vfs: fix --vfs-cache-mode minimal,writes ignoring cached files

Before this change, with --vfs-cache-mode minimal,writes if files were
opened they would always be read from the remote, regardless of
whether they were in the cache or not.

This change checks to see if the file is in the cache when opening a
file with --vfs-cache-mode >= minimal and if so then it uses it from
the cache.

This makes --vfs-cache-mode writes in particular much more
efficient. No longer is a file uploaded (with write mode) then
immediately downloaded (with read only mode).

Fixes #3330

vfs/cache.go

@@ -263,6 +263,19 @@ func (c *cache) cacheDir(name string) {
 	}
 }
 
+// exists checks to see if the file exists in the cache or not
+func (c *cache) exists(name string) bool {
+	osPath := c.toOSPath(name)
+	fi, err := os.Stat(osPath)
+	if err != nil {
+		return false
+	}
+	if fi.IsDir() {
+		return false
+	}
+	return true
+}
+
 // _close marks name as closed - must be called with the lock held
 func (c *cache) _close(isFile bool, name string) {
 	for {

vfs/file.go

@@ -546,11 +546,9 @@ func (f *File) Open(flags int) (fd Handle, err error) {
 		write = true
 	}
 
-	// FIXME discover if file is in cache or not?
-
 	// Open the correct sort of handle
 	CacheMode := f.d.vfs.Opt.CacheMode
-	if CacheMode >= CacheModeMinimal && f.d.vfs.cache.opens(f.Path()) > 0 {
+	if CacheMode >= CacheModeMinimal && (f.d.vfs.cache.opens(f.Path()) > 0 || f.d.vfs.cache.exists(f.Path())) {
 		fd, err = f.openRW(flags)
 	} else if read && write {
 		if CacheMode >= CacheModeMinimal {