vfs: fix download loop when file size shrunk

Before this change, if a file shrunk in size on the remote then rclone
could get into an loop trying to download the file forever.

The symptom was repeating errors like this:

    vfs cache: restart download failed: failed to start downloader: failed to open downloader: vfs reader: failed to open source file: invalid seek position

The fix was to check that file size in various places and makes sure
that we weren't trying to download too much data.

This was a problems with backends (like s3) which update the size of
the object on Open to the actual size of the object.

vfs/vfscache/downloaders/downloaders.go

@@ -359,6 +359,10 @@ func (dls *Downloaders) _ensureDownloader(r ranges.Range) (err error) {
 	if !startNew {
 		return nil
 	}
+	// Size can be 0 here if file shrinks - no need to download
+	if r.Size == 0 {
+		return nil
+	}
 	// Downloader not found so start a new one
 	_, err = dls._newDownloader(r)
 	if err != nil {
@@ -389,7 +393,10 @@ func (dls *Downloaders) _dispatchWaiters() {
 
 	newWaiters := dls.waiters[:0]
 	for _, waiter := range dls.waiters {
-		if dls.item.HasRange(waiter.r) {
+		// Clip the size against the actual size in case it has shrunk
+		r := waiter.r
+		r.Clip(dls.src.Size())
+		if dls.item.HasRange(r) {
 			waiter.errChan <- nil
 		} else {
 			newWaiters = append(newWaiters, waiter)

vfs/vfscache/item.go

@@ -1279,6 +1279,15 @@ func (item *Item) readAt(b []byte, off int64) (n int, err error) {
 		return 0, err
 	}
 
+	// Check to see if object has shrunk - if so don't read too much.
+	if item.o != nil && !item.info.Dirty && item.o.Size() != item.info.Size {
+		fs.Debugf(item.o, "Size has changed from %d to %d", item.info.Size, item.o.Size())
+		err = item._truncate(item.o.Size())
+		if err != nil {
+			return 0, err
+		}
+	}
+
 	item.info.ATime = time.Now()
 	// Do the reading with Item.mu unlocked and cache protected by preAccess
 	n, err = item.fd.ReadAt(b, off)