Additional logs and tweak to tests

src/service.js

@@ -108,6 +108,8 @@ let Service = function Service (Transports, config) {
 
       function go (data) {
         // Handle both callbacks and promises here.
+        // It is expected that the CALLER only calls the callback OR
+        // returns a promise. If they do both, there will be two callbacks
         let callbackHandler = function (err, response) {
           if (err) return cb(err)
           return finish(response)
@@ -116,7 +118,6 @@ let Service = function Service (Transports, config) {
         let response = _tmp.fn(data, callbackHandler)
 
         if (response && typeof response.then === 'function') {
-          callbackHandler = null
           return response.then(finish, cb)
           // if an exception was thrown from finish(), let it bubble up
           // as an unhandled rejection, rather than also call cb()

test/z-integration.js

@@ -11,9 +11,9 @@ const Transports = require('../src/transports');
 
 describe("Integrations", function () {
   it("does not double call when callback throws", function (done) {
-    var transport = new Transports.Local();
-    var service = new Service(transport);
-    var client = new Client({ s: transport });
+    const transport = new Transports.Local();
+    const service = new Service(transport);
+    const client = new Client({ s: transport });
 
     let countCalled = 0
 
@@ -32,7 +32,7 @@ describe("Integrations", function () {
       switch (countCalled) {
         case 0: throw Error('did not get called at all')
         case 1: return done()
-        case 2: throw Error('called multiple times')
+        default: throw Error('called multiple times')
       }
     }, 100)
   });