fix(execute-operation): return promise on session support check

3.3.0 introduced an extra roundtrip of selection in order to
accurately determine if sessions were supported by the topology.
Unfortunately, the fix only supported callbacks. So, if a server
wasn't actually available immediately, we would return `undefined`
in cases when promises were requested for operation execution.

NODE-2136

Author: mbroadst

lib/operations/execute_operation.js

@@ -35,18 +35,7 @@ function executeOperation(topology, operation, callback) {
     !operation.hasAspect(Aspect.SKIP_SESSION) &&
     topology.shouldCheckForSessionSupport()
   ) {
-    // TODO: this is only supported for unified topology, the first part of this check
-    //       should go away when we drop legacy topology types.
-    topology.selectServer(ReadPreference.primaryPreferred, err => {
-      if (err) {
-        callback(err);
-        return;
-      }
-
-      executeOperation(topology, operation, callback);
-    });
-
-    return;
+    return selectServerForSessionSupport(topology, operation, callback);
   }
 
   const Promise = topology.s.promiseLibrary;
@@ -179,4 +168,31 @@ function executeWithServerSelection(topology, operation, callback) {
   });
 }
 
+// TODO: This is only supported for unified topology, it should go away once
+//       we remove support for legacy topology types.
+function selectServerForSessionSupport(topology, operation, callback) {
+  const Promise = topology.s.promiseLibrary;
+
+  let result;
+  if (typeof callback !== 'function') {
+    result = new Promise((resolve, reject) => {
+      callback = (err, result) => {
+        if (err) return reject(err);
+        resolve(result);
+      };
+    });
+  }
+
+  topology.selectServer(ReadPreference.primaryPreferred, err => {
+    if (err) {
+      callback(err);
+      return;
+    }
+
+    executeOperation(topology, operation, callback);
+  });
+
+  return result;
+}
+
 module.exports = executeOperation;