Allow empty selection sets - flag version

[martinbonnin]

See graphql/graphql-spec#1227

There is a new (and already deprecated!) parser/validation option allowEmptySelectionSets. It is true by default (new behaviour). Set it to false to switch back to the old behaviour.

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[magicmark]

🥳

[benjie]

Solid changes 👍

[benjie]

LGTM; the options object does add some garbage collection overhead so @yaacovCR might want to trigger the benchmarks, but I imagine it's an incredibly lightweight cost so unlikely to be a big deal.

[glen-84]

With the parser now accepting { }, subscription { } parses and validates cleanly, but I think it should still fail validation.

The spec's Single Root Field rule says the collected fields map "must have exactly one entry", and this PR doesn't change that rule. SingleFieldSubscriptionsRule only checks the upper bound though:

if (groupedFieldSet.size > 1) {

That was enough while the grammar guaranteed at least one selection. Now it lets an empty subscription through, and the same goes for subscription { ...Empty } with an empty fragment, since that also collects to zero fields.

Suggestion: report when groupedFieldSet.size === 0 as well, e.g. Subscription "X" must select a top level field. (or fold both into size !== 1), plus a case in SingleFieldSubscriptionsRule-test.ts for the empty operation and the empty-fragment variant.

For reference, Hot Chocolate's implementation of this change (ChilliCream/graphql-platform#10337) allows empty selection sets on query and mutation roots and on composite fields, but keeps rejecting an empty subscription root.

Drafted by Claude (Anthropic AI assistant).

[martinbonnin]

@glen-84 sorry I forgot to reply here but the case where a subscription has no root field is now handled in this PR. Thank you and Claude for catching it.

[martinbonnin]

@yaacovCR the spec is merged, can we merge this one?