Change error handling strategy and add verbose errors

[surister]

Summary of the changes / Why this is an improvement

Issue #2

This is one of the first pieces required to finish the feature and its only implemented in Python for early feedback/validation.

Main changes of PR:

• Throwing antlr4 parse errors as exceptions is now only enabled by raise_exception API influenced by DRF Validators

• Default behaviour is not to raise an exception, the reason is that it forces clients to try/catch errors, when you already work with CrateDB in the HTTP protocol you are used to check the json response, rather than catching exceptions, this change of behaviour aims to make integrating with this library as similar as possible as with CrateDB.

• Implements an ExceptionCollectorListener that collects all errors that are caught, (remember that we use parser.statements, to support running several queries on the frontends, so several errors can happen simultaneously)

• Adds more verbose errors

The main issue I would like help with the review: <-------------------------------------

• Whenever we collect the errors I just add the errors to a list for further processing:

class ExceptionCollectorListener(ErrorListener):
    """
    Error listener that collects all errors into errors for further processing.
    Based partially on https://github.com/antlr/antlr4/issues/396
    """

    def __init__(self):
        self.errors = []

    def syntaxError(self, recognizer, offendingSymbol, line, column, msg, e):
        error = ParsingException(
            msg=msg,
            offending_token=offendingSymbol,
            e=e,
            query=e.ctx.parser.getTokenStream().getText(e.ctx.start, e.offendingToken.tokenIndex),
        )

        self.errors.append(error)

we lose the information on which statement the error belongs to, I tried with different ErrorStrategies and Error listeners but to no avail, also didn't find anything online.

My best effort is to try to match the error's offendingToken query, to any of the parsed statements:

# Shortened for clarity purposes.
def find_suitable_error(statement, errors):
    for error in errors[:]:
        if error.query == statement.query:
            statement.exception = error
            errors.pop(errors.index(error))

There are a couple of edge cases that I covered but in short, it works reasonable well, even though I'm not a fan of it, maybe you folks way more experience in antlr4 have an idea?

Checklist

• Link to issue this PR refers to (if applicable):
• CLA is signed

[amotl]

Thank you very much. Also adding @matriv and @seut as optional reviewers, in order to a) give them a chance to spot nitty errors I am regularly missing because my reviewing style may be too lenient, and b) also give them a chance to respond to my first suggestion about API style matters.

Edit: Apologies, I missed to spot this PR is still in draft mode. So, please consider my suggestions as early feedback only.

[surister]

cc @proddata I'd love your feedback as well as any input on this

[proddata]

I read through it and generally like the idea of not raising an exception. However, I'm not sure about the defaults.

Regarding the "main issue," I don't have a strong opinion as I lack details on the mechanics. If the approach works, though, 👍.

[matriv]

Thx for the work on this @surister !

Left a comment, but I will also checkout the branch and play around a bit, so I can understand better the code.

[matriv]

thx @surister ! LGTM