Fix bugs relating to optional fields with custom (un)marshalers (#116)

## Summary:
There were a few bugs here, one of which Craig came across when pulling
the custom-unmarshaler change into webapp:
1. If you have an optional field with a custom unmarshaler, and the
   server omits the field from the response entirely (i.e. does not
   write `"myField": null`), we would still call your unmarshaler with
   an input of `[]byte(nil)`.  This is just wrong; it's our job to do
   the nil-check.  (This is the one Craig found; in practice gqlgen
   servers do not do this and I think the spec says not to although it's
   a bit fuzzy on the matter of serialization.  But in practice we have
   mocks that do it -- for required fields even! -- and it seems better
   to handle it than pass you data on which you'll probably err or even
   panic.)
2. If you have an optional field with a custom unmarshaler, and the
   server returns an explicit null (i.e. `"myField": null`), we would
   call your unmarshaler with `[]byte("null")`.  In principle the intent
   was you're supposed to implement that, as [`json.Unmarshaler`
   advises][1].  But (a) I forgot to document that, and (b) in practice
   `json.Unmarshal` [does *not* call you in that case][2], i.e. its
   advice is unnecessary.  So I think it's better for us to just match
   it, and not call you.  (And in that case I see no reason to bother
   documenting the advice.)
3. If you have an optional, `pointer: true` field with a custom
   marshaler, the reverse of (2) applies: if the pointer is nil, we
   shouldn't really call you.  (Indeed if you were a real
   `json.Marshaler` with a value-method rather than a pointer-method,
   trying to call you might panic!)  Note we don't need to explicitly
   write "null"; we just leave the `json.RawMessage` as nil, and
   `json.Marshal` [handles that][3].
4. We handle interface types effectively the same as custom
   unmarshalers, just we generate the unmarshaler.  So if you have an
   optional field with interface type, (1) would also apply there; our
   generated unmarshaler returns an error in this case.
5. While (2) doesn't apply to such optional interface fields (because we
   do the customary `if string(b) == "null"` check -- this I at least
   thought to test), if you set `pointer: true` on the field, we would
   still call the unmarshaler on the value, and it would no-op, but only
   *after* we initialized the pointer.  Put more simply, we'd return a
   non-nil pointer to nil interface, rather than a nil pointer; this is
   wrong since the whole point of `pointer: true` is you only get a
   non-nil pointer if your value is nil!  Of course, in practice there's
   little reason to use `pointer: true` on interface fields, and indeed
   this stuff gets so confusing my test was even wrong.

In this commit I fix all the bugs, by adding appropriate nil-checks to
wrap the unmarshaler-calls.  The templates are, as always, a bit
confusing, but the generated code makes it clear what changed.

Note we'll want to land this before cutting a release with custom
marshaler/unmarshaler support, because the first three bugs are
potentially quite noticeable.  (The latter two are in `v0.1.0`, but
presumably quite rare.)

[1]: https://pkg.go.dev/encoding/json#Unmarshaler
[2]: https://play.golang.org/p/Pw6zNN8trGO
[3]: https://play.golang.org/p/crTfnT7ePte

Issue: https://phabricator.khanacademy.org/D74453#inline-558571

## Test plan:
make tesc


Author: benjaminjkraft

Reviewers: csilvers, StevenACoffman, benjaminjkraft, aberkan, dnerdy, jvoll, mahtabsabet, MiguelCastillo

Required Reviewers: 

Approved By: csilvers, StevenACoffman

Checks: ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint, ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint

Pull Request URL: https://github.com/Khan/genqlient/pull/116

internal/integration/generated.go

@@ -40,11 +40,13 @@ func (v *AnimalFields) UnmarshalJSON(b []byte) error {
 	{
 		dst := &v.Owner
 		src := firstPass.Owner
-		err = __unmarshalAnimalFieldsOwnerBeing(
-			src, dst)
-		if err != nil {
-			return fmt.Errorf(
-				"Unable to unmarshal AnimalFields.Owner: %w", err)
+		if len(src) != 0 && string(src) != "null" {
+			err = __unmarshalAnimalFieldsOwnerBeing(
+				src, dst)
+			if err != nil {
+				return fmt.Errorf(
+					"Unable to unmarshal AnimalFields.Owner: %w", err)
+			}
 		}
 	}
 	return nil
@@ -314,6 +316,39 @@ func (v *__queryWithCustomMarshalInput) MarshalJSON() ([]byte, error) {
 	return json.Marshal(&fullObject)
 }
 
+// __queryWithCustomMarshalOptionalInput is used internally by genqlient
+type __queryWithCustomMarshalOptionalInput struct {
+	Date *time.Time `json:"-"`
+	Id   *string    `json:"id"`
+}
+
+func (v *__queryWithCustomMarshalOptionalInput) MarshalJSON() ([]byte, error) {
+
+	var fullObject struct {
+		*__queryWithCustomMarshalOptionalInput
+		Date json.RawMessage `json:"date"`
+		graphql.NoMarshalJSON
+	}
+	fullObject.__queryWithCustomMarshalOptionalInput = v
+
+	{
+
+		dst := &fullObject.Date
+		src := v.Date
+		if src != nil {
+			var err error
+			*dst, err = testutil.MarshalDate(
+				src)
+			if err != nil {
+				return nil, fmt.Errorf(
+					"Unable to marshal __queryWithCustomMarshalOptionalInput.Date: %w", err)
+			}
+		}
+	}
+
+	return json.Marshal(&fullObject)
+}
+
 // __queryWithCustomMarshalSliceInput is used internally by genqlient
 type __queryWithCustomMarshalSliceInput struct {
 	Dates []time.Time `json:"-"`
@@ -396,6 +431,51 @@ type failingQueryResponse struct {
 	Me   failingQueryMeUser `json:"me"`
 }
 
+// queryWithCustomMarshalOptionalResponse is returned by queryWithCustomMarshalOptional on success.
+type queryWithCustomMarshalOptionalResponse struct {
+	UserSearch []queryWithCustomMarshalOptionalUserSearchUser `json:"userSearch"`
+}
+
+// queryWithCustomMarshalOptionalUserSearchUser includes the requested fields of the GraphQL type User.
+type queryWithCustomMarshalOptionalUserSearchUser struct {
+	Id        string    `json:"id"`
+	Name      string    `json:"name"`
+	Birthdate time.Time `json:"-"`
+}
+
+func (v *queryWithCustomMarshalOptionalUserSearchUser) UnmarshalJSON(b []byte) error {
+
+	if string(b) == "null" {
+		return nil
+	}
+
+	var firstPass struct {
+		*queryWithCustomMarshalOptionalUserSearchUser
+		Birthdate json.RawMessage `json:"birthdate"`
+		graphql.NoUnmarshalJSON
+	}
+	firstPass.queryWithCustomMarshalOptionalUserSearchUser = v
+
+	err := json.Unmarshal(b, &firstPass)
+	if err != nil {
+		return err
+	}
+
+	{
+		dst := &v.Birthdate
+		src := firstPass.Birthdate
+		if len(src) != 0 && string(src) != "null" {
+			err = testutil.UnmarshalDate(
+				src, dst)
+			if err != nil {
+				return fmt.Errorf(
+					"Unable to unmarshal queryWithCustomMarshalOptionalUserSearchUser.Birthdate: %w", err)
+			}
+		}
+	}
+	return nil
+}
+
 // queryWithCustomMarshalResponse is returned by queryWithCustomMarshal on success.
 type queryWithCustomMarshalResponse struct {
 	UsersBornOn []queryWithCustomMarshalUsersBornOnUser `json:"usersBornOn"`
@@ -434,11 +514,13 @@ func (v *queryWithCustomMarshalSliceUsersBornOnDatesUser) UnmarshalJSON(b []byte
 	{
 		dst := &v.Birthdate
 		src := firstPass.Birthdate
-		err = testutil.UnmarshalDate(
-			src, dst)
-		if err != nil {
-			return fmt.Errorf(
-				"Unable to unmarshal queryWithCustomMarshalSliceUsersBornOnDatesUser.Birthdate: %w", err)
+		if len(src) != 0 && string(src) != "null" {
+			err = testutil.UnmarshalDate(
+				src, dst)
+			if err != nil {
+				return fmt.Errorf(
+					"Unable to unmarshal queryWithCustomMarshalSliceUsersBornOnDatesUser.Birthdate: %w", err)
+			}
 		}
 	}
 	return nil
@@ -472,11 +554,13 @@ func (v *queryWithCustomMarshalUsersBornOnUser) UnmarshalJSON(b []byte) error {
 	{
 		dst := &v.Birthdate
 		src := firstPass.Birthdate
-		err = testutil.UnmarshalDate(
-			src, dst)
-		if err != nil {
-			return fmt.Errorf(
-				"Unable to unmarshal queryWithCustomMarshalUsersBornOnUser.Birthdate: %w", err)
+		if len(src) != 0 && string(src) != "null" {
+			err = testutil.UnmarshalDate(
+				src, dst)
+			if err != nil {
+				return fmt.Errorf(
+					"Unable to unmarshal queryWithCustomMarshalUsersBornOnUser.Birthdate: %w", err)
+			}
 		}
 	}
 	return nil
@@ -513,11 +597,13 @@ func (v *queryWithFragmentsBeingsAnimal) UnmarshalJSON(b []byte) error {
 	{
 		dst := &v.Owner
 		src := firstPass.Owner
-		err = __unmarshalqueryWithFragmentsBeingsAnimalOwnerBeing(
-			src, dst)
-		if err != nil {
-			return fmt.Errorf(
-				"Unable to unmarshal queryWithFragmentsBeingsAnimal.Owner: %w", err)
+		if len(src) != 0 && string(src) != "null" {
+			err = __unmarshalqueryWithFragmentsBeingsAnimalOwnerBeing(
+				src, dst)
+			if err != nil {
+				return fmt.Errorf(
+					"Unable to unmarshal queryWithFragmentsBeingsAnimal.Owner: %w", err)
+			}
 		}
 	}
 	return nil
@@ -722,11 +808,13 @@ func (v *queryWithFragmentsResponse) UnmarshalJSON(b []byte) error {
 			len(src))
 		for i, src := range src {
 			dst := &(*dst)[i]
-			err = __unmarshalqueryWithFragmentsBeingsBeing(
-				src, dst)
-			if err != nil {
-				return fmt.Errorf(
-					"Unable to unmarshal queryWithFragmentsResponse.Beings: %w", err)
+			if len(src) != 0 && string(src) != "null" {
+				err = __unmarshalqueryWithFragmentsBeingsBeing(
+					src, dst)
+				if err != nil {
+					return fmt.Errorf(
+						"Unable to unmarshal queryWithFragmentsResponse.Beings: %w", err)
+				}
 			}
 		}
 	}
@@ -846,11 +934,13 @@ func (v *queryWithInterfaceListFieldResponse) UnmarshalJSON(b []byte) error {
 			len(src))
 		for i, src := range src {
 			dst := &(*dst)[i]
-			err = __unmarshalqueryWithInterfaceListFieldBeingsBeing(
-				src, dst)
-			if err != nil {
-				return fmt.Errorf(
-					"Unable to unmarshal queryWithInterfaceListFieldResponse.Beings: %w", err)
+			if len(src) != 0 && string(src) != "null" {
+				err = __unmarshalqueryWithInterfaceListFieldBeingsBeing(
+					src, dst)
+				if err != nil {
+					return fmt.Errorf(
+						"Unable to unmarshal queryWithInterfaceListFieldResponse.Beings: %w", err)
+				}
 			}
 		}
 	}
@@ -970,12 +1060,14 @@ func (v *queryWithInterfaceListPointerFieldResponse) UnmarshalJSON(b []byte) err
 			len(src))
 		for i, src := range src {
 			dst := &(*dst)[i]
-			*dst = new(queryWithInterfaceListPointerFieldBeingsBeing)
-			err = __unmarshalqueryWithInterfaceListPointerFieldBeingsBeing(
-				src, *dst)
-			if err != nil {
-				return fmt.Errorf(
-					"Unable to unmarshal queryWithInterfaceListPointerFieldResponse.Beings: %w", err)
+			if len(src) != 0 && string(src) != "null" {
+				*dst = new(queryWithInterfaceListPointerFieldBeingsBeing)
+				err = __unmarshalqueryWithInterfaceListPointerFieldBeingsBeing(
+					src, *dst)
+				if err != nil {
+					return fmt.Errorf(
+						"Unable to unmarshal queryWithInterfaceListPointerFieldResponse.Beings: %w", err)
+				}
 			}
 		}
 	}
@@ -1097,11 +1189,13 @@ func (v *queryWithInterfaceNoFragmentsResponse) UnmarshalJSON(b []byte) error {
 	{
 		dst := &v.Being
 		src := firstPass.Being
-		err = __unmarshalqueryWithInterfaceNoFragmentsBeing(
-			src, dst)
-		if err != nil {
-			return fmt.Errorf(
-				"Unable to unmarshal queryWithInterfaceNoFragmentsResponse.Being: %w", err)
+		if len(src) != 0 && string(src) != "null" {
+			err = __unmarshalqueryWithInterfaceNoFragmentsBeing(
+				src, dst)
+			if err != nil {
+				return fmt.Errorf(
+					"Unable to unmarshal queryWithInterfaceNoFragmentsResponse.Being: %w", err)
+			}
 		}
 	}
 	return nil
@@ -1262,11 +1356,13 @@ func (v *queryWithNamedFragmentsResponse) UnmarshalJSON(b []byte) error {
 			len(src))
 		for i, src := range src {
 			dst := &(*dst)[i]
-			err = __unmarshalqueryWithNamedFragmentsBeingsBeing(
-				src, dst)
-			if err != nil {
-				return fmt.Errorf(
-					"Unable to unmarshal queryWithNamedFragmentsResponse.Beings: %w", err)
+			if len(src) != 0 && string(src) != "null" {
+				err = __unmarshalqueryWithNamedFragmentsBeingsBeing(
+					src, dst)
+				if err != nil {
+					return fmt.Errorf(
+						"Unable to unmarshal queryWithNamedFragmentsResponse.Beings: %w", err)
+				}
 			}
 		}
 	}
@@ -1474,6 +1570,37 @@ query queryWithCustomMarshalSlice ($dates: [Date!]!) {
 	return &retval, err
 }
 
+func queryWithCustomMarshalOptional(
+	ctx context.Context,
+	client graphql.Client,
+	date *time.Time,
+	id *string,
+) (*queryWithCustomMarshalOptionalResponse, error) {
+	__input := __queryWithCustomMarshalOptionalInput{
+		Date: date,
+		Id:   id,
+	}
+	var err error
+
+	var retval queryWithCustomMarshalOptionalResponse
+	err = client.MakeRequest(
+		ctx,
+		"queryWithCustomMarshalOptional",
+		`
+query queryWithCustomMarshalOptional ($date: Date, $id: ID) {
+	userSearch(birthdate: $date, id: $id) {
+		id
+		name
+		birthdate
+	}
+}
+`,
+		&retval,
+		&__input,
+	)
+	return &retval, err
+}
+
 func queryWithInterfaceNoFragments(
 	ctx context.Context,
 	client graphql.Client,

internal/integration/integration_test.go

@@ -175,6 +175,44 @@ func TestCustomMarshalSlice(t *testing.T) {
 	assert.Len(t, resp.UsersBornOnDates, 0)
 }
 
+func TestCustomMarshalOptional(t *testing.T) {
+	_ = `# @genqlient
+	query queryWithCustomMarshalOptional(
+		# @genqlient(pointer: true)
+		$date: Date,
+		# @genqlient(pointer: true)
+		$id: ID,
+	) {
+		userSearch(birthdate: $date, id: $id) { id name birthdate }
+	}`
+
+	ctx := context.Background()
+	server := server.RunServer()
+	defer server.Close()
+	client := graphql.NewClient(server.URL, http.DefaultClient)
+
+	date := time.Date(2025, time.January, 1, 12, 34, 56, 789, time.UTC)
+	resp, err := queryWithCustomMarshalOptional(ctx, client, &date, nil)
+	require.NoError(t, err)
+
+	assert.Len(t, resp.UserSearch, 1)
+	user := resp.UserSearch[0]
+	assert.Equal(t, "1", user.Id)
+	assert.Equal(t, "Yours Truly", user.Name)
+	assert.Equal(t,
+		time.Date(2025, time.January, 1, 0, 0, 0, 0, time.UTC),
+		user.Birthdate)
+
+	id := "2"
+	resp, err = queryWithCustomMarshalOptional(ctx, client, nil, &id)
+	require.NoError(t, err)
+	assert.Len(t, resp.UserSearch, 1)
+	user = resp.UserSearch[0]
+	assert.Equal(t, "2", user.Id)
+	assert.Equal(t, "Raven", user.Name)
+	assert.Zero(t, user.Birthdate)
+}
+
 func TestInterfaceNoFragments(t *testing.T) {
 	_ = `# @genqlient
 	query queryWithInterfaceNoFragments($id: ID!) {
@@ -322,7 +360,7 @@ func TestInterfaceListPointerField(t *testing.T) {
 	assert.Equal(t, "3", animal.Id)
 	assert.Equal(t, "Fido", animal.Name)
 
-	assert.Nil(t, *resp.Beings[2])
+	assert.Nil(t, resp.Beings[2])
 }
 
 func TestFragments(t *testing.T) {

internal/integration/schema.graphql

@@ -8,6 +8,7 @@ type Query {
   lotteryWinner(number: Int!): Lucky
   usersBornOn(date: Date!): [User!]!
   usersBornOnDates(dates: [Date!]!): [User!]!
+  userSearch(birthdate: Date, id: ID): [User]
   fail: Boolean
 }
 

internal/integration/server/gqlgen_exec.go

@@ -65,6 +65,7 @@ type ComplexityRoot struct {
 		LotteryWinner    func(childComplexity int, number int) int
 		Me               func(childComplexity int) int
 		User             func(childComplexity int, id *string) int
+		UserSearch       func(childComplexity int, birthdate *string, id *string) int
 		UsersBornOn      func(childComplexity int, date string) int
 		UsersBornOnDates func(childComplexity int, dates []string) int
 	}
@@ -86,6 +87,7 @@ type QueryResolver interface {
 	LotteryWinner(ctx context.Context, number int) (Lucky, error)
 	UsersBornOn(ctx context.Context, date string) ([]*User, error)
 	UsersBornOnDates(ctx context.Context, dates []string) ([]*User, error)
+	UserSearch(ctx context.Context, birthdate *string, id *string) ([]*User, error)
 	Fail(ctx context.Context) (*bool, error)
 }
 
@@ -215,6 +217,18 @@ func (e *executableSchema) Complexity(typeName, field string, childComplexity in
 
 		return e.complexity.Query.User(childComplexity, args["id"].(*string)), true
 
+	case "Query.userSearch":
+		if e.complexity.Query.UserSearch == nil {
+			break
+		}
+
+		args, err := ec.field_Query_userSearch_args(context.TODO(), rawArgs)
+		if err != nil {
+			return 0, false
+		}
+
+		return e.complexity.Query.UserSearch(childComplexity, args["birthdate"].(*string), args["id"].(*string)), true
+
 	case "Query.usersBornOn":
 		if e.complexity.Query.UsersBornOn == nil {
 			break
@@ -334,6 +348,7 @@ type Query {
   lotteryWinner(number: Int!): Lucky
   usersBornOn(date: Date!): [User!]!
   usersBornOnDates(dates: [Date!]!): [User!]!
+  userSearch(birthdate: Date, id: ID): [User]
   fail: Boolean
 }
 
@@ -438,6 +453,30 @@ func (ec *executionContext) field_Query_lotteryWinner_args(ctx context.Context,
 	return args, nil
 }
 
+func (ec *executionContext) field_Query_userSearch_args(ctx context.Context, rawArgs map[string]interface{}) (map[string]interface{}, error) {
+	var err error
+	args := map[string]interface{}{}
+	var arg0 *string
+	if tmp, ok := rawArgs["birthdate"]; ok {
+		ctx := graphql.WithPathContext(ctx, graphql.NewPathWithField("birthdate"))
+		arg0, err = ec.unmarshalODate2ᚖstring(ctx, tmp)
+		if err != nil {
+			return nil, err
+		}
+	}
+	args["birthdate"] = arg0
+	var arg1 *string
+	if tmp, ok := rawArgs["id"]; ok {
+		ctx := graphql.WithPathContext(ctx, graphql.NewPathWithField("id"))
+		arg1, err = ec.unmarshalOID2ᚖstring(ctx, tmp)
+		if err != nil {
+			return nil, err
+		}
+	}
+	args["id"] = arg1
+	return args, nil
+}
+
 func (ec *executionContext) field_Query_user_args(ctx context.Context, rawArgs map[string]interface{}) (map[string]interface{}, error) {
 	var err error
 	args := map[string]interface{}{}
@@ -1032,6 +1071,45 @@ func (ec *executionContext) _Query_usersBornOnDates(ctx context.Context, field g
 	return ec.marshalNUser2ᚕᚖgithubᚗcomᚋKhanᚋgenqlientᚋinternalᚋintegrationᚋserverᚐUserᚄ(ctx, field.Selections, res)
 }
 
+func (ec *executionContext) _Query_userSearch(ctx context.Context, field graphql.CollectedField) (ret graphql.Marshaler) {
+	defer func() {
+		if r := recover(); r != nil {
+			ec.Error(ctx, ec.Recover(ctx, r))
+			ret = graphql.Null
+		}
+	}()
+	fc := &graphql.FieldContext{
+		Object:     "Query",
+		Field:      field,
+		Args:       nil,
+		IsMethod:   true,
+		IsResolver: true,
+	}
+
+	ctx = graphql.WithFieldContext(ctx, fc)
+	rawArgs := field.ArgumentMap(ec.Variables)
+	args, err := ec.field_Query_userSearch_args(ctx, rawArgs)
+	if err != nil {
+		ec.Error(ctx, err)
+		return graphql.Null
+	}
+	fc.Args = args
+	resTmp, err := ec.ResolverMiddleware(ctx, func(rctx context.Context) (interface{}, error) {
+		ctx = rctx // use context from middleware stack in children
+		return ec.resolvers.Query().UserSearch(rctx, args["birthdate"].(*string), args["id"].(*string))
+	})
+	if err != nil {
+		ec.Error(ctx, err)
+		return graphql.Null
+	}
+	if resTmp == nil {
+		return graphql.Null
+	}
+	res := resTmp.([]*User)
+	fc.Result = res
+	return ec.marshalOUser2ᚕᚖgithubᚗcomᚋKhanᚋgenqlientᚋinternalᚋintegrationᚋserverᚐUser(ctx, field.Selections, res)
+}
+
 func (ec *executionContext) _Query_fail(ctx context.Context, field graphql.CollectedField) (ret graphql.Marshaler) {
 	defer func() {
 		if r := recover(); r != nil {
@@ -2628,6 +2706,17 @@ func (ec *executionContext) _Query(ctx context.Context, sel ast.SelectionSet) gr
 				}
 				return res
 			})
+		case "userSearch":
+			field := field
+			out.Concurrently(i, func() (res graphql.Marshaler) {
+				defer func() {
+					if r := recover(); r != nil {
+						ec.Error(ctx, ec.Recover(ctx, r))
+					}
+				}()
+				res = ec._Query_userSearch(ctx, field)
+				return res
+			})
 		case "fail":
 			field := field
 			out.Concurrently(i, func() (res graphql.Marshaler) {
@@ -3516,6 +3605,46 @@ func (ec *executionContext) marshalOString2ᚖstring(ctx context.Context, sel as
 	return graphql.MarshalString(*v)
 }
 
+func (ec *executionContext) marshalOUser2ᚕᚖgithubᚗcomᚋKhanᚋgenqlientᚋinternalᚋintegrationᚋserverᚐUser(ctx context.Context, sel ast.SelectionSet, v []*User) graphql.Marshaler {
+	if v == nil {
+		return graphql.Null
+	}
+	ret := make(graphql.Array, len(v))
+	var wg sync.WaitGroup
+	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 wg.Done()
+			}
+			ret[i] = ec.marshalOUser2ᚖgithubᚗcomᚋKhanᚋgenqlientᚋinternalᚋintegrationᚋserverᚐUser(ctx, sel, v[i])
+		}
+		if isLen1 {
+			f(i)
+		} else {
+			go f(i)
+		}
+
+	}
+	wg.Wait()
+	return ret
+}
+
 func (ec *executionContext) marshalOUser2ᚖgithubᚗcomᚋKhanᚋgenqlientᚋinternalᚋintegrationᚋserverᚐUser(ctx context.Context, sel ast.SelectionSet, v *User) graphql.Marshaler {
 	if v == nil {
 		return graphql.Null

internal/integration/server/server.go

@@ -107,6 +107,17 @@ func (r *queryResolver) UsersBornOnDates(ctx context.Context, dates []string) ([
 	return usersByBirthdates(dates), nil
 }
 
+func (r *queryResolver) UserSearch(ctx context.Context, birthdate *string, id *string) ([]*User, error) {
+	switch {
+	case birthdate == nil && id != nil:
+		return []*User{userByID(*id)}, nil
+	case birthdate != nil && id == nil:
+		return usersByBirthdates([]string{*birthdate}), nil
+	default:
+		return nil, fmt.Errorf("need exactly one of birthdate or id")
+	}
+}
+
 func (r *queryResolver) Fail(ctx context.Context) (*bool, error) {
 	f := true
 	return &f, fmt.Errorf("oh no")

internal/testutil/types.go

@@ -36,10 +36,6 @@ func MarshalDate(t *time.Time) ([]byte, error) {
 
 func UnmarshalDate(b []byte, t *time.Time) error {
 	// (modified from time.Time.UnmarshalJSON)
-	if string(b) == "null" {
-		return nil
-	}
-
 	var err error
 	*t, err = time.Parse(`"`+dateFormat+`"`, string(b))
 	return err