Support @defer directive

[morrys]

Hello to all,
this PR allows to support the @defer directive. Although the implementation is mostly complete, the PR is in draft to discuss it together and evaluate its integration.

In the PR I also created tests and I performed the integration tests with Relay, express-graphql & fetch-multipart-graphql with positive results.

In summary how I implemented defer:

• @defer has two arguments:
  • if (optional, default true) the fragment is defer when true.
  • label (required) used to distinguish deferred fragments
• @defer currently provides only FRAGMENT_SPREAD & INLINE_FRAGMENT (Relay) as location, but support for FIELD (Apollo) is commented in the PR

  graphql-js/src/execution/execute.js

  Lines 663 to 668 in 4226f23

  	/*
  	in order to support defer on field
  	const isFieldDefer = shouldDeferNode(exeContext, selection);
  	if (deferDirective && !isFieldDefer) {
  	*/
  	if (deferDirective) {

• I added resolverResult: ResultResolver new property in ExecutionContext which allows the management of the results to be returned at the end of the execution
• modified execute function now its return type is AsyncIterable<Promise<ExecutionResult> | PromiseOrValue<ExecutionResult>; . Using isAsyncIterable (result) it is possible to distinguish if there are deferred results.
• fields that are marked as deferred are resolved after the resolution of the previous result. I also implemented an optimization to avoid solving the same field multiple times.

this is a slow motion gif of how it works.

Let me know if more information is needed.

Thank you
Lorenzo

[wtrocki]

Some additional context from Apollo Defer: apollographql/apollo-server#2700

[morrys]

Looking at the Relay test schema
https://github.com/facebook/relay/blob/5da3be070283c6dcd42774ba33c1590db65fe3c7/packages/relay-test-utils-internal/testschema.graphql#L13-L16 i noticed that defer is also supported for inline_fragment, the last commit support it and add some tests

[morrys]

In this branch I created support for @stream:
https://github.com/morrys/graphql-js/tree/support-stream-directive

Some checks and optimizations are missing. But the first tests with Relay were positive:

[IvanGoncharov]

@morrys Thanks for your effort 👍
I didn't have enough time to read through actual code but on the surface, it looks like both #2318 and #2319 add very similar functionality especially since both of them Relay compatible.
Or I'm missing something?

About the next steps for merging this PR please see my answer in #2319:
#2319

@morrys @lilianammmatos Can you please review each other PRs and figure out how to collaborate?

[morrys]

@IvanGoncharov yes the two PR add the same functionality.
Tomorrow I integrate the branch where I implemented the stream and then I dedicate myself to making a review of @lilianammmatos 's PR