fix: pipeline with transactions causes unhandled warnings (#884)

Pipeline#exec() is a little tricky: it has a init process when
called first time (indicated by whether Pipeline#nodeifiedPromise
is true). Transaction#exec() is similar to that, it has a init
process that parse the result from Pipeline#exec before returning
to users. However, #nodeifiedPromise is not checked here.

Closes: #883

lib/transaction.ts

@@ -31,6 +31,12 @@ export function addTransactionSupport (redis) {
       if (this._transactions > 0) {
         exec.call(pipeline)
       }
+
+      // Returns directly when the pipeline
+      // has been called multiple times (retries).
+      if (this.nodeifiedPromise) {
+        return exec.call(pipeline)
+      }
       const promise = exec.call(pipeline)
       return asCallback(promise.then(function (result) {
         const execResult = result[result.length - 1]

test/functional/cluster/transaction.js

@@ -36,6 +36,7 @@ describe('cluster:transaction', function () {
     var cluster = new Redis.Cluster([
       { host: '127.0.0.1', port: '30001' }
     ]);
+
     cluster.multi().get('foo').set('foo', 'bar').exec(function (err, result) {
       expect(err).to.eql(null);
       expect(result[0]).to.eql([null, 'bar']);
@@ -97,4 +98,45 @@ describe('cluster:transaction', function () {
       done();
     });
   });
+
+  it('should not print unhandled warnings', function (done) {
+    const errorMessage = 'Connection is closed.'
+    var slotTable = [
+      [0, 16383, ['127.0.0.1', 30001]]
+    ];
+    new MockServer(30001, function (argv) {
+      if (argv[0] === 'exec' || argv[1] === 'foo') {
+        return new Error(errorMessage)
+      }
+    }, slotTable);
+
+    var cluster = new Redis.Cluster([
+      { host: '127.0.0.1', port: '30001' }
+    ], {
+      maxRedirections: 3
+    });
+
+    let isDoneCalled = false
+    const wrapDone = function (error) {
+      if (isDoneCalled) {
+        return
+      }
+      isDoneCalled = true
+      process.removeAllListeners('unhandledRejection')
+      done(error)
+    }
+
+    process.on('unhandledRejection', err => {
+      wrapDone(new Error('got unhandledRejection: ' + err.message))
+    })
+    cluster.multi().get('foo').set('foo', 'bar').exec(function (err) {
+      expect(err).to.have.property('message', errorMessage)
+      cluster.on('end', function () {
+        // Wait for the end event to ensure the transaction
+        // promise has been resolved.
+        wrapDone();
+      })
+      cluster.disconnect();
+    });
+  });
 });