Harmonize MaterialX XML syntax

[ashwinbhat]

MaterialX uses XML syntax for transportation of material data. There is desire to ensure that the MaterialX XML syntax supports the following for cleaner data interop:

• Validation using standard XML validation tools
• Support Canonical XML Version 1.1 (https://www.w3.org/TR/xml-c14n11/)
• Support an XML Schema (XSD)

Also related: #2648

[kwokcb]

A note from experience for trying to support json / gltf schema that there are an unlimited number of element types since node instances use element category names. This can make validation hard to be deterministic.

The approach taken for gltf was to instead have 1 node type and category and data type as mandatory. This matches a similar approach in USD where node instances are Shaders but with exact definition specification by node id.

This would be a big change for xml so just putting this up for comment.

[dgovil]

It would be really great to make the mtlx files be fully xml spec compliant.
We often get requests to use a more spec compliant XML parser for performance and security reasons, or we just use different readers because we aren't using the MaterialX library directly for other reasons (analysis , processing etc)

Moving away from the PugiXML specific quirks and making sure they can be read independently would be a huge boon for this

[kwokcb]

Added link to MaterialX namespaces which break validation since they are being treated as undeclared XML namespaces.

[kwokcb]

Here is a simple checker using Python XML:

"""
XML Compliance Checker for MaterialX (.mtlx) files
"""

import os
import sys
import argparse
from xml.etree import ElementTree as ET
import re

class XMLComplianceChecker:
    """
    @brief Checks XML compliance of .mtlx files and reports errors with line numbers and content.    
    """
    def __init__(self):
        """
        @brief Initializes the XMLComplianceChecker with an empty error list.
        """
        self.errors = []
    
    def check_file(self, filepath):
        """
        @brief Checks the XML compliance of a single file and records any errors found.
        @param filepath The path to the file to check.
        @return True if the file is compliant, False otherwise.
        """
        self.errors = []
        
        if not os.path.exists(filepath):
            self.errors.append({
                'file': filepath,
                'line': None,
                'message': f"File not found"
            })
            return False
        
        try:
            ET.parse(filepath)
            return True
        except ET.ParseError as e:
            error_msg = str(e)
            line_num = self._get_line_number(error_msg)
            
            # Get the actual line from the file
            line_content = None
            if line_num:
                with open(filepath, 'r', encoding='utf-8') as f:
                    lines = f.readlines()
                    if line_num <= len(lines):
                        line_content = lines[line_num - 1].rstrip('\n\r')
            
            self.errors.append({
                'file': filepath,
                'line': line_num,
                'line_content': line_content,
                'message': error_msg
            })
            return False
    
    def _get_line_number(self, error_msg):
        """
        @brief Extracts the line number from an XML parsing error message.
        @param error_msg The error message from which to extract the line number.
        @return The line number if found, None otherwise.
        """
        match = re.search(r'line\s+(\d+)', error_msg, re.IGNORECASE)
        return int(match.group(1)) if match else None


def main():
    """
    @brief Command to check XML compliance of MateraiLX files. 
    @param path The file or folder path to check for .mtlx files.
    @return Exits with code 0 if compliant, 1 if errors are found.
    """
    parser = argparse.ArgumentParser(description='Check XML compliance of .mtlx files')
    parser.add_argument('path', help='File or folder path')
    args = parser.parse_args()
    
    # Find all .mtlx files
    files = []
    if os.path.isfile(args.path) and args.path.endswith('.mtlx'):
        files = [args.path]
    elif os.path.isdir(args.path):
        for root, _, filenames in os.walk(args.path):
            files.extend(os.path.join(root, f) for f in filenames if f.endswith('.mtlx'))
    
    # Check each file
    checker = XMLComplianceChecker()
    errors_found = False
    
    for filepath in files:
        if not checker.check_file(filepath):
            print(f"* Error(s) found in file: {filepath}")
            errors_found = True
            for err in checker.errors:
                if err['line']:
                    print(f"   * Line {err['line']}: {err['message']}")
                    if err['line_content']:
                        print(f"   * {err['line_content']}")
                else:
                    print(f"   * {err['message']}")
    
    sys.exit(1 if errors_found else 0)

if __name__ == "__main__":
    main()

• Running on libraries shows no errors.
• Running on resources shows the non-escaped character and namespace errors:

* Error(s) found in file: ..\..\..\ILM_materialx\resources\Materials\TestSuite\nprlib\toon_shade.mtlx
   * Line 19: unbound prefix: line 19, column 2
   *   <npr_test:toon_shade name="toon_shade1" type="color3">
* Error(s) found in file: ..\..\..\ILM_materialx\resources\Materials\TestSuite\stdlib\texture\udim.mtlx
   * Line 4: not well-formed (invalid token): line 4, column 78
   *     <input name="file" type="filename" value="resources/Images/grid_udim/grid.<UDIM>.png" colorspace="srgb_texture" />