Fix bug when calling freeze when freezing

Previously, when an attacher (or transformer?) called `.freeze`,
this could cause plugins to be invoked multiple times.
This commit fixed that. 😬

Closes remarkjs/remark-lint#131.

index.js

@@ -47,6 +47,7 @@ function unified() {
   var transformers = trough();
   var namespace = {};
   var frozen = false;
+  var freezeIndex = -1;
 
   /* Data management. */
   processor.data = data;
@@ -95,7 +96,6 @@ function unified() {
    * In essence, always invoke this when exporting a
    * processor. */
   function freeze() {
-    var index = -1;
     var values;
     var plugin;
     var options;
@@ -105,8 +105,8 @@ function unified() {
       return processor;
     }
 
-    while (++index < attachers.length) {
-      values = attachers[index];
+    while (++freezeIndex < attachers.length) {
+      values = attachers[freezeIndex];
       plugin = values[0];
       options = values[1];
       transformer = null;
@@ -127,6 +127,7 @@ function unified() {
     }
 
     frozen = true;
+    freezeIndex = Infinity;
 
     return processor;
   }

test/freeze.js

@@ -78,5 +78,26 @@ test('freeze()', function (t) {
     '`process` can be invoked on frozen interfaces'
   );
 
+  t.test('should freeze once, even for nested calls', function (st) {
+    st.plan(2);
+
+    var index = 0;
+    var p = unified()
+      .use(plugin)
+      .use({plugins: [freezingPlugin]})
+      .use({plugins: [freezingPlugin]})
+      .freeze();
+
+    p().freeze();
+
+    function plugin() {
+      st.pass('Expected: ' + String(index++));
+    }
+
+    function freezingPlugin() {
+      this.freeze();
+    }
+  });
+
   t.end();
 });