fix an incorrect condition when checking for available server workers

Also, log when the RPC queue has a backlog and when it is drained

lib/workers/index.js

@@ -67,13 +67,14 @@ Workers.initialize = function (Env, config, _cb) {
     but this is a relatively easy way to make sure it's always up to date.
     We'll see how it performs in practice before optimizing.
 */
-            if (workers[temp] && Object.keys(workers[temp]).length < MAX_JOBS) {
+            if (workers[temp] && Object.keys(workers[temp].tasks || {}).length < MAX_JOBS) {
                 return temp;
             }
         }
         return -1;
     };
 
+    var drained = true;
     var sendCommand = function (msg, _cb) {
         var index = getAvailableWorkerIndex();
 
@@ -85,6 +86,13 @@ Workers.initialize = function (Env, config, _cb) {
                 msg: msg,
                 cb: _cb,
             });
+            if (drained) {
+                drained = false;
+                Log.debug('WORKER_QUEUE_BACKLOG', {
+                    workers: workers.length,
+                });
+            }
+
             return;
         }
 
@@ -117,7 +125,15 @@ Workers.initialize = function (Env, config, _cb) {
         if (!res.txid) { return; }
         response.handle(res.txid, [res.error, res.value]);
         delete state.tasks[res.txid];
-        if (!queue.length) { return; }
+        if (!queue.length) {
+            if (!drained) {
+                drained = true;
+                Log.debug('WORKER_QUEUE_DRAINED', {
+                    workers: workers.length,
+                });
+            }
+            return;
+        }
 
         var nextMsg = queue.shift();
 /*  `nextMsg` was at the top of the queue.