vfs: decouple writeback from Item so it can be tested

vfs/vfscache/item.go

@@ -608,7 +608,9 @@ func (item *Item) Close(storeFn StoreFn) (err error) {
 		} else {
 			// asynchronous writeback
 			item.mu.Unlock()
-			item.c.writeback.add(item, item.name, item.modified, storeFn)
+			item.c.writeback.add(item, item.name, item.modified, func(ctx context.Context) error {
+				return item.store(ctx, storeFn)
+			})
 			item.mu.Lock()
 		}
 	}

vfs/vfscache/writeback.go

@@ -17,6 +17,9 @@ const (
 	maxUploadDelay = 5 * time.Minute // max delay betwen upload attempts
 )
 
+// putFn is the interface that item provides to store the data
+type putFn func(context.Context) error
+
 // writeBack keeps track of the items which need to be written back to the disk at some point
 type writeBack struct {
 	mu      sync.Mutex
@@ -61,7 +64,7 @@ type writeBackItem struct {
 	onHeap    bool               // true if this item is on the items heap
 	cancel    context.CancelFunc // To cancel the upload with
 	done      chan struct{}      // closed when the cancellation completes
-	storeFn   StoreFn            // To write the object back with
+	putFn     putFn              // To write the object data
 	tries     int                // number of times we have tried to upload
 	delay     time.Duration      // delay between upload attempts
 }
@@ -204,7 +207,7 @@ func (wb *writeBack) _resetTimer() {
 // is already there.
 //
 // if modified is false then it it doesn't a pending upload
-func (wb *writeBack) add(item *Item, name string, modified bool, storeFn StoreFn) {
+func (wb *writeBack) add(item *Item, name string, modified bool, putFn putFn) {
 	wb.mu.Lock()
 	defer wb.mu.Unlock()
 
@@ -219,7 +222,7 @@ func (wb *writeBack) add(item *Item, name string, modified bool, storeFn StoreFn
 		// Kick the timer on
 		wb.items._update(wbItem, wb._newExpiry())
 	}
-	wbItem.storeFn = storeFn
+	wbItem.putFn = putFn
 	wb._resetTimer()
 }
 
@@ -264,11 +267,11 @@ func (wb *writeBack) _kickUploader() {
 func (wb *writeBack) upload(ctx context.Context, wbItem *writeBackItem) {
 	wb.mu.Lock()
 	defer wb.mu.Unlock()
-	item := wbItem.item
+	putFn := wbItem.putFn
 	wbItem.tries++
 
 	wb.mu.Unlock()
-	err := item.store(ctx, wbItem.storeFn)
+	err := putFn(ctx)
 	wb.mu.Lock()
 
 	wbItem.cancel() // cancel context to release resources since store done