sia: fix corrupted files being created when the source ends early

If the source supplied fewer bytes than its declared size, the upload
request failed but a retry could report success even though the stored
file was truncated, because the retry re-sent an already exhausted
reader.

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

This was found by the new FsPutShortEOF integration test.

backend/sia/sia.go

@@ -24,6 +24,7 @@ import (
 	"github.com/rclone/rclone/fs/hash"
 	"github.com/rclone/rclone/lib/encoder"
 	"github.com/rclone/rclone/lib/pacer"
+	"github.com/rclone/rclone/lib/readers"
 	"github.com/rclone/rclone/lib/rest"
 )
 
@@ -176,11 +177,12 @@ func (o *Object) Open(ctx context.Context, options ...fs.OpenOption) (in io.Read
 // Update the object with the contents of the io.Reader
 func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, options ...fs.OpenOption) (err error) {
 	size := src.Size()
+	counter := readers.NewCountingReader(in)
 	var resp *http.Response
 	opts := rest.Opts{
 		Method:        "POST",
 		Path:          path.Join("/renter/uploadstream/", o.fs.opt.Enc.FromStandardPath(path.Join(o.fs.root, o.remote))),
-		Body:          in,
+		Body:          counter,
 		ContentLength: &size,
 		Parameters:    url.Values{},
 	}
@@ -191,6 +193,14 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 		return o.fs.shouldRetry(resp, err)
 	})
 
+	// Check the source supplied the number of bytes it declared
+	// otherwise a truncated file would be stored as a good upload.
+	// The partially uploaded file is cleaned up by Put which copes
+	// with it appearing asynchronously after the failed upload.
+	if err == nil && size >= 0 && int64(counter.BytesRead()) != size {
+		return fmt.Errorf("expected %d bytes in input, but got %d: %w", size, counter.BytesRead(), io.ErrUnexpectedEOF)
+	}
+
 	if err == nil {
 		err = o.readMetaData(ctx)
 	}