fix(cluster): prefer master when there're two same node for a slot

lib/cluster/ConnectionPool.ts

@@ -123,7 +123,13 @@ export default class ConnectionPool extends EventEmitter {
     debug("Reset with %O", nodes);
     const newNodes = {};
     nodes.forEach(node => {
-      newNodes[getNodeKey(node)] = node;
+      const key = getNodeKey(node);
+
+      // Don't override the existing (master) node
+      // when the current one is slave.
+      if (!(node.readOnly && newNodes[key])) {
+        newNodes[key] = node;
+      }
     });
 
     Object.keys(this.nodes.all).forEach(key => {

test/unit/clusters/ConnectionPool.ts

@@ -0,0 +1,28 @@
+import * as sinon from "sinon";
+import { expect } from "chai";
+import ConnectionPool from "../../../lib/cluster/ConnectionPool";
+
+describe("ConnectionPool", () => {
+  describe("#reset", () => {
+    it("prefers to master if there are two same node for a slot", () => {
+      const pool = new ConnectionPool({});
+      const stub = sinon.stub(pool, "findOrCreate");
+
+      pool.reset([
+        { host: "127.0.0.1", port: 30001, readOnly: true },
+        { host: "127.0.0.1", port: 30001, readOnly: false }
+      ]);
+
+      expect(stub.callCount).to.eql(1);
+      expect(stub.firstCall.args[1]).to.eql(false);
+
+      pool.reset([
+        { host: "127.0.0.1", port: 30001, readOnly: false },
+        { host: "127.0.0.1", port: 30001, readOnly: true }
+      ]);
+
+      expect(stub.callCount).to.eql(2);
+      expect(stub.firstCall.args[1]).to.eql(false);
+    });
+  });
+});

test/unit/cluster.ts → test/unit/clusters/index.ts

@@ -1,5 +1,5 @@
-import { nodeKeyToRedisOptions } from "../../lib/cluster/util";
-import { Cluster } from "../../lib";
+import { nodeKeyToRedisOptions } from "../../../lib/cluster/util";
+import { Cluster } from "../../../lib";
 import * as sinon from "sinon";
 import { expect } from "chai";