vfs: fix file leaks with --vfs-cache-mode full and --buffer-size 0

Before this change using --vfs-cache-mode full and --buffer-size 0
together caused the vfs downloader to open more and more downloaders.

This is fixed by introducing a minimum size of 1M for the window to
look for an existing downloader.

Fixes #4892

vfs/vfscache/downloaders/downloaders.go

@@ -26,6 +26,9 @@ const (
 	backgroundKickerInterval = 5 * time.Second
 	// maximum number of errors before declaring dead
 	maxErrorCount = 10
+	// If a downloader is within this range or --buffer-size
+	// whichever is the larger, we will reuse the downloader
+	minWindow = 1024 * 1024
 )
 
 // Item is the interface that an item to download must obey
@@ -327,6 +330,11 @@ func (dls *Downloaders) _ensureDownloader(r ranges.Range) (err error) {
 		r.Size = 0
 	}
 
+	// If buffer size is less than minWindow then make it that
+	if window < minWindow {
+		window = minWindow
+	}
+
 	var dl *downloader
 	// Look through downloaders to find one in range
 	// If there isn't one then start a new one