vfs: fix modtime changing when reading file into cache - fixes #5277

Before this change but after:

aea8776a43 vfs: fix modtimes not updating when writing via cache #4763

When a file was opened read-only the modtime was read from the cached
file. However this modtime wasn't correct leading to an incorrect
result.

This change fixes the definition of `item.IsDirty` to be true only
when the data is dirty. This fixes the problem as a read only file
isn't considered dirty.

vfs/vfscache/cache.go

@@ -549,7 +549,7 @@ func (c *Cache) purgeClean(quota int64) {
 
 	// Make a slice of clean cache files
 	for _, item := range c.item {
-		if !item.IsDataDirty() {
+		if !item.IsDirty() {
 			items = append(items, item)
 		}
 	}

vfs/vfscache/item.go

@@ -450,15 +450,8 @@ func (item *Item) Dirty() {
 	item.mu.Unlock()
 }
 
-// IsDirty returns true if the item is dirty
+// IsDirty returns true if the item data is dirty
 func (item *Item) IsDirty() bool {
-	item.mu.Lock()
-	defer item.mu.Unlock()
-	return item.metaDirty || item.info.Dirty
-}
-
-// IsDataDirty returns true if the item's data is dirty
-func (item *Item) IsDataDirty() bool {
 	item.mu.Lock()
 	defer item.mu.Unlock()
 	return item.info.Dirty