http-request.xpl

<?xml version="1.0" encoding="UTF-8"?>
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc"
  xmlns:c="http://www.w3.org/ns/xproc-step" version="1.0">
  
  <p:documentation>Checks the availability of a resource given in the href option.
  
  URL that works (HTTP status 200): http://www.le-tex.de/
  
  URL that is unavailable (404): http://www.le-tex.de/foo
  
  URL that causes an error that is caught: http://www.le-tex
  
  URL that causes an uncaught exception: http://www.journalofosteopathicmedicine.com/
  ERROR: org.apache.http.client.ClientProtocolException
  ERROR: Underlying exception: null
  
  After digging a bit, @lwittmar got hold of the root cause:
  Caused by: org.apache.http.client.CircularRedirectException: 
  Circular redirect to 'http://www.journalofosteopathicmedicine.com/action/consumeSharedSessionAction?JSESSIONID=…'
  
  </p:documentation>
  
  <p:option name="href">
    <p:documentation>URL to check</p:documentation>
  </p:option>
  
  <p:output port="result" primary="true"/>
  
  <p:add-attribute name="create-request" match="/*" attribute-name="href">
    <p:input port="source">
      <p:inline>
        <c:request method="head" detailed="true" status-only="true"/>
      </p:inline>
    </p:input>
    <p:with-option name="attribute-value" select="$href"/>
  </p:add-attribute>
  
  <p:try name="send-request">
    <p:group>
      <p:http-request/>    
    </p:group>
    <p:catch name="catch">
      <p:identity>
        <p:input port="source">
          <p:pipe port="error" step="catch"/>
        </p:input>
      </p:identity>
    </p:catch>
  </p:try>
  
</p:declare-step>