vfs: make --vfs-download-first flag WIP FIXME

Needs
- tests
- docs

vfs/read_write.go

@@ -113,6 +113,17 @@ func (fh *RWFileHandle) openPending() (err error) {
 	}
 	fh.opened = true
 	fh.d.addObject(fh.file) // make sure the directory has this object in it now
+
+	// If we are downloading the file first, then do that here
+	if fh.d.vfs.Opt.DownloadFirst {
+		fs.Errorf(fh.logPrefix(), "downloading %v file completely to cache", fs.SizeSuffix(size))
+		err = fh.item.Ensure()
+		if err != nil {
+			fs.Errorf(fh.logPrefix(), "failed to download file to cache: %v", err)
+			return fmt.Errorf("failed to download file to cache: %w", err)
+		}
+		fs.Errorf(fh.logPrefix(), "downloaded %v file completely to cache", fs.SizeSuffix(size))
+	}
 	return nil
 }
 

vfs/vfscache/item.go

@@ -642,6 +642,16 @@ func (item *Item) store(ctx context.Context, storeFn StoreFn) (err error) {
 	return item._store(ctx, storeFn)
 }
 
+// Ensure item is entirely on disk
+func (item *Item) Ensure() (err error) {
+	item.mu.Lock()
+	defer item.mu.Unlock()
+	if item.o == nil {
+		return nil
+	}
+	return item._ensure(0, item.info.Size)
+}
+
 // Close the cache file
 func (item *Item) Close(storeFn StoreFn) (err error) {
 	// defer log.Trace(item.o, "Item.Close")("err=%v", &err)

vfs/vfscommon/options.go

@@ -145,6 +145,11 @@ var OptionsInfo = fs.Options{{
 	Default: false,
 	Help:    "Use fast (less accurate) fingerprints for change detection",
 	Groups:  "VFS",
+}, {
+	Name:    "vfs_download_first",
+	Default: false,
+	Help:    "If set and --vfs-cache-mode full then download files completely on first access.",
+	Groups:  "VFS",
 }, {
 	Name:    "vfs_disk_space_total_size",
 	Default: fs.SizeSuffix(-1),
@@ -203,6 +208,7 @@ type Options struct {
 	ReadAhead          fs.SizeSuffix `config:"vfs_read_ahead"`       // bytes to read ahead in cache mode "full"
 	UsedIsSize         bool          `config:"vfs_used_is_size"`     // if true, use the `rclone size` algorithm for Used size
 	FastFingerprint    bool          `config:"vfs_fast_fingerprint"` // if set use fast fingerprints
+	DownloadFirst      bool          `config:"vfs_download_first"`   // if set download files to VFS cache first
 	DiskSpaceTotalSize fs.SizeSuffix `config:"vfs_disk_space_total_size"`
 }