lib/cache: fix locking so we don't try to create the item many times

Before this fix, if several Get requests were submitted very quickly,
this could run the item create function multiple times due to the
unlock of the mutex in the creation code.

This fixes the problem by having a mutex in each cache entry which is
held when the item is being created.

lib/cache/cache_test.go

@@ -3,6 +3,8 @@ package cache
 import (
 	"errors"
 	"fmt"
+	"sync"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -11,7 +13,7 @@ import (
 )
 
 var (
-	called      = 0
+	called      = int32(0)
 	errSentinel = errors.New("an error")
 	errCached   = errors.New("a cached error")
 )
@@ -19,17 +21,19 @@ var (
 func setup(t *testing.T) (*Cache, CreateFunc) {
 	called = 0
 	create := func(path string) (interface{}, bool, error) {
-		assert.Equal(t, 0, called)
-		called++
+		newCalled := atomic.AddInt32(&called, 1)
+		assert.Equal(t, int32(1), newCalled)
 		switch path {
 		case "/":
+			time.Sleep(100 * time.Millisecond)
 			return "/", true, nil
 		case "/file.txt":
 			return "/file.txt", true, errCached
 		case "/error":
 			return nil, false, errSentinel
 		}
-		panic(fmt.Sprintf("Unknown path %q", path))
+		assert.Fail(t, fmt.Sprintf("Unknown path %q", path))
+		return nil, false, nil
 	}
 	c := New()
 	return c, create
@@ -51,6 +55,24 @@ func TestGet(t *testing.T) {
 	assert.Equal(t, f, f2)
 }
 
+func TestGetConcurrent(t *testing.T) {
+	c, create := setup(t)
+	assert.Equal(t, 0, len(c.cache))
+
+	var wg sync.WaitGroup
+	for i := 0; i < 100; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			_, err := c.Get("/", create)
+			require.NoError(t, err)
+		}()
+	}
+	wg.Wait()
+
+	assert.Equal(t, 1, len(c.cache))
+}
+
 func TestGetFile(t *testing.T) {
 	c, create := setup(t)