build(aio): add checkContentRules processor (#22759)

This processor will enable us to write rules about
how the content should appear, such as:

* no headings in markdown content
* only one sentence per line
* no single character parameter names
* etc.

PR Close #22759

aio/tools/transforms/angular-base-package/processors/checkContentRules.js

@@ -0,0 +1,69 @@
+
+/**
+ * A processor that can run arbitrary checking rules against properties of documents
+
+ * The configuration for the processor is via the `docTypeRules`.
+ * This is a hash of docTypes to rulesets.
+ * Each rules set is a hash of properties to rule functions.
+ *
+ * The processor will run each rule function against each matching property of each matching doc.
+ *
+ * An example rule might look like:
+ *
+ * ```
+ * function noMarkdownHeadings(doc, prop, value) {
+ *   const match = /^\s?#+\s+.*$/m.exec(value);
+ *   if (match) {
+ *     return `Headings not allowed in "${prop}" property. Found "${match[0]}"`;
+ *   }
+ * }
+ * ```
+ *
+ */
+module.exports = function checkContentRules(log, createDocMessage) {
+  return {
+    /**
+     * {
+     *   [docType]: {
+     *     [property]: Array<(doc: Document, property: string, value: any) => string|undefined>
+     *   }
+     * }
+     */
+    docTypeRules: {},
+    failOnContentErrors: false,
+    $runAfter: ['tags-extracted'],
+    $runBefore: ['processing-docs'],
+    $process(docs) {
+      const errors = [];
+      docs.forEach(doc => {
+        const docErrors = [];
+        const rules = this.docTypeRules[doc.docType] || {};
+        if (rules) {
+          Object.keys(rules).forEach(property => {
+            const ruleFns = rules[property];
+            ruleFns.forEach(ruleFn => {
+              const error = ruleFn(doc, property, doc[property]);
+              if (error) {
+                docErrors.push(error);
+              }
+            });
+          });
+        }
+        if (docErrors.length) {
+          errors.push({ doc, errors: docErrors });
+        }
+      });
+
+      if (errors.length) {
+        log.error('Content contains errors');
+        errors.forEach(docError => {
+          const errors = docError.errors.join('\n        ');
+          log.error(createDocMessage(errors + '\n        ', docError.doc));
+        });
+        if (this.failOnContentErrors) {
+          throw new Error('Stopping due to content errors.');
+        }
+      }
+    }
+  };
+};