Resolve requests for federation entities in parallel

In apollo federation, we may be asked for data about a list of entities.
These can typically be resolved in parallel, just as with sibling fields
in ordinary GraphQL queries.  Now we do!

Note that I kept the behavior where if one lookup fails, we cancel the
whole assembly.  It's not clear to me if that's the ideal behavior --
there are arguments both ways -- but I figured it was easiest to retain
it for now.  (I used x/sync/errgroup to do so; it's already a transitive
dep via x/tools but if the new direct dep is an issue I can do it with
sync.Waitgroup instead, at the cost of some extra code.)

The examples probably give the clearest picture of the changes. (And the
clearest test; the changed functionality is already exercised by
`integration-test.js` as watching the test server logs will attest.)

Fixes 99designs#1278.

go.mod

@@ -25,6 +25,7 @@ require (
 	github.com/urfave/cli/v2 v2.1.1
 	github.com/vektah/dataloaden v0.2.1-0.20190515034641-a19b9a6e7c9e
 	github.com/vektah/gqlparser/v2 v2.1.0
+	golang.org/x/sync v0.0.0-20190423024810-112230192c58
 	golang.org/x/tools v0.0.0-20200114235610-7ae403b6b589
 	gopkg.in/yaml.v2 v2.2.4
 	sourcegraph.com/sourcegraph/appdash v0.0.0-20180110180208-2cc67fd64755

go.sum

@@ -82,7 +82,9 @@ golang.org/x/net v0.0.0-20190311183353-d8887717615a/go.mod h1:t9HGtf8HONx5eT2rtn
 golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
 golang.org/x/net v0.0.0-20190620200207-3b0461eec859 h1:R/3boaszxrf1GEUWTVDzSKVwLmSJpwZ1yqXm8j0v2QI=
 golang.org/x/net v0.0.0-20190620200207-3b0461eec859/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
+golang.org/x/sync v0.0.0-20190423024810-112230192c58 h1:8gQV6CLnAEikrhgkHFbMAEhagSSnXWGV915qUMm9mrU=
 golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sync v0.0.0-20200625203802-6e8e738ad208 h1:qwRHBd0NqMbJxfbotnDhm2ByMI1Shq4Y6oRJo21SGJA=
 golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20190222072716-a9d3bda3a223/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20190412213103-97732733099d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=

plugin/federation/federation.gotpl

@@ -3,6 +3,7 @@
 {{ reserveImport "fmt"  }}
 {{ reserveImport "strings"  }}
 
+{{ reserveImport "golang.org/x/sync/errgroup" }}
 {{ reserveImport "github.com/99designs/gqlgen/plugin/federation/fedruntime" }}
 
 func (ec *executionContext) __resolve__service(ctx context.Context) (fedruntime.Service, error) {
@@ -26,11 +27,11 @@ func (ec *executionContext) __resolve__service(ctx context.Context) (fedruntime.
 
 {{if .Entities}}
 func (ec *executionContext) __resolve_entities(ctx context.Context, representations []map[string]interface{}) ([]fedruntime.Entity, error) {
-	list := []fedruntime.Entity{}
-	for _, rep := range representations {
+	list := make([]fedruntime.Entity, len(representations))
+	resolveEntity := func(ctx context.Context, i int, rep map[string]interface{}) error {
 		typeName, ok := rep["__typename"].(string)
 		if !ok {
-			return nil, errors.New("__typename must be an existing string")
+			return errors.New("__typename must be an existing string")
 		}
 		switch typeName {
 		{{ range .Entities }}
@@ -39,31 +40,52 @@ func (ec *executionContext) __resolve_entities(ctx context.Context, representati
 				{{ range $i, $keyField := .KeyFields -}}
 					id{{$i}}, err := ec.{{.TypeReference.UnmarshalFunc}}(ctx, rep["{{$keyField.Field.Name}}"])
 					if err != nil {
-						return nil, errors.New(fmt.Sprintf("Field %s undefined in schema.", "{{$keyField.Field.Name}}"))
+						return errors.New(fmt.Sprintf("Field %s undefined in schema.", "{{$keyField.Field.Name}}"))
 					}
 				{{end}}
 
 				entity, err := ec.resolvers.Entity().{{.ResolverName | go}}(ctx,
 					{{ range $i, $_ := .KeyFields -}} id{{$i}}, {{end}})
 				if err != nil {
-					return nil, err
+					return err
 				}
 
 				{{ range .Requires }}
 					{{ range .Fields}}
 						entity.{{.NameGo}}, err = ec.{{.TypeReference.UnmarshalFunc}}(ctx, rep["{{.Name}}"])
 						if err != nil {
-							return nil, err
+							return err
 						}
 					{{ end }}
 				{{ end }}
-				list = append(list, entity)
+                list[i] = entity
+				return nil
 			{{ end }}
 		{{ end }}
 		default:
-			return nil, errors.New("unknown type: "+typeName)
+			return errors.New("unknown type: "+typeName)
 		}
 	}
-	return list, nil
+
+	// if there are multiple entities to resolve, parallelize (similar to
+	// graphql.FieldSet.Dispatch)
+	switch len(representations) {
+	case 0:
+		return list, nil
+	case 1:
+		err := resolveEntity(ctx, 0, representations[0])
+		return list, err
+	default:
+		eg, gCtx := errgroup.WithContext(ctx)
+		for i, rep := range representations {
+			i, rep := i, rep
+			eg.Go(func() error { return resolveEntity(gCtx, i, rep) })
+		}
+		err := eg.Wait()
+		if err != nil {
+			return nil, err
+		}
+		return list, nil
+	}
 }
 {{end}}