From 4b9610c09c7c6a0ca784db750e4a47c6499f8905 Mon Sep 17 00:00:00 2001
From: Will Norris <wnorris@twitter.com>
Date: Tue, 30 Mar 2021 13:27:13 -0700
Subject: [PATCH] add 'extends' property for extending ruleset

---
 lib/config.js        | 28 ++++++++++++++++++++++++++--
 rulesets/schema.json |  4 ++++
 2 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/lib/config.js b/lib/config.js
index 645a118d..db25cece 100644
--- a/lib/config.js
+++ b/lib/config.js
@@ -6,6 +6,7 @@ const fetch = require('node-fetch')
 const findConfig = require('find-config')
 const fs = require('fs')
 const jsonfile = require('jsonfile')
+const lodash = require('lodash')
 const path = require('path')
 const yaml = require('js-yaml')
 
@@ -26,11 +27,12 @@ function isAbsoluteURL(url) {
 /**
  * Load a ruleset config from the specified location.
  *
- * @param {string} configLocation A URL, file location, or directory containing a repolinter config file
+ * @param {string} [configLocation] A URL, file location, or directory containing a repolinter config file
+ * @param {array} [processed] list of config files already processed, used to prevent loops
  * @returns {Object} The loaded repolinter json config
  * @throws Will throw an error if unable to parse config or if config is invalid
  */
-async function loadConfig(configLocation) {
+async function loadConfig(configLocation, processed = []) {
   if (configLocation == null) {
     throw new Error('must specify config location')
   }
@@ -74,6 +76,28 @@ async function loadConfig(configLocation) {
     }
   }
 
+  // merge extended rulesets
+  if (ruleset.extends) {
+    processed.push(configLocation)
+    if (processed.length > 20) {
+      // safeguard against infinite loops. expose as flag one day if needed
+      throw new Error('exceeded maximum 20 ruleset extensions')
+    }
+
+    let parent
+    if (isAbsoluteURL(ruleset.extends)) {
+      parent = ruleset.extends
+    } else if (isAbsoluteURL(configLocation)) {
+      parent = new URL(ruleset.extends, configLocation)
+    } else {
+      parent = path.resolve(path.dirname(configLocation), ruleset.extends)
+    }
+    if (!processed.includes(parent)) {
+      const parentRuleset = await loadConfig(parent, processed)
+      ruleset = lodash.merge({}, parentRuleset, ruleset)
+    }
+  }
+
   return ruleset
 }
 
diff --git a/rulesets/schema.json b/rulesets/schema.json
index 9e4a176e..8a9922b6 100644
--- a/rulesets/schema.json
+++ b/rulesets/schema.json
@@ -14,6 +14,10 @@
     "properties": {
       "$schema": { "type": "string" },
       "version": { "const": 2 },
+      "extends": {
+        "type": "string",
+        "title": "URL or path of ruleset file this ruleset extends"
+      },
       "axioms": {
         "type": "object",
         "title": "The axioms schema",