[xsl] how to avoid losing data when the transformation fails

["Wolfhart Totschnig wolfhart.totschnig@xxxxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

Dear list,

I would like to ask for your advice on the following problem: I have a 
script that modifies an existing XML file, drawing on external data, 
changing or complementing the existing data in the file. This means that 
the source and the output of the transformation are the same file. Now, 
it has happened that the transformation fails because the external data 
does not have the form that the script expects it to have. In these 
cases, the transformation is terminated with an error message and the 
source file is overwritten with a truncated or even completely empty 
file. This is a problem because I might lose (and have lost) data in 
this way, namely when the back-up copy of the source file that I have is 
not the latest version.

What is the proper way to deal with this problem? Is there a way to 
instruct Saxon (I am using Saxon 9 HE) not to write an output when the 
transformation fails? Or should the transformation be organized differently?

Thank you in advance for your help!
Wolfhart