filelu: fix truncated files being uploaded successfully when the source ends early

If the source supplied fewer bytes than its declared size, the
truncated file was stored and the upload reported success with the
object claiming the declared size.

Count the bytes actually read from the source and if they do not match
the declared size, remove the truncated file and return an error.

This was found by the new TestRcatSizeShortEOF integration test.

backend/filelu/filelu_object.go

@@ -14,6 +14,7 @@ import (
 
 	"github.com/rclone/rclone/fs"
 	"github.com/rclone/rclone/fs/hash"
+	"github.com/rclone/rclone/lib/readers"
 	"github.com/rclone/rclone/lib/rest"
 )
 
@@ -141,6 +142,8 @@ func (o *Object) Open(ctx context.Context, options ...fs.OpenOption) (io.ReadClo
 // Update updates the object with new data
 func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, options ...fs.OpenOption) error {
 	size := src.Size()
+	counter := readers.NewCountingReader(in)
+	in = counter
 
 	if size <= int64(o.fs.opt.UploadCutoff) {
 		err := o.fs.uploadFile(ctx, in, o.remote)
@@ -155,6 +158,15 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 		}
 	}
 
+	// Check the source supplied the number of bytes it declared
+	// otherwise a truncated file would be stored as a good upload
+	if size >= 0 && int64(counter.BytesRead()) != size {
+		if removeErr := o.Remove(ctx); removeErr != nil {
+			fs.Errorf(o, "Failed to remove partially transferred object: %v", removeErr)
+		}
+		return fmt.Errorf("expected %d bytes in input, but got %d: %w", size, counter.BytesRead(), io.ErrUnexpectedEOF)
+	}
+
 	o.size = size
 	o.modTime = src.ModTime(ctx)
 	return nil