🏗 Ignore markdown templates in link checker by convention (ampproject…

…#25226)

Also some modernizing and cleanup.

(Offshoot of ampproject#25164)

build-system/tasks/check-links.js

@@ -51,68 +51,51 @@ function getMarkdownFiles() {
 async function checkLinks() {
   maybeUpdatePackages();
   const markdownFiles = getMarkdownFiles();
-  const linkCheckers = markdownFiles.map(function(markdownFile) {
-    return runLinkChecker(markdownFile);
-  });
-  return BBPromise.all(linkCheckers).then(function(allResults) {
-    let deadLinksFound = false;
-    const filesWithDeadLinks = [];
-    allResults.map(function(results, index) {
-      // Skip files that were deleted by the PR.
-      if (!fs.existsSync(markdownFiles[index])) {
-        return;
-      }
+  const allResults = await Promise.all(markdownFiles.map(runLinkChecker));
+
+  const filesWithDeadLinks = allResults
+    .map((results, index) => {
       let deadLinksFoundInFile = false;
-      results.forEach(function(result) {
+      for (const {link, status, statusCode} of results || []) {
         // Skip links to files that were introduced by the PR.
-        if (isLinkToFileIntroducedByPR(result.link)) {
-          return;
+        if (isLinkToFileIntroducedByPR(link)) {
+          continue;
         }
-        if (result.status === 'dead') {
-          deadLinksFound = true;
+        if (status === 'dead') {
           deadLinksFoundInFile = true;
-          log(`[${red('✖')}] ${result.link} (${red(result.statusCode)})`);
+          log(`[${red('✖')}] ${link} (${red(statusCode)})`);
         } else if (!isTravisBuild()) {
-          log(`[${green('✔')}] ${result.link}`);
+          log(`[${green('✔')}] ${link}`);
         }
-      });
+      }
+      const filename = markdownFiles[index];
       if (deadLinksFoundInFile) {
-        filesWithDeadLinks.push(markdownFiles[index]);
-        log(
-          red('ERROR'),
-          'Possible dead link(s) found in',
-          magenta(markdownFiles[index])
-        );
-      } else {
-        log(
-          green('SUCCESS'),
-          'All links in',
-          magenta(markdownFiles[index]),
-          'are alive.'
-        );
+        log(red('ERROR'), 'Possible dead link(s) found in', magenta(filename));
+        return filename;
       }
-    });
-    if (deadLinksFound) {
-      log(
-        red('ERROR'),
-        'Please update dead link(s) in',
-        magenta(filesWithDeadLinks.join(',')),
-        'or whitelist them in build-system/tasks/check-links.js'
-      );
-      log(
-        yellow('NOTE'),
-        'If the link(s) above are not meant to resolve to a real webpage',
-        'surrounding them with backticks will exempt them from the link',
-        'checker.'
-      );
-      process.exitCode = 1;
-    } else {
-      log(
-        green('SUCCESS'),
-        'All links in all markdown files in this branch are alive.'
-      );
-    }
-  });
+      log(green('SUCCESS'), 'All links in', magenta(filename), 'are alive.');
+    })
+    .filter(filenameOrUndef => filenameOrUndef);
+
+  if (filesWithDeadLinks.length > 0) {
+    log(
+      red('ERROR'),
+      'Please update dead link(s) in',
+      magenta(filesWithDeadLinks.join(',')),
+      'or add them to allow-list in build-system/tasks/check-links.js'
+    );
+    log(
+      yellow('NOTE'),
+      'If the link(s) above are not meant to resolve to a real webpage,',
+      'surrounding them with backticks will exempt them from the link checker.'
+    );
+    process.exitCode = 1;
+    return;
+  }
+  log(
+    green('SUCCESS'),
+    'All links in all markdown files in this branch are alive.'
+  );
 }
 
 /**
@@ -128,12 +111,12 @@ function isLinkToFileIntroducedByPR(link) {
 }
 
 /**
- * Filters out whitelisted links before running the link checker.
+ * Filters out links in allow-list before running the link checker.
  *
  * @param {string} markdown Original markdown.
- * @return {string} Markdown after filtering out whitelisted links.
+ * @return {string} Markdown after filtering out allowed links.
  */
-function filterWhitelistedLinks(markdown) {
+function filterAllowedLinks(markdown) {
   let filteredMarkdown = markdown;
 
   // localhost links optionally preceded by ( or [ (not served on Travis)
@@ -151,7 +134,7 @@ function filterWhitelistedLinks(markdown) {
   // Links inside a <pre> block (illustrative, and not always valid)
   filteredMarkdown = filteredMarkdown.replace(/<pre>([^]*?)<\/pre>/g, '');
 
-  // After all whitelisting is done, clean up any remaining empty blocks bounded
+  // After allow-listing is done, clean up any remaining empty blocks bounded
   // by backticks. Otherwise, `` will be treated as the start of a code block
   // and confuse the link extractor.
   filteredMarkdown = filteredMarkdown.replace(/\ \`\`\ /g, '');
@@ -167,12 +150,17 @@ function filterWhitelistedLinks(markdown) {
  * @return {Promise} Used to wait until the async link checker is done.
  */
 function runLinkChecker(markdownFile) {
+  // `.template.md` is a common suffix for files that may have interpolation
+  // tokens, possibly as part of their links. So we skip them.
+  if (path.basename(markdownFile).endsWith('.template.md')) {
+    return Promise.resolve();
+  }
   // Skip files that were deleted by the PR.
   if (!fs.existsSync(markdownFile)) {
     return Promise.resolve();
   }
   const markdown = fs.readFileSync(markdownFile).toString();
-  const filteredMarkdown = filterWhitelistedLinks(markdown);
+  const filteredMarkdown = filterAllowedLinks(markdown);
   const opts = {
     baseUrl: 'file://' + path.dirname(path.resolve(markdownFile)),
   };