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

generate/testdata/snapshots/TestGenerate-ComplexInlineFragments.graphql-ComplexInlineFragments.graphql.go

@@ -206,11 +206,13 @@ func (v *ComplexInlineFragmentsNestedStuffTopic) UnmarshalJSON(b []byte) error {
 			len(src))
 		for i, src := range src {
 			dst := &(*dst)[i]
-			err = __unmarshalComplexInlineFragmentsNestedStuffTopicChildrenContent(
-				src, dst)
-			if err != nil {
-				return fmt.Errorf(
-					"Unable to unmarshal ComplexInlineFragmentsNestedStuffTopic.Children: %w", err)
+			if len(src) != 0 && string(src) != "null" {
+				err = __unmarshalComplexInlineFragmentsNestedStuffTopicChildrenContent(
+					src, dst)
+				if err != nil {
+					return fmt.Errorf(
+						"Unable to unmarshal ComplexInlineFragmentsNestedStuffTopic.Children: %w", err)
+				}
 			}
 		}
 	}
@@ -257,11 +259,13 @@ func (v *ComplexInlineFragmentsNestedStuffTopicChildrenArticleParentContentParen
 			len(src))
 		for i, src := range src {
 			dst := &(*dst)[i]
-			err = __unmarshalComplexInlineFragmentsNestedStuffTopicChildrenArticleParentContentParentTopicChildrenContent(
-				src, dst)
-			if err != nil {
-				return fmt.Errorf(
-					"Unable to unmarshal ComplexInlineFragmentsNestedStuffTopicChildrenArticleParentContentParentTopic.Children: %w", err)
+			if len(src) != 0 && string(src) != "null" {
+				err = __unmarshalComplexInlineFragmentsNestedStuffTopicChildrenArticleParentContentParentTopicChildrenContent(
+					src, dst)
+				if err != nil {
+					return fmt.Errorf(
+						"Unable to unmarshal ComplexInlineFragmentsNestedStuffTopicChildrenArticleParentContentParentTopic.Children: %w", err)
+				}
 			}
 		}
 	}
@@ -819,44 +823,52 @@ func (v *ComplexInlineFragmentsResponse) UnmarshalJSON(b []byte) error {
 	{
 		dst := &v.RandomItem
 		src := firstPass.RandomItem
-		err = __unmarshalComplexInlineFragmentsRandomItemContent(
-			src, dst)
-		if err != nil {
-			return fmt.Errorf(
-				"Unable to unmarshal ComplexInlineFragmentsResponse.RandomItem: %w", err)
+		if len(src) != 0 && string(src) != "null" {
+			err = __unmarshalComplexInlineFragmentsRandomItemContent(
+				src, dst)
+			if err != nil {
+				return fmt.Errorf(
+					"Unable to unmarshal ComplexInlineFragmentsResponse.RandomItem: %w", err)
+			}
 		}
 	}
 
 	{
 		dst := &v.RepeatedStuff
 		src := firstPass.RepeatedStuff
-		err = __unmarshalComplexInlineFragmentsRepeatedStuffContent(
-			src, dst)
-		if err != nil {
-			return fmt.Errorf(
-				"Unable to unmarshal ComplexInlineFragmentsResponse.RepeatedStuff: %w", err)
+		if len(src) != 0 && string(src) != "null" {
+			err = __unmarshalComplexInlineFragmentsRepeatedStuffContent(
+				src, dst)
+			if err != nil {
+				return fmt.Errorf(
+					"Unable to unmarshal ComplexInlineFragmentsResponse.RepeatedStuff: %w", err)
+			}
 		}
 	}
 
 	{
 		dst := &v.ConflictingStuff
 		src := firstPass.ConflictingStuff
-		err = __unmarshalComplexInlineFragmentsConflictingStuffContent(
-			src, dst)
-		if err != nil {
-			return fmt.Errorf(
-				"Unable to unmarshal ComplexInlineFragmentsResponse.ConflictingStuff: %w", err)
+		if len(src) != 0 && string(src) != "null" {
+			err = __unmarshalComplexInlineFragmentsConflictingStuffContent(
+				src, dst)
+			if err != nil {
+				return fmt.Errorf(
+					"Unable to unmarshal ComplexInlineFragmentsResponse.ConflictingStuff: %w", err)
+			}
 		}
 	}
 
 	{
 		dst := &v.NestedStuff
 		src := firstPass.NestedStuff
-		err = __unmarshalComplexInlineFragmentsNestedStuffContent(
-			src, dst)
-		if err != nil {
-			return fmt.Errorf(
-				"Unable to unmarshal ComplexInlineFragmentsResponse.NestedStuff: %w", err)
+		if len(src) != 0 && string(src) != "null" {
+			err = __unmarshalComplexInlineFragmentsNestedStuffContent(
+				src, dst)
+			if err != nil {
+				return fmt.Errorf(
+					"Unable to unmarshal ComplexInlineFragmentsResponse.NestedStuff: %w", err)
+			}
 		}
 	}
 	return nil

generate/testdata/snapshots/TestGenerate-ComplexNamedFragments.graphql-ComplexNamedFragments.graphql.go

@@ -173,33 +173,39 @@ func (v *InnerQueryFragment) UnmarshalJSON(b []byte) error {
 	{
 		dst := &v.RandomItem
 		src := firstPass.RandomItem
-		err = __unmarshalInnerQueryFragmentRandomItemContent(
-			src, dst)
-		if err != nil {
-			return fmt.Errorf(
-				"Unable to unmarshal InnerQueryFragment.RandomItem: %w", err)
+		if len(src) != 0 && string(src) != "null" {
+			err = __unmarshalInnerQueryFragmentRandomItemContent(
+				src, dst)
+			if err != nil {
+				return fmt.Errorf(
+					"Unable to unmarshal InnerQueryFragment.RandomItem: %w", err)
+			}
 		}
 	}
 
 	{
 		dst := &v.RandomLeaf
 		src := firstPass.RandomLeaf
-		err = __unmarshalInnerQueryFragmentRandomLeafLeafContent(
-			src, dst)
-		if err != nil {
-			return fmt.Errorf(
-				"Unable to unmarshal InnerQueryFragment.RandomLeaf: %w", err)
+		if len(src) != 0 && string(src) != "null" {
+			err = __unmarshalInnerQueryFragmentRandomLeafLeafContent(
+				src, dst)
+			if err != nil {
+				return fmt.Errorf(
+					"Unable to unmarshal InnerQueryFragment.RandomLeaf: %w", err)
+			}
 		}
 	}
 
 	{
 		dst := &v.OtherLeaf
 		src := firstPass.OtherLeaf
-		err = __unmarshalInnerQueryFragmentOtherLeafLeafContent(
-			src, dst)
-		if err != nil {
-			return fmt.Errorf(
-				"Unable to unmarshal InnerQueryFragment.OtherLeaf: %w", err)
+		if len(src) != 0 && string(src) != "null" {
+			err = __unmarshalInnerQueryFragmentOtherLeafLeafContent(
+				src, dst)
+			if err != nil {
+				return fmt.Errorf(
+					"Unable to unmarshal InnerQueryFragment.OtherLeaf: %w", err)
+			}
 		}
 	}
 	return nil
@@ -703,11 +709,13 @@ func (v *MoreVideoFieldsParentTopic) UnmarshalJSON(b []byte) error {
 			len(src))
 		for i, src := range src {
 			dst := &(*dst)[i]
-			err = __unmarshalMoreVideoFieldsParentTopicChildrenContent(
-				src, dst)
-			if err != nil {
-				return fmt.Errorf(
-					"Unable to unmarshal MoreVideoFieldsParentTopic.Children: %w", err)
+			if len(src) != 0 && string(src) != "null" {
+				err = __unmarshalMoreVideoFieldsParentTopicChildrenContent(
+					src, dst)
+				if err != nil {
+					return fmt.Errorf(
+						"Unable to unmarshal MoreVideoFieldsParentTopic.Children: %w", err)
+				}
 			}
 		}
 	}

generate/testdata/snapshots/TestGenerate-CustomMarshal.graphql-CustomMarshal.graphql.go

@@ -49,11 +49,13 @@ func (v *CustomMarshalUsersBornOnUser) UnmarshalJSON(b []byte) error {
 	{
 		dst := &v.Birthdate
 		src := firstPass.Birthdate
-		err = testutil.UnmarshalDate(
-			src, dst)
-		if err != nil {
-			return fmt.Errorf(
-				"Unable to unmarshal CustomMarshalUsersBornOnUser.Birthdate: %w", err)
+		if len(src) != 0 && string(src) != "null" {
+			err = testutil.UnmarshalDate(
+				src, dst)
+			if err != nil {
+				return fmt.Errorf(
+					"Unable to unmarshal CustomMarshalUsersBornOnUser.Birthdate: %w", err)
+			}
 		}
 	}
 	return nil

generate/testdata/snapshots/TestGenerate-CustomMarshalSlice.graphql-CustomMarshalSlice.graphql.go

@@ -82,12 +82,14 @@ func (v *__CustomMarshalSliceInput) MarshalJSON() ([]byte, error) {
 					len(src))
 				for i, src := range src {
 					dst := &(*dst)[i]
-					var err error
-					*dst, err = testutil.MarshalDate(
-						src)
-					if err != nil {
-						return nil, fmt.Errorf(
-							"Unable to marshal __CustomMarshalSliceInput.Datesssp: %w", err)
+					if src != nil {
+						var err error
+						*dst, err = testutil.MarshalDate(
+							src)
+						if err != nil {
+							return nil, fmt.Errorf(
+								"Unable to marshal __CustomMarshalSliceInput.Datesssp: %w", err)
+						}
 					}
 				}
 			}

generate/testdata/snapshots/TestGenerate-InterfaceListField.graphql-InterfaceListField.graphql.go

@@ -50,11 +50,13 @@ func (v *InterfaceListFieldRootTopic) UnmarshalJSON(b []byte) error {
 			len(src))
 		for i, src := range src {
 			dst := &(*dst)[i]
-			err = __unmarshalInterfaceListFieldRootTopicChildrenContent(
-				src, dst)
-			if err != nil {
-				return fmt.Errorf(
-					"Unable to unmarshal InterfaceListFieldRootTopic.Children: %w", err)
+			if len(src) != 0 && string(src) != "null" {
+				err = __unmarshalInterfaceListFieldRootTopicChildrenContent(
+					src, dst)
+				if err != nil {
+					return fmt.Errorf(
+						"Unable to unmarshal InterfaceListFieldRootTopic.Children: %w", err)
+				}
 			}
 		}
 	}
@@ -209,11 +211,13 @@ func (v *InterfaceListFieldWithPointerTopic) UnmarshalJSON(b []byte) error {
 			len(src))
 		for i, src := range src {
 			dst := &(*dst)[i]
-			err = __unmarshalInterfaceListFieldWithPointerTopicChildrenContent(
-				src, dst)
-			if err != nil {
-				return fmt.Errorf(
-					"Unable to unmarshal InterfaceListFieldWithPointerTopic.Children: %w", err)
+			if len(src) != 0 && string(src) != "null" {
+				err = __unmarshalInterfaceListFieldWithPointerTopicChildrenContent(
+					src, dst)
+				if err != nil {
+					return fmt.Errorf(
+						"Unable to unmarshal InterfaceListFieldWithPointerTopic.Children: %w", err)
+				}
 			}
 		}
 	}

generate/testdata/snapshots/TestGenerate-InterfaceListOfListsOfListsField.graphql-InterfaceListOfListsOfListsField.graphql.go

@@ -185,11 +185,13 @@ func (v *InterfaceListOfListOfListsFieldResponse) UnmarshalJSON(b []byte) error
 					len(src))
 				for i, src := range src {
 					dst := &(*dst)[i]
-					err = __unmarshalInterfaceListOfListOfListsFieldListOfListsOfListsOfContent(
-						src, dst)
-					if err != nil {
-						return fmt.Errorf(
-							"Unable to unmarshal InterfaceListOfListOfListsFieldResponse.ListOfListsOfListsOfContent: %w", err)
+					if len(src) != 0 && string(src) != "null" {
+						err = __unmarshalInterfaceListOfListOfListsFieldListOfListsOfListsOfContent(
+							src, dst)
+						if err != nil {
+							return fmt.Errorf(
+								"Unable to unmarshal InterfaceListOfListOfListsFieldResponse.ListOfListsOfListsOfContent: %w", err)
+						}
 					}
 				}
 			}
@@ -214,12 +216,14 @@ func (v *InterfaceListOfListOfListsFieldResponse) UnmarshalJSON(b []byte) error
 					len(src))
 				for i, src := range src {
 					dst := &(*dst)[i]
-					*dst = new(InterfaceListOfListOfListsFieldWithPointerContent)
-					err = __unmarshalInterfaceListOfListOfListsFieldWithPointerContent(
-						src, *dst)
-					if err != nil {
-						return fmt.Errorf(
-							"Unable to unmarshal InterfaceListOfListOfListsFieldResponse.WithPointer: %w", err)
+					if len(src) != 0 && string(src) != "null" {
+						*dst = new(InterfaceListOfListOfListsFieldWithPointerContent)
+						err = __unmarshalInterfaceListOfListOfListsFieldWithPointerContent(
+							src, *dst)
+						if err != nil {
+							return fmt.Errorf(
+								"Unable to unmarshal InterfaceListOfListOfListsFieldResponse.WithPointer: %w", err)
+						}
 					}
 				}
 			}

generate/testdata/snapshots/TestGenerate-InterfaceNesting.graphql-InterfaceNesting.graphql.go

@@ -48,11 +48,13 @@ func (v *InterfaceNestingRootTopic) UnmarshalJSON(b []byte) error {
 			len(src))
 		for i, src := range src {
 			dst := &(*dst)[i]
-			err = __unmarshalInterfaceNestingRootTopicChildrenContent(
-				src, dst)
-			if err != nil {
-				return fmt.Errorf(
-					"Unable to unmarshal InterfaceNestingRootTopic.Children: %w", err)
+			if len(src) != 0 && string(src) != "null" {
+				err = __unmarshalInterfaceNestingRootTopicChildrenContent(
+					src, dst)
+				if err != nil {
+					return fmt.Errorf(
+						"Unable to unmarshal InterfaceNestingRootTopic.Children: %w", err)
+				}
 			}
 		}
 	}
@@ -196,11 +198,13 @@ func (v *InterfaceNestingRootTopicChildrenContentParentTopic) UnmarshalJSON(b []
 			len(src))
 		for i, src := range src {
 			dst := &(*dst)[i]
-			err = __unmarshalInterfaceNestingRootTopicChildrenContentParentTopicChildrenContent(
-				src, dst)
-			if err != nil {
-				return fmt.Errorf(
-					"Unable to unmarshal InterfaceNestingRootTopicChildrenContentParentTopic.Children: %w", err)
+			if len(src) != 0 && string(src) != "null" {
+				err = __unmarshalInterfaceNestingRootTopicChildrenContentParentTopicChildrenContent(
+					src, dst)
+				if err != nil {
+					return fmt.Errorf(
+						"Unable to unmarshal InterfaceNestingRootTopicChildrenContentParentTopic.Children: %w", err)
+				}
 			}
 		}
 	}

generate/testdata/snapshots/TestGenerate-InterfaceNoFragments.graphql-InterfaceNoFragments.graphql.go

@@ -275,34 +275,40 @@ func (v *InterfaceNoFragmentsQueryResponse) UnmarshalJSON(b []byte) error {
 	{
 		dst := &v.RandomItem
 		src := firstPass.RandomItem
-		err = __unmarshalInterfaceNoFragmentsQueryRandomItemContent(
-			src, dst)
-		if err != nil {
-			return fmt.Errorf(
-				"Unable to unmarshal InterfaceNoFragmentsQueryResponse.RandomItem: %w", err)
+		if len(src) != 0 && string(src) != "null" {
+			err = __unmarshalInterfaceNoFragmentsQueryRandomItemContent(
+				src, dst)
+			if err != nil {
+				return fmt.Errorf(
+					"Unable to unmarshal InterfaceNoFragmentsQueryResponse.RandomItem: %w", err)
+			}
 		}
 	}
 
 	{
 		dst := &v.RandomItemWithTypeName
 		src := firstPass.RandomItemWithTypeName
-		err = __unmarshalInterfaceNoFragmentsQueryRandomItemWithTypeNameContent(
-			src, dst)
-		if err != nil {
-			return fmt.Errorf(
-				"Unable to unmarshal InterfaceNoFragmentsQueryResponse.RandomItemWithTypeName: %w", err)
+		if len(src) != 0 && string(src) != "null" {
+			err = __unmarshalInterfaceNoFragmentsQueryRandomItemWithTypeNameContent(
+				src, dst)
+			if err != nil {
+				return fmt.Errorf(
+					"Unable to unmarshal InterfaceNoFragmentsQueryResponse.RandomItemWithTypeName: %w", err)
+			}
 		}
 	}
 
 	{
 		dst := &v.WithPointer
 		src := firstPass.WithPointer
-		*dst = new(InterfaceNoFragmentsQueryWithPointerContent)
-		err = __unmarshalInterfaceNoFragmentsQueryWithPointerContent(
-			src, *dst)
-		if err != nil {
-			return fmt.Errorf(
-				"Unable to unmarshal InterfaceNoFragmentsQueryResponse.WithPointer: %w", err)
+		if len(src) != 0 && string(src) != "null" {
+			*dst = new(InterfaceNoFragmentsQueryWithPointerContent)
+			err = __unmarshalInterfaceNoFragmentsQueryWithPointerContent(
+				src, *dst)
+			if err != nil {
+				return fmt.Errorf(
+					"Unable to unmarshal InterfaceNoFragmentsQueryResponse.WithPointer: %w", err)
+			}
 		}
 	}
 	return nil

generate/testdata/snapshots/TestGenerate-MultipleDirectives.graphql-MultipleDirectives.graphql.go

@@ -40,12 +40,14 @@ func (v *MyInput) MarshalJSON() ([]byte, error) {
 
 		dst := &fullObject.Birthdate
 		src := v.Birthdate
-		var err error
-		*dst, err = testutil.MarshalDate(
-			src)
-		if err != nil {
-			return nil, fmt.Errorf(
-				"Unable to marshal MyInput.Birthdate: %w", err)
+		if src != nil {
+			var err error
+			*dst, err = testutil.MarshalDate(
+				src)
+			if err != nil {
+				return nil, fmt.Errorf(
+					"Unable to marshal MyInput.Birthdate: %w", err)
+			}
 		}
 	}
 
@@ -127,12 +129,14 @@ func (v *UserQueryInput) MarshalJSON() ([]byte, error) {
 
 		dst := &fullObject.Birthdate
 		src := v.Birthdate
-		var err error
-		*dst, err = testutil.MarshalDate(
-			src)
-		if err != nil {
-			return nil, fmt.Errorf(
-				"Unable to marshal UserQueryInput.Birthdate: %w", err)
+		if src != nil {
+			var err error
+			*dst, err = testutil.MarshalDate(
+				src)
+			if err != nil {
+				return nil, fmt.Errorf(
+					"Unable to marshal UserQueryInput.Birthdate: %w", err)
+			}
 		}
 	}
 

generate/testdata/snapshots/TestGenerate-Pointers.graphql-Pointers.graphql.go

@@ -95,12 +95,14 @@ func (v *UserQueryInput) MarshalJSON() ([]byte, error) {
 
 		dst := &fullObject.Birthdate
 		src := v.Birthdate
-		var err error
-		*dst, err = testutil.MarshalDate(
-			src)
-		if err != nil {
-			return nil, fmt.Errorf(
-				"Unable to marshal UserQueryInput.Birthdate: %w", err)
+		if src != nil {
+			var err error
+			*dst, err = testutil.MarshalDate(
+				src)
+			if err != nil {
+				return nil, fmt.Errorf(
+					"Unable to marshal UserQueryInput.Birthdate: %w", err)
+			}
 		}
 	}
 

generate/testdata/snapshots/TestGenerate-SimpleInlineFragment.graphql-SimpleInlineFragment.graphql.go

@@ -152,11 +152,13 @@ func (v *SimpleInlineFragmentResponse) UnmarshalJSON(b []byte) error {
 	{
 		dst := &v.RandomItem
 		src := firstPass.RandomItem
-		err = __unmarshalSimpleInlineFragmentRandomItemContent(
-			src, dst)
-		if err != nil {
-			return fmt.Errorf(
-				"Unable to unmarshal SimpleInlineFragmentResponse.RandomItem: %w", err)
+		if len(src) != 0 && string(src) != "null" {
+			err = __unmarshalSimpleInlineFragmentRandomItemContent(
+				src, dst)
+			if err != nil {
+				return fmt.Errorf(
+					"Unable to unmarshal SimpleInlineFragmentResponse.RandomItem: %w", err)
+			}
 		}
 	}
 	return nil

generate/testdata/snapshots/TestGenerate-SimpleNamedFragment.graphql-SimpleNamedFragment.graphql.go

@@ -269,22 +269,26 @@ func (v *SimpleNamedFragmentResponse) UnmarshalJSON(b []byte) error {
 	{
 		dst := &v.RandomItem
 		src := firstPass.RandomItem
-		err = __unmarshalSimpleNamedFragmentRandomItemContent(
-			src, dst)
-		if err != nil {
-			return fmt.Errorf(
-				"Unable to unmarshal SimpleNamedFragmentResponse.RandomItem: %w", err)
+		if len(src) != 0 && string(src) != "null" {
+			err = __unmarshalSimpleNamedFragmentRandomItemContent(
+				src, dst)
+			if err != nil {
+				return fmt.Errorf(
+					"Unable to unmarshal SimpleNamedFragmentResponse.RandomItem: %w", err)
+			}
 		}
 	}
 
 	{
 		dst := &v.RandomLeaf
 		src := firstPass.RandomLeaf
-		err = __unmarshalSimpleNamedFragmentRandomLeafLeafContent(
-			src, dst)
-		if err != nil {
-			return fmt.Errorf(
-				"Unable to unmarshal SimpleNamedFragmentResponse.RandomLeaf: %w", err)
+		if len(src) != 0 && string(src) != "null" {
+			err = __unmarshalSimpleNamedFragmentRandomLeafLeafContent(
+				src, dst)
+			if err != nil {
+				return fmt.Errorf(
+					"Unable to unmarshal SimpleNamedFragmentResponse.RandomLeaf: %w", err)
+			}
 		}
 	}
 	return nil

generate/testdata/snapshots/TestGenerate-StructOption.graphql-StructOption.graphql.go

@@ -86,11 +86,13 @@ func (v *StructOptionRootTopicChildrenContentParentTopic) UnmarshalJSON(b []byte
 			len(src))
 		for i, src := range src {
 			dst := &(*dst)[i]
-			err = __unmarshalStructOptionRootTopicChildrenContentParentTopicInterfaceChildrenContent(
-				src, dst)
-			if err != nil {
-				return fmt.Errorf(
-					"Unable to unmarshal StructOptionRootTopicChildrenContentParentTopic.InterfaceChildren: %w", err)
+			if len(src) != 0 && string(src) != "null" {
+				err = __unmarshalStructOptionRootTopicChildrenContentParentTopicInterfaceChildrenContent(
+					src, dst)
+				if err != nil {
+					return fmt.Errorf(
+						"Unable to unmarshal StructOptionRootTopicChildrenContentParentTopic.InterfaceChildren: %w", err)
+				}
 			}
 		}
 	}

generate/testdata/snapshots/TestGenerate-TypeNames.graphql-TypeNames.graphql.go

@@ -153,11 +153,13 @@ func (v *Resp) UnmarshalJSON(b []byte) error {
 	{
 		dst := &v.RandomItem
 		src := firstPass.RandomItem
-		err = __unmarshalItem(
-			src, dst)
-		if err != nil {
-			return fmt.Errorf(
-				"Unable to unmarshal Resp.RandomItem: %w", err)
+		if len(src) != 0 && string(src) != "null" {
+			err = __unmarshalItem(
+				src, dst)
+			if err != nil {
+				return fmt.Errorf(
+					"Unable to unmarshal Resp.RandomItem: %w", err)
+			}
 		}
 	}
 	return nil

generate/testdata/snapshots/TestGenerate-UnionNoFragments.graphql-UnionNoFragments.graphql.go

@@ -100,11 +100,13 @@ func (v *UnionNoFragmentsQueryResponse) UnmarshalJSON(b []byte) error {
 	{
 		dst := &v.RandomLeaf
 		src := firstPass.RandomLeaf
-		err = __unmarshalUnionNoFragmentsQueryRandomLeafLeafContent(
-			src, dst)
-		if err != nil {
-			return fmt.Errorf(
-				"Unable to unmarshal UnionNoFragmentsQueryResponse.RandomLeaf: %w", err)
+		if len(src) != 0 && string(src) != "null" {
+			err = __unmarshalUnionNoFragmentsQueryRandomLeafLeafContent(
+				src, dst)
+			if err != nil {
+				return fmt.Errorf(
+					"Unable to unmarshal UnionNoFragmentsQueryResponse.RandomLeaf: %w", err)
+			}
 		}
 	}
 	return nil