Latest Data Dump has invalid XML and invalid characters

Question

As I have been looking through the latest StackExchange data dump, it seems like a non-compliant XML serializer was used. There are numerous escape sequences that are simply invalid XML such as &#x1E or even &#x00. Here you can see the escape sequence in question near the beginning of the selected line:

Here is that same PostHistory record (from 3dprinting) in the April 2024 dump where these characters are not present:

These issues break most XML parsers.

Further, if we ignore the invalid escape sequences, there is even an escape sequence for a character that is not valid UTF-8:

This character has no representation in the Unicode code page. This breaks any attempts to serialize this using APIs such as C#'s Encoding.UTF8.GetBytes.

Answer 1

That is very unfortunate. Until a better solution appears I've crafted a CleaningStreamReader (C#) that replaces any illegal character sequence with the space sequence so that at least the XmlReader can process the whole file.

The implementation really needs some StateMachine for cases where the characters are split across two reads. But that is for vNext and when we agree this is something that can work. Alternatively the FileStream itself can be cleaned but then you have similar problems but at the UTF8 Encoding for up to 4 bytes.

public class CleaningStreamReader:StreamReader
{
    public CleaningStreamReader(Stream fs, Encoding enc):base(fs,enc) {
    }
    
    public override int Read (char[] buffer, int index, int count)
    {
        var charcount = base.Read(buffer, index,count);
     
        for( var i = index; i < index+charcount; i++) {
            // this really needs a state machine because we might not have all chars we need in this current buffer
            // check if &#x01; till &#x1F; exists, replace 1F with 20
            if ((i + 5 < index + charcount) && buffer[i] == '&' && buffer[i+1] == '#' && buffer[i+2] == 'x' && buffer[i+5] == ';')
            {
               if ((buffer[i+3] == '0' || buffer[i+3] == '1' ) && (buffer[i+4] >= '0' && buffer[i+4] <= 'F')) {
                  buffer[i+3] = '2'; buffer[i+4] = '0';
               }
            }
        }
        return charcount;
    }
}

And here is how you use that class:

var filename = @"\some\path\3dprinting.stackexchange.com\posthistory.xml";
 
 var nodes = new Dictionary<XmlNodeType, int>();
 
 using(var fs = File.OpenRead(filename)) 
 using(var sr = new CleaningStreamReader(fs, Encoding.UTF8))
 using(var xr = XmlReader.Create(sr)) {
     while(xr.Read()) {
         if (nodes.Keys.Contains(xr.NodeType)) {
            nodes[xr.NodeType] =  nodes[xr.NodeType] +1 ;
         } else {
            nodes.Add(xr.NodeType, 1); 
         }
      
         if (xr.NodeType == XmlNodeType.Element) {
         
             bool ha = xr.MoveToFirstAttribute();
             while(ha) {
                 if (nodes.Keys.Contains(xr.NodeType)) {
                     nodes[xr.NodeType] =  nodes[xr.NodeType] +1 ;
                 } else {
                     nodes.Add(xr.NodeType, 1); 
                 }
                 ha = xr.MoveToNextAttribute();
             }
         }
     }
 }
 nodes.Dump();

This is what statistics of NodeTypes will look like when you processed the 3d printing PostHistory.xml successfully:

Answer 2

I was working on the dump recently and I faced the similar issue, I found the default XML python parser fails to parse, but there is a working solution with lxml below with using recover=True:

from lxml import etree

def skip_exceptions(it):
    while True:
      try:
          yield next(it)
      except StopIteration:
          break
      except Exception as e:
          print('Skipping iteration because of exception {}'.format(e))    

if __name__ == "__main__":
    xml_file = "<your_xml_filepath>.xml"
    f = open(xml_file, "rb")
    xmlit = iter(etree.iterparse(f, events=('start', 'end'), recover=True))
    for (event, elem) in xmlit:
          do what you want...

Answer 3

This change has been made. Please see this post for more details on other fixes and changes related to data dumps.