Re: [xml] propper error reporting

[Daniel Veillard <veillard redhat com>]

On Tue, Jan 09, 2007 at 02:53:20PM +0100, Petr Pajas wrote:
> Hi Daniel, All,
>
> I'm struggling with fixing error reporting from RelaxNG validation 
> in the Perl XML::LibXML bindings. I would like to get a flat 
> string, actually the very same string that xmllint spits only in 
> memory, not on stderr. 
>
> The problem is that if I use xmlRelaxNGSetValidErrors (like xmllint 
> does, only with a different function than fprintf), my handler does 
> not get the filename and line number of the error. So basically, if 
> I use fprintf as my handler (like xmllint), the info is there, but 
> if I e.g. wrap vsprintf in my own function, the info is absent.
>
> I assume that this is because __xmlRaiseError makes itself this 
> distinction:
>
>     else if ((channel == (xmlGenericErrorFunc) fprintf) ||
>              (channel == xmlGenericErrorDefaultFunc))
>         xmlReportError(to, ctxt, str, channel, data);
>     else
>         channel(data, "%s", str);
>
> So, fprintf and xmlGenericErrorDefaultFunc get the full message 
> produced by xmlReportError, while other (flat) channels only get 
> str.

  Some clients wanted messages in one string which xmlReportError
doesn't do it output it in multiple chunks (for example they emitted 
a network packet on each callback on the channel). When we see fprintf or
the default function we know that this is not a problem and go ahead.

> So my next idea was to use xmlRelaxNGSetValidStructuredErrors and do 
> something like
>
> xmlReportError(error, NULL, error->message, my_flat_channel, 
> my_user_data);
>
> to create a formatted error message (exactly the same as that of 
> xmllint) and pass it to my error reporting code.
>
> However, xmlReportError is static, so that won't work. So my 
> question is: is there any public API that would produce formatted 
> error strings from xmlError or do I need to copy-paste the insides 
> of xmlReportError to my code?

  People want flexibility, so there is the structured error API which 
gives all the pieces and let them rearrange their messaging as they want.
Use the structured error API, which should work with RNG too. That's the
only way to cope with the various conflicting requirements that people
put on error reporting.
  You could also somehow copy xmlReportError() too in your own code if
you don't want to switch to the new APIs.

> Also, is there any actual reason for 
> the distinction in __xmlRaiseError? Why would fprintf and some 
> channel want to get different strings?

  See before.

Daniel

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