sftp: stop uploads re-using the same ssh connection to improve performance

Before this change we released the ssh connection back to the pool
before the upload was finished.

This meant that uploads were re-using the same ssh connection which
reduces throughput.

This releases the ssh connection back to the pool only after the
upload has finished, or on error state.

See: https://forum.rclone.org/t/sftp-backend-opens-less-connection-than-expected/40245

backend/sftp/sftp.go

@@ -1992,9 +1992,10 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 	if err != nil {
 		return fmt.Errorf("Update: %w", err)
 	}
+	// Hang on to the connection for the whole upload so it doesn't get re-used while we are uploading
 	file, err := c.sftpClient.OpenFile(o.path(), os.O_WRONLY|os.O_CREATE|os.O_TRUNC)
-	o.fs.putSftpConnection(&c, err)
 	if err != nil {
+		o.fs.putSftpConnection(&c, err)
 		return fmt.Errorf("Update Create failed: %w", err)
 	}
 	// remove the file if upload failed
@@ -2014,14 +2015,18 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 	}
 	_, err = file.ReadFrom(&sizeReader{Reader: in, size: src.Size()})
 	if err != nil {
+		o.fs.putSftpConnection(&c, err)
 		remove()
 		return fmt.Errorf("Update ReadFrom failed: %w", err)
 	}
 	err = file.Close()
 	if err != nil {
+		o.fs.putSftpConnection(&c, err)
 		remove()
 		return fmt.Errorf("Update Close failed: %w", err)
 	}
+	// Release connection only when upload has finished so we don't upload multiple files on the same connection
+	o.fs.putSftpConnection(&c, err)
 
 	// Set the mod time - this stats the object if o.fs.opt.SetModTime == true
 	err = o.SetModTime(ctx, src.ModTime(ctx))