[Proposal] Add defer for field resolvers

[h-khodadadeh]

What happened?

Running field resolver on tests with mock using github.com/stretchr/testify results in dead-lock, if the called function is not defined.

What did you expect?

My test should run smoothly, no dead-lock.

Minimal graphql.schema and models to reproduce

Create a type like below and define
type Timeout {
Name: String!
}

define a resolver on the field. If you have a resolver, a goroutine will be created to run the resolver. If inside this goroutine runtime.Goexit() is called , then we'll have a dead-lock.

versions

• go run github.com/99designs/gqlgen version? v0.17.41
• go version? go version go1.21.0 linux/amd64

Long Version

I have a web server which has many types. Many of these types have resolver on their fields. If you takes a look at the code, you will see that

gqlgen/graphql/fieldset.go

Lines 37 to 57 in 24ea195

	func (m *FieldSet) Dispatch(ctx context.Context) {
	if len(m.delayed) == 1 {
	// only one concurrent task, no need to spawn a goroutine or deal create waitgroups
	d := m.delayed[0]
	m.Values[d.i] = d.f(ctx)
	} else if len(m.delayed) > 1 {
	// more than one concurrent task, use the main goroutine to do one, only spawn goroutines for the others
	var wg sync.WaitGroup
	for _, d := range m.delayed[1:] {
	wg.Add(1)
	go func(d delayedResult) {
	m.Values[d.i] = d.f(ctx)
	wg.Done()
	}(d)
	}
	m.Values[m.delayed[0].i] = m.delayed[0].f(ctx)
	wg.Wait()
	}
	}

a wait group is defined and it will run resolver functions in a separate goroutine. Once all the resolver functions are finished, wait group allows the code for move forward.

But Sometime Inside the resolver,I call a service which during a test a mock will be used. When the mock doesn't know what to return, it calls FailNow function on *testing.T which in turn calls runtime.Goexit() and it will kill the goroutine. This means that wg.Done() is not called and the application gets stuck at wg.Wait(). To fix this issue, I need to change:

gqlgen/graphql/fieldset.go

Lines 48 to 51 in 24ea195

	go func(d delayedResult) {
	m.Values[d.i] = d.f(ctx)
	wg.Done()
	}(d)

to

                       go func(d delayedResult) {
                               defer wg.Done()
				m.Values[d.i] = d.f(ctx)
			}(d)

to make sure even if runtime.Goexit() is called, the test will exit instead of running forever.

[UnAfraid]

Nice catch, i've open up a PR to fix it #2861