Support parsing `cel_expr_string` from XDS CelExpression

[isker]

This field is not handled by Envoy here:

envoy/source/extensions/filters/common/expr/evaluator.cc

Lines 224 to 243 in 9192140

	absl::StatusOr<CompiledExpression>
	CompiledExpression::Create(const BuilderInstanceSharedConstPtr& builder,
	const xds::type::v3::CelExpression& xds_expr) {
	// First try to get expression from the new CEL canonical format.
	if (xds_expr.has_cel_expr_checked()) {
	return Create(builder, xds_expr.cel_expr_checked().expr());
	} else if (xds_expr.has_cel_expr_parsed()) {
	return Create(builder, xds_expr.cel_expr_parsed().expr());
	}
	// Fallback to handling legacy formats for backward compatibility.
	switch (xds_expr.expr_specifier_case()) {
	case xds::type::v3::CelExpression::ExprSpecifierCase::kParsedExpr:
	return Create(builder, xds_expr.parsed_expr().expr());
	case xds::type::v3::CelExpression::ExprSpecifierCase::kCheckedExpr:
	return Create(builder, xds_expr.checked_expr().expr());
	default:
	return absl::InvalidArgumentError("CEL expression not set.");
	}
	PANIC_DUE_TO_CORRUPT_ENUM;
	}

[isker]

Ah, @wbpcode added this to xds here.

[agrawroh]

@isker Is it okay to close this as it's already supported?

[isker]

Supported where? My previous message was just pinging a maintainer (? or at least a contributor) that I thought might have context on the missing support in Envoy.

[isker]

I might be inferring too much, but it looks like this field was added to the proto in xds without being supported in Envoy. Maybe he knows why.

[kyessenov]

I think it can be supported, but the guidance is to parse expressions at the control plane. Basically, do as much validation as possible prior to the data plane configuration. But CEL text expressions would be fine on a standalone proxy.

[isker]

For sure, yeah. Handwriting the parsed expression is a bit painful.

[agrawroh]

Supported where? My previous message was just pinging a maintainer (? or at least a contributor) that I thought might have context on the missing support in Envoy.

Sorry, I thought you were saying that it's already implemented.