Re: FR: Option to terminate page rendering from an Action nicely

Hi Nagy, see below for my fix, it probably is a good idea to put that
into some lib shipped with roundup and document the behaviour.

On Fri, Aug 06, 2021 at 09:12:50PM +0200, Nagy Gabor wrote:
> 
> Based on ExportCSVAction, I wrote an ExportXLSAction using xslxwriter.
> Everything worked fine on Linux systems, but MS Office always reported
> corrupted xlsx file, which seemed OK after applying Office's repair.
> Mystery...
> 
> After hours of debugging, I found the bug:
> 
> First I realized that the problem is that my generated .xlsx file is
> correct, but it is served with a '\n' character added to the end of the
> file. Analogously to cgi.actions.ExportCSVAction.handle(), the last line of
> my handle method was also   return '\n'.
> We are getting closer: Yes, the returned string is appended to the
> served content, this is verified for different strings. At this point I
> believed that I am done, but neither return '' nor return None worked.
> In these cases the issue.index template file is rendered into the xlsx
> binary. Hmmm. So I have to looked into Roundup's code deeper, and I
> found the relevant code in cgi/client.py (lines 685-696, inner_main()
> method of Client class):
> 
>                 if csrf_ok:
>                     # csrf checks pass. Run actions etc.
>                     # possibly handle a form submit action (may change
>                     # self.classname and self.template, and may also
>                     # append error/ok_messages)
>                     html = self.handle_action()
>                 else:
>                     html = None
> 
>                 if html:
>                     self.write_html(html)
>                     return
>                 # now render the page
>                 # we don't want clients caching our dynamic pages
> 
> This means that if the action's handle() returns anything which
> evaluates to True in the "if html" condition, then it is added to the
> page, and the rendering is done; otherwise the rendering goes on.
> 
> My feature request is to add a nice possibility for the Action to
> terminate the page rendering. (Maybe by raising an exception, which is
> handled by the except part of inner_main.) I've just added an
> "if html == True:
>      return"
> test before the "if html:" test, and my Action returns True. (This
> is an ugly solution, but it should work, because self.write_html(True)
> induces an error, so probably no other Action returns True, because
> with the old code, that would lead to an exception.)

I'm using the following class in my actions:

class True_Value (type ("")):
    """ A class that evaluates to True but can return a zero-length string.
        We use this as return value from a handle routine where the
        output happend to the file descriptor
    """
    def __bool__ (self):
        return True

Note that for Python2 you want to add
    __nonzero__ = __bool__

I'm returning an instance of that class which appends an empty string.

Ralf
-- 
Dr. Ralf Schlatterbeck                  Tel:   +43/2243/26465-16
Open Source Consulting                  www:   www.runtux.com
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