutes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Extract preview URL logic into bin/preview-url-from-vercel.js

Moves the inline script from the workflow into a standalone file
for easier maintenance. The workflow now requires it and calls
resolvePreviewUrl with the actions/github-script context.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: lennessyy

.github/workflows/docs-preview-links.yml

@@ -28,24 +28,14 @@ jobs:
 
       - name: Compute preview base URL
         id: preview-url
-        env:
-          REPO_PREVIEW_BASE_URL: ${{ vars.DOCS_PREVIEW_BASE_URL }}
-          REPO_PREVIEW_TEMPLATE: ${{ vars.DOCS_PREVIEW_BASE_URL_TEMPLATE }}
-        run: |
-          BRANCH_NAME="${GITHUB_HEAD_REF:-${GITHUB_REF_NAME}}"
-          BRANCH_SLUG=$(echo "$BRANCH_NAME" | tr '[:upper:]' '[:lower:]' | tr -d '_' | sed 's/[^a-z0-9]/-/g' | sed 's/-\+/-/g' | sed 's/^-//' | sed 's/-$//')
-
-          if [ -n "$REPO_PREVIEW_BASE_URL" ]; then
-            BASE_URL="$REPO_PREVIEW_BASE_URL"
-          elif [ -n "$REPO_PREVIEW_TEMPLATE" ]; then
-            BASE_URL="${REPO_PREVIEW_TEMPLATE//\{branch\}/$BRANCH_SLUG}"
-          else
-            BASE_URL="https://temporal-documentation-git-${BRANCH_SLUG}.preview.thundergun.io"
-          fi
-
-          echo "DOCS_PREVIEW_BASE_URL=$BASE_URL" >> "$GITHUB_ENV"
-          echo "BRANCH_SLUG=$BRANCH_SLUG" >> "$GITHUB_ENV"
-          echo "base_url=$BASE_URL" >> "$GITHUB_OUTPUT"
+        uses: actions/github-script@v7
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          script: |
+            const { resolvePreviewUrl } = require('./bin/preview-url-from-vercel.js');
+            const baseUrl = await resolvePreviewUrl({ github, context, core });
+            core.exportVariable('DOCS_PREVIEW_BASE_URL', baseUrl);
+            core.setOutput('base_url', baseUrl);
 
       - name: Generate docs preview list
         env:

bin/preview-url-from-vercel.js

@@ -0,0 +1,74 @@
+#!/usr/bin/env node
+
+const MAX_SUBDOMAIN_LENGTH = 63;
+const MAX_ATTEMPTS = 18;
+const INTERVAL_MS = 10_000;
+const PREVIEW_DOMAIN = 'preview.thundergun.io';
+
+function branchToSlug(branchName) {
+  return branchName
+    .toLowerCase()
+    .replace(/_/g, '')
+    .replace(/[^a-z0-9]/g, '-')
+    .replace(/-+/g, '-')
+    .replace(/^-/, '')
+    .replace(/-$/, '');
+}
+
+function buildSubdomain(branchSlug) {
+  return `temporal-documentation-git-${branchSlug}`;
+}
+
+function extractPreviewUrlFromComment(body) {
+  const match = body.match(/\[vc\]:\s*#[^:]*:(eyJ[A-Za-z0-9+/=]+)/);
+  if (!match) return null;
+
+  const payload = JSON.parse(Buffer.from(match[1], 'base64').toString('utf8'));
+  return payload.projects?.[0]?.previewUrl || null;
+}
+
+async function resolvePreviewUrl({ github, context, core }) {
+  const branchName = process.env.GITHUB_HEAD_REF || process.env.GITHUB_REF_NAME;
+  const branchSlug = branchToSlug(branchName);
+  const subdomain = buildSubdomain(branchSlug);
+
+  if (subdomain.length <= MAX_SUBDOMAIN_LENGTH) {
+    const baseUrl = `https://${subdomain}.${PREVIEW_DOMAIN}`;
+    core.info(`Branch name is short enough (${subdomain.length} chars), using constructed URL: ${baseUrl}`);
+    return baseUrl;
+  }
+
+  core.info(`Subdomain would be ${subdomain.length} chars (exceeds ${MAX_SUBDOMAIN_LENGTH}), polling for Vercel comment...`);
+
+  for (let attempt = 1; attempt <= MAX_ATTEMPTS; attempt++) {
+    core.info(`Polling for Vercel comment (attempt ${attempt}/${MAX_ATTEMPTS})...`);
+    const { data: comments } = await github.rest.issues.listComments({
+      owner: context.repo.owner,
+      repo: context.repo.repo,
+      issue_number: context.issue.number,
+      per_page: 100,
+    });
+
+    const vercelComment = comments.find(
+      (c) => c.user?.login === 'vercel[bot]' && c.body?.includes('[vc]:'),
+    );
+
+    if (vercelComment) {
+      const previewUrl = extractPreviewUrlFromComment(vercelComment.body);
+      if (previewUrl) {
+        const baseUrl = `https://${previewUrl}`;
+        core.info(`Found Vercel preview URL: ${baseUrl}`);
+        return baseUrl;
+      }
+    }
+
+    if (attempt < MAX_ATTEMPTS) {
+      await new Promise((resolve) => setTimeout(resolve, INTERVAL_MS));
+    }
+  }
+
+  core.warning('Vercel comment not found after polling, using constructed URL as fallback (links may be broken for this long branch name)');
+  return `https://${subdomain}.${PREVIEW_DOMAIN}`;
+}
+
+module.exports = { resolvePreviewUrl, branchToSlug, buildSubdomain, extractPreviewUrlFromComment };