fix(HttpClient): semaphore emitResult

lib/HttpClient.js

@@ -289,6 +289,10 @@ function rawRequest(opts, cb) {
     // the inflight request counter only ever decrements once for a given
     // request.
     var hasDecrementedInflightCounter = false;
+    // We emit result when response is ended or request has an error. Race
+    // condition can happen between the two, so we use a semaphore to ensure
+    // it is only emitted once.
+    var hasResultEmitted = false;
     req = proto.request(opts, function onResponse(res) {
         var latency = Date.now() - requestTime;
         res.headers['x-request-received'] = requestTime;
@@ -343,7 +347,10 @@ function rawRequest(opts, cb) {
             emitAfter(req, res, err);
         });
 
-        emitResult((err || null), req, res);
+        if (!hasResultEmitted) {
+            hasResultEmitted = true;
+            emitResult((err || null), req, res);
+        }
 
         // The 'readable' event listener has to be added after we emit the
         // 'result' event to keep the response stream readable in flowing mode.
@@ -423,7 +430,10 @@ function rawRequest(opts, cb) {
         cb(realErr, req);
 
         process.nextTick(function () {
-            emitResult(realErr, req, null);
+            if (!hasResultEmitted) {
+                hasResultEmitted = true;
+                emitResult(realErr, req, null);
+            }
             emitAfter(req, null, realErr);
         });
     });