fix(transaction): only rollback if connection was acquired (#9051)

lib/transaction.js

@@ -121,13 +121,15 @@ class Transaction {
         this.connection = connection;
         this.connection.uuid = this.id;
       })
-      .then(() => this.begin())
-      .then(() => this.setDeferrable())
-      .then(() => this.setIsolationLevel())
-      .then(() => this.setAutocommit())
-      .catch(setupErr => this.rollback().finally(() => {
-        throw setupErr;
-      }))
+      .then(() => {
+        return this.begin()
+          .then(() => this.setDeferrable())
+          .then(() => this.setIsolationLevel())
+          .then(() => this.setAutocommit())
+          .catch(setupErr => this.rollback().finally(() => {
+            throw setupErr;
+          }));
+      })
       .tap(() => {
         if (useCLS && this.sequelize.constructor._cls) {
           this.sequelize.constructor._cls.set('transaction', this);

test/integration/pool.test.js

@@ -33,4 +33,22 @@ describe(Support.getTestDialectTeaser('Pooling'), function() {
     return expect(this.testInstance.authenticate())
       .to.eventually.be.rejectedWith('ResourceRequest timed out');
   });
+  
+  it('should not result in unhandled promise rejection when unable to acquire connection', () => {
+    this.testInstance = new Sequelize('localhost', 'ffd', 'dfdf', {
+      dialect,
+      databaseVersion: '1.2.3',
+      pool: {
+        acquire: 1000,
+        max: 1
+      }
+    });
+
+    this.sinon.stub(this.testInstance.connectionManager, '_connect')
+      .returns(new Sequelize.Promise(() => {}));
+
+    return expect(this.testInstance.transaction()
+      .then(() => this.testInstance.transaction()))
+      .to.eventually.be.rejectedWith('ResourceRequest timed out');
+  });
 });