Program: 23450
Project: 23450
Buggy source:
private StructureDefinition checkExtension(List<ValidationMessage> errors,String path,Element element,ElementDefinition def,StructureDefinition profile,NodeStack stack){
  String url=element.getNamedChildValue("String_Node_Str");
  boolean isModifier=element.getName().equals("String_Node_Str");
  long t=System.nanoTime();
  StructureDefinition ex=context.fetchResource(StructureDefinition.class,url);
  sdTime=sdTime + (System.nanoTime() - t);
  if (ex == null) {
    if (!rule(errors,IssueType.STRUCTURE,element.line(),element.col(),path,allowUnknownExtension(url),"String_Node_Str" + url + "String_Node_Str"))     warning(errors,IssueType.STRUCTURE,element.line(),element.col(),path,allowUnknownExtension(url),"String_Node_Str" + url);
  }
 else {
    if (def.getIsModifier())     rule(errors,IssueType.STRUCTURE,element.line(),element.col(),path + "String_Node_Str" + url+ "String_Node_Str",ex.getSnapshot().getElement().get(0).getIsModifier(),"String_Node_Str");
 else     rule(errors,IssueType.STRUCTURE,element.line(),element.col(),path + "String_Node_Str" + url+ "String_Node_Str",!ex.getSnapshot().getElement().get(0).getIsModifier(),"String_Node_Str");
    checkExtensionContext(errors,element,ex,stack,ex.getUrl());
    if (isModifier)     rule(errors,IssueType.STRUCTURE,element.line(),element.col(),path + "String_Node_Str" + url+ "String_Node_Str",ex.getSnapshot().getElement().get(0).getIsModifier(),"String_Node_Str" + url + "String_Node_Str");
 else     rule(errors,IssueType.STRUCTURE,element.line(),element.col(),path + "String_Node_Str" + url+ "String_Node_Str",!ex.getSnapshot().getElement().get(0).getIsModifier(),"String_Node_Str" + url + "String_Node_Str");
  }
  return ex;
}

Fixed version (ground truth):
private StructureDefinition checkExtension(List<ValidationMessage> errors,String path,Element element,ElementDefinition def,StructureDefinition profile,NodeStack stack) throws FHIRException {
  String url=element.getNamedChildValue("String_Node_Str");
  boolean isModifier=element.getName().equals("String_Node_Str");
  long t=System.nanoTime();
  StructureDefinition ex=context.fetchResource(StructureDefinition.class,url);
  sdTime=sdTime + (System.nanoTime() - t);
  if (ex == null) {
    if (!rule(errors,IssueType.STRUCTURE,element.line(),element.col(),path,allowUnknownExtension(url),"String_Node_Str" + url + "String_Node_Str"))     warning(errors,IssueType.STRUCTURE,element.line(),element.col(),path,allowUnknownExtension(url),"String_Node_Str" + url);
  }
 else {
    if (def.getIsModifier())     rule(errors,IssueType.STRUCTURE,element.line(),element.col(),path + "String_Node_Str" + url+ "String_Node_Str",ex.getSnapshot().getElement().get(0).getIsModifier(),"String_Node_Str");
 else     rule(errors,IssueType.STRUCTURE,element.line(),element.col(),path + "String_Node_Str" + url+ "String_Node_Str",!ex.getSnapshot().getElement().get(0).getIsModifier(),"String_Node_Str");
    checkExtensionContext(errors,element,ex,stack,ex.getUrl());
    if (isModifier)     rule(errors,IssueType.STRUCTURE,element.line(),element.col(),path + "String_Node_Str" + url+ "String_Node_Str",ex.getSnapshot().getElement().get(0).getIsModifier(),"String_Node_Str" + url + "String_Node_Str");
 else     rule(errors,IssueType.STRUCTURE,element.line(),element.col(),path + "String_Node_Str" + url+ "String_Node_Str",!ex.getSnapshot().getElement().get(0).getIsModifier(),"String_Node_Str" + url + "String_Node_Str");
    validateElement(errors,ex,ex.getSnapshot().getElement().get(0),null,null,element,element,"String_Node_Str",stack,false);
  }
  return ex;
}