add worker_limit option for server code generation

[OldBigBuddha]

Overview

When executing a child resolver, a goroutine is created each time if there are multiple fields.

For example, if two fields are in 10,000 objects, and they are required to execute a child resolver, 20,000 goroutines will be created at once with a single request. Since each goroutine requires at least 2 KiB, each request requires approximately 41 MiB for ONLY goroutines.

In this PR, as discussed in #3371, I introduced an option to prevent unintended big memory pressure by restricting goroutines generated at once using a semaphore. To minimize the difference in automatically generated code between before and after this patch is merged, if worker_limit is not set, there is no diff from the previously generated code.

exec:
  filename: graph/generated.go
  package: graph
  worker_limit: 1000

If you set it as above, the following code will be generated.

ret := make(graphql.Array, len(v))
var wg sync.WaitGroup
sm := semaphore.NewWeighted(1000)
isLen1 := len(v) == 1
if !isLen1 {
wg.Add(len(v))
}
for i := range v {
i := i
fc := &graphql.FieldContext{
	Index:  &i,
	Result: &v[i],
}
ctx := graphql.WithFieldContext(ctx, fc)
f := func(i int) {
	defer func() {
		if r := recover(); r != nil {
			ec.Error(ctx, ec.Recover(ctx, r))
			ret = nil
		}
	}()
	if !isLen1 {
		defer func() {
			sm.Release(1)
			wg.Done()
		}()
	}
	ret[i] = ec.marshalNTodo2ᚖgithubᚗcomᚋOldBigBuddhaᚋgqlgenᚑplaygroundᚋgraphᚋmodelᚐTodo(ctx, sel, v[i])
}
if isLen1 {
	f(i)
} else {
	if err := sm.Acquire(ctx, 1); err != nil {
		ec.Error(ctx, ctx.Err())
	} else {
		go f(i)
	}
}

}
wg.Wait()

for _, e := range ret {
if e == graphql.Null {
	return graphql.Null
}
}

return ret

Note that worker_limit is an option that reduces memory consumption at the expense of concurrency, so excessive restrictions will unnecessarily prolong execution times and degrade application performance.

I appreciate @StevenACoffman for sharing this idea with me.

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[coveralls]

coverage: 59.578%. remained the same
when pulling 35e6a64 on OldBigBuddha:worker_limit
into 288848a on 99designs:master.