"pegen.py python" should validate the output file.

I ran pegen python -o grammar_parser.py after making an incorrect change to metagrammar.gram. This was invoked by make regen-pegen-metaparser, or build.bat --regen on Windows.
The resulting output file was syntactically correct, but it could not correctly parse the metagrammar file. So when I ran the make again later, it failed. The call to GeneratedParser.start() raised an exception.

The previous (correctly working) version of grammar_parser.py is now lost, unless I could get it back from Git.

Desired behavior

pegen must be sure that the newly generated output file will work correctly, before writing it out to the filesystem.

Generation of the output file begins in pegen/build.py,

def build_python_generator(
    grammar: Grammar,
    grammar_file: str,
    output_file: str,
    skip_actions: bool = False,
) -> ParserGenerator:
    with open(output_file, "w") as file:
        gen: ParserGenerator = PythonParserGenerator(grammar, file)  # TODO: skip_actions
        gen.generate(grammar_file)
    return gen

This creates a new output file. Even if the body of the with raises an exception, the output file is closed and remains in the filesystem, overwriting the previous version.
Instead,

1. Use a StringIO as the file, instead of a real open() file. Call it result.
2. Generate the file, which fills in result.
3. Validate result (see more below).
4. If validation succeeds, then write the result to a new open(output_file, "w").
5. Otherwise, do nothing more. The file designated by output_file will still be intact.

Validating the output file

To validate the output file result, go through the entire process of generating result, except using
result instead of the grammar_parser module. This process is the entire code of build_python_parser_and_generator, not just the build_python_generator function.

grammar_parser is used to import the class GeneratedParser. For validation purposes, exec(result) with an empty globals/locals dict, then use the ['GeneratedParser'] item. Note, in build.py, this class is imported as the name GrammarParser.
This process should generate a new_result StringIO, and it should be identical to result. Perhaps there may be some insignificant differences between the two, and if so, they shouldn't be compared.
If the generation of new_result raises an exception, then the validation has failed.

Linked PRs

• gh-101937

Start in pegen/build.py, especially build_python_generator and build_python_parser_and_generator. Trace how generated parser output is produced and validated, then preserve the existing output file when validation fails and confirm successful output is written only after validation passes. Review linked PR gh-101937 for work already under way.