test: introduce defer helper for resource cleanup in tests

This adds a new `defer` method to the mocha test context which
allows us to call arbitrary async functions directly after a test
has completed running.

Author: mbroadst

test/runner/index.js

@@ -95,5 +95,6 @@ before(function(_done) {
 after(() => mock.cleanup());
 
 // optionally enable test runner-wide plugins
+require('./plugins/deferred');
 require('./plugins/session_leak_checker');
 require('./plugins/client_leak_checker');

test/runner/plugins/deferred.js

@@ -0,0 +1,50 @@
+'use strict';
+const kDeferred = Symbol('deferred');
+
+(mocha => {
+  const Context = mocha.Context;
+  function makeExecuteDeferred(test) {
+    return () => {
+      const deferredActions = test[kDeferred];
+
+      return Promise.all(
+        Array.from(deferredActions).map(action => {
+          if (action.length > 0) {
+            // assume these are async methods with provided `done`
+            return new Promise((resolve, reject) => {
+              function done(err) {
+                if (err) return reject(err);
+                resolve();
+              }
+
+              action(done);
+            });
+          }
+
+          // otherwise assume a Promise is returned
+          action();
+        })
+      ).then(() => {
+        test[kDeferred].clear();
+      });
+    };
+  }
+
+  Context.prototype.defer = function(fn) {
+    const test = this.test;
+    if (test[kDeferred] == null) {
+      test[kDeferred] = new Set();
+
+      const parentSuite = test.parent;
+      const afterEachHooks = parentSuite._afterEach;
+      if (afterEachHooks[0] == null || afterEachHooks[0].title !== kDeferred) {
+        const deferredHook = parentSuite._createHook('"deferred" hook', makeExecuteDeferred(test));
+
+        afterEachHooks.unshift(deferredHook);
+      }
+    }
+
+    test[kDeferred].add(fn);
+    return this;
+  };
+})(require('mocha'));