[Xsltforms-support] Content negotiation problem: HTTP "Accept" header specifies non-XML content types

[Conal T. <con...@ve...>]

I have been using XSLTForms recently, for editing XML metadata records.

In our project, we have a RESTful web service (based on a Fedora 
repository) which uses HTTP Content Negotiation to publish different 
representations of a resource (i.e. using the same HTTP URL, but in 
response to different HTTP Accept headers).

Each resource may be represented as an XML metadata record or as an 
XForm, depending on the content-type requested. If a browser sends a 
request for an HTML page, we want to return an XForm embedded in HTML, 
but if the browser doesn't want HTML, then we send the "raw" XML record.

If the browser accepts HTML, then it receives an XForm, and the XForm 
itself then issues a request (to the same URL!) to retrieve the XML 
instance. The problem is that XSLTForms doesn't specify an HTTP Accept 
header, and the browser (Firefox 5 for example) sends its default Accept 
header, which includes "text/html", etc. Hence the instance document 
retrieved is not the "raw" XML representation, but rather an XForm (i.e. 
the XForm has loaded itself as a model instance!)

I think it's a bug for XSLTForms to say that it accepts 
"text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8" (which 
is the default Accept header in Firefox). It seems sensible to me to set 
an Accept header listing "text/xml" and "application/xml", so that 
XSLTForms can be used with services such as ours, which use content 
negotiation.

I found I could fix the problem by explicitly setting the Accept header 
in xsltforms.js, in two places (there is a case for Internet Explorer 
and a case for other browsers). See below:

> if (window.XMLHttpRequest) {
>         Core.openRequest = function(method, uri, async) {
>                 // 
> netscape.security.PrivilegeManager.enablePrivilege("UniversalBrowserRead");
>                 var req = new XMLHttpRequest();
>                 try {
>                         req.open(method, Core.constructURI(uri), async);
>                 } catch (e) {
>                         try {
>                                  req = new 
> ActiveXObject("Msxml2.XMLHTTP.3.0");
>                            } catch (e) {
>                                 try {
>                                          req = new 
> ActiveXObject("Msxml2.XMLHTTP");
>                                 } catch (e) {
>                                            throw new Error("This 
> browser does not support XHRs(Ajax)! \n Cause: " + (e.message || 
> e.description || e) + " \n Enable Javascript or ActiveX controls (on 
> IE) or lower security restrictions.");
>                                 }
>                         }
>                         req.open(method, Core.constructURI(uri), async);
>                 }
>                 // Added by con...@ve... so xsltforms 
> doesn't accept "text/html" (so that HTTP content negotiation can be used)
>                 req.setRequestHeader("Accept", 
> "text/xml;q=0.5,application/xml;q=0.6,application/xhtml+xml;q=0.4");
>                 if (Core.isMozilla) {
>                         req.overrideMimeType("text/xml");
>                    }
>                 return req;
>         };
> } else if (window.ActiveXObject) {
>         Core.openRequest = function(method, uri, async) {
>                 try {
>                         req = new ActiveXObject("Msxml2.XMLHTTP.3.0");
>                 } catch (e) {
>                         try {
>                                 req = new ActiveXObject("Msxml2.XMLHTTP");
>                         } catch (e) {
>                                 throw new Error("This browser does not 
> support XHRs(Ajax)! \n Cause: " + (e.message || e.description || e) + 
> " \n Enable Javascript or ActiveX controls (on IE) or lower security 
> restrictions.");
>                         }
>                 }
>                 req.open(method, Core.constructURI(uri), async);
>                 // Added by con...@ve... so xsltforms 
> doesn't accept "text/html" (so that HTTP content negotiation can be used)
>                 req.setRequestHeader("Accept", 
> "text/xml;q=0.5,application/xml;q=0.6,application/xhtml+xml;q=0.4");
>                 return req;
>         };
> } else {
>         throw new Error("This browser does not support XHRs(Ajax)! \n 
> Enable Javascript or ActiveX controls (on IE) or lower security 
> restrictions.");
> }
-- 
Conal Tuohy
eResearch Business Analyst
Victorian eResearch Strategic Initiative
+61-466324297