Avoid env var reference loops and support $parent. prefix

Fixes #2099

packages/node_modules/@node-red/runtime/lib/nodes/flows/Flow.js

@@ -68,6 +68,20 @@ class Flow {
         })
     }
 
+    /**
+     * Log an error-level message from this flow
+     * @param  {[type]} msg [description]
+     * @return {[type]}     [description]
+     */
+    error(msg) {
+        Log.log({
+            id: this.id||"global",
+            level: Log.ERROR,
+            type:this.TYPE,
+            msg:msg
+        })
+    }
+
     /**
      * Log a info-level message from this flow
      * @param  {[type]} msg [description]

packages/node_modules/@node-red/runtime/lib/nodes/flows/Subflow.js

@@ -263,17 +263,37 @@ class Subflow extends Flow {
      * @return {Object}  val    value of env var
      */
     getSetting(name) {
+        this.trace("getSetting:"+name);
+        if (!/^\$parent\./.test(name)) {
             var env = this.env;
             if (env && env.hasOwnProperty(name)) {
                 var val = env[name];
+                // If this is an env type property we need to be careful not
+                // to get into lookup loops.
+                // 1. if the value to lookup is the same as this one, go straight to parent
+                // 2. otherwise, check if it is a compound env var ("foo $(bar)")
+                //    and if so, substitute any instances of `name` with $parent.name
+                // See https://github.com/node-red/node-red/issues/2099
+                if (val.type !== 'env' || val.value !== name) {
+                    let value = val.value;
+                    if (val.type === 'env') {
+                        value = value.replace(new RegExp("\\${"+name+"}","g"),"${$parent."+name+"}");
+                    }
                     try {
-                var ret = redUtil.evaluateNodeProperty(val.value, val.type, this.node, null, null);
+                        var ret = redUtil.evaluateNodeProperty(value, val.type, this.node, null, null);
                         return ret;
                     }
                     catch (e) {
                         this.error(e);
                         return undefined;
                     }
+                } else {
+                    // This _is_ an env property pointing at itself - go to parent
+                }
+            }
+        } else {
+            // name starts $parent. ... so delegate to parent automatically
+            name = name.substring(8);
         }
         var parent = this.parent;
         if (parent) {