Fix data race in downloads gathering (#217)

* Fix data race in downloads gathering

* Run tests with race flag

Makefile

@@ -47,4 +47,4 @@ lint: ## Lint the code
 	golint `go list ./... | grep -v /vendor/`
 
 test: ## Run tests.
-	go test -v `go list ./... | grep -v /vendor/`
+	go test -v -race ./...

models/line_item.go

@@ -285,7 +285,9 @@ func (i *LineItem) Process(config *conf.Configuration, userClaims map[string]int
 		i.AddonItems[index].Price = lowestPrice.cents
 	}
 
+	order.ModificationLock.Lock()
 	order.Downloads = append(order.Downloads, i.MissingDownloads(order, meta)...)
+	order.ModificationLock.Unlock()
 
 	return i.calculatePrice(userClaims, meta.Prices, order.Currency)
 }

models/order.go

@@ -3,6 +3,7 @@ package models
 import (
 	"encoding/json"
 	"fmt"
+	"sync"
 	"time"
 
 	"github.com/jinzhu/gorm"
@@ -104,6 +105,8 @@ type Order struct {
 	CreatedAt time.Time  `json:"created_at" sql:"index"`
 	UpdatedAt time.Time  `json:"updated_at"`
 	DeletedAt *time.Time `json:"-" sql:"index"`
+
+	ModificationLock sync.Mutex `json:"-" sql:"-"`
 }
 
 // TableName returns the database table name for the Order model.