Add support for interfaces, part 1: the simplest cases

In this commit I begin the journey to add the long-awaited support for
interfaces (part of #8).  Well, it's not the beginning: I already had
some half-written broken code around.  But it's the first fully
functional support, and especially, the first *tested* support; it's
probably best to review the nontrivially-changed code as if it were new.

Conceptually, the code so far is pretty simple: we generate an interface
type, and the implementations.  (That code is in fact mostly unchanged.)
The complexity comes in because encoding/json doesn't know how to
unmarshal that.  So we have to add an UnmarshalJSON method, which
actually has to be on the types with interface-type fields, that knows
how.  I factored it into two methods, such that that UnmarshalJSON
method is just glue, and then there's a separate function, corresponding
to each interface-type, that actually does all the work.  (If only one
could just write it as an actual method!)  The method uses the same
trick suggested to me by a few others in another context to deserialize
all but one field, then handle that field specially, which is discussed
in the code.

This still has some limitations, which will be lifted in future commits:
- it doesn't allow for list-of-interface fields
- it requires that you manually ask for `__typename`
- it doesn't support fragments, i.e. you can only query for interface
  fields, not concrete-type-specific ones
But it works, even in integration tests, which is progress!

As a part of this, I added a proper config option for the "allow broken
features" flag, since I need to be able to set it from the integration
tests which are in a separate package (and actually shell out via `go
generate`).  I also renamed what was to be the first case
(InterfaceNoFragments), and replaced it with a further-simplified
version (avoiding list-of-interface fields.

[1] https://github.com/benjaminjkraft/notes/blob/master/go-json-interfaces.md

Issue: #8

Test plan: make tesc

generate/config.go

@@ -73,6 +73,13 @@ type Config struct {
 	// and UnmarshalJSON methods, or otherwise be convertible to JSON.
 	Scalars map[string]string `yaml:"scalars"`
 
+	// Set to true to use features that aren't fully ready to use.
+	//
+	// This is primarily intended for genqlient's own tests.  These features
+	// are likely BROKEN and come with NO EXPECTATION OF COMPATIBBILITY.  Use
+	// them at your own risk!
+	AllowBrokenFeatures bool `yaml:"allow_broken_features"`
+
 	// Set automatically to the filename of the config file itself.
 	configFilename string
 }

generate/convert.go

@@ -25,7 +25,7 @@ func (g *generator) baseTypeForOperation(operation ast.Operation) (*ast.Definiti
 	case ast.Mutation:
 		return g.schema.Mutation, nil
 	case ast.Subscription:
-		if !allowBrokenFeatures {
+		if !g.Config.AllowBrokenFeatures {
 			return nil, errorf(nil, "genqlient does not yet support subscriptions")
 		}
 		return g.schema.Subscription, nil
@@ -255,10 +255,23 @@ func (g *generator) convertDefinition(
 		return goType, nil
 
 	case ast.Interface, ast.Union:
-		if !allowBrokenFeatures {
+		if !g.Config.AllowBrokenFeatures {
 			return nil, errorf(pos, "not implemented: %v", def.Kind)
 		}
 
+		// We need to request __typename so we know which concrete type to use.
+		hasTypename := false
+		for _, selection := range selectionSet {
+			field, ok := selection.(*ast.Field)
+			if ok && field.Name == "__typename" {
+				hasTypename = true
+			}
+		}
+		if !hasTypename {
+			// TODO(benkraft): Instead, modify the query to add __typename.
+			return nil, errorf(pos, "union/interface type %s must request __typename", def.Name)
+		}
+
 		implementationTypes := g.schema.GetPossibleTypes(def)
 		goType := &goInterfaceType{
 			GoName:          name,

generate/generate.go

@@ -17,9 +17,6 @@ import (
 	"golang.org/x/tools/imports"
 )
 
-// Set to true to test features that aren't yet really ready.
-var allowBrokenFeatures = false
-
 // generator is the context for the codegen process (and ends up getting passed
 // to the template).
 type generator struct {
@@ -230,7 +227,7 @@ func Generate(config *Config) (map[string][]byte, error) {
 			strings.Join(config.Operations, ", "))
 	}
 
-	if len(document.Fragments) > 0 && !allowBrokenFeatures {
+	if len(document.Fragments) > 0 && !config.AllowBrokenFeatures {
 		return nil, errorf(document.Fragments[0].Position,
 			"genqlient does not yet support fragments")
 	}

generate/generate_test.go

@@ -30,9 +30,6 @@ const (
 // update the snapshots.  Make sure to check that the output is sensible; the
 // snapshots don't even get compiled!
 func TestGenerate(t *testing.T) {
-	// we can test parts of features even if they're not done yet!
-	allowBrokenFeatures = true
-
 	files, err := ioutil.ReadDir(dataDir)
 	if err != nil {
 		t.Fatal(err)
@@ -59,6 +56,7 @@ func TestGenerate(t *testing.T) {
 					"Junk":        "interface{}",
 					"ComplexJunk": "[]map[string]*[]*map[string]interface{}",
 				},
+				AllowBrokenFeatures: true,
 			})
 			if err != nil {
 				t.Fatal(err)
@@ -73,9 +71,9 @@ func TestGenerate(t *testing.T) {
 			t.Run("Build", func(t *testing.T) {
 				if testing.Short() {
 					t.Skip("skipping build due to -short")
-				} else if sourceFilename == "InterfaceNesting.graphql" ||
-					sourceFilename == "InterfaceNoFragments.graphql" ||
-					sourceFilename == "Omitempty.graphql" {
+				} else if sourceFilename == "InterfaceNesting.graphql" || // #8
+					sourceFilename == "InterfaceListField.graphql" || // #8
+					sourceFilename == "Omitempty.graphql" { // #43
 					t.Skip("TODO: enable these once they build")
 				}
 
@@ -109,9 +107,6 @@ func TestGenerate(t *testing.T) {
 }
 
 func TestGenerateErrors(t *testing.T) {
-	// we can test parts of features even if they're not done yet!
-	allowBrokenFeatures = true
-
 	files, err := ioutil.ReadDir(errorsDir)
 	if err != nil {
 		t.Fatal(err)
@@ -138,6 +133,7 @@ func TestGenerateErrors(t *testing.T) {
 					"ValidScalar":   "string",
 					"InvalidScalar": "bogus",
 				},
+				AllowBrokenFeatures: true,
 			})
 			if err == nil {
 				t.Fatal("expected an error")

generate/testdata/errors/MissingTypeName.go

@@ -0,0 +1,5 @@
+package errors
+
+const _ = `# @genqlient
+	query MyQuery { i { f } }
+`

generate/testdata/errors/MissingTypeName.graphql

@@ -0,0 +1 @@
+query MyQuery { i { f } }

generate/testdata/errors/MissingTypeName.schema.graphql

@@ -0,0 +1,11 @@
+type Query {
+  i: I
+}
+
+type T implements I {
+  f: String!
+}
+
+interface I {
+  f: String!
+}

generate/testdata/queries/InterfaceListField.graphql

@@ -0,0 +1,11 @@
+query InterfaceNoFragmentsQuery {
+  root {
+    id
+    name
+    children {
+      __typename
+      id
+      name
+    }
+  }
+}

generate/testdata/queries/InterfaceNesting.graphql

@@ -2,10 +2,13 @@ query InterfaceNesting {
   root {
     id
     children {
+      __typename
       id
       parent {
+        __typename
         id
         children {
+          __typename
           id
         }
       }

generate/testdata/queries/InterfaceNoFragments.graphql

@@ -1,10 +1,4 @@
 query InterfaceNoFragmentsQuery {
-  root {
-    id
-    name
-    children {
-      id
-      name
-    }
-  }
+  root { id name }       # (make sure sibling fields work)
+  randomItem { __typename id name }
 }

generate/testdata/queries/schema.graphql

@@ -104,6 +104,7 @@ type Query {
   """usersWithRole looks a user up by role."""
   usersWithRole(role: Role!): [User!]!
   root: Topic!
+  randomItem: Content!
   randomLeaf: LeafContent!
   convert(dt: DateTime!, tz: String): DateTime!
   maybeConvert(dt: DateTime, tz: String): DateTime

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

@@ -0,0 +1,9 @@
+{
+  "operations": [
+    {
+      "operationName": "InterfaceNoFragmentsQuery",
+      "query": "\nquery InterfaceNoFragmentsQuery {\n\troot {\n\t\tid\n\t\tname\n\t\tchildren {\n\t\t\t__typename\n\t\t\tid\n\t\t\tname\n\t\t}\n\t}\n}\n",
+      "sourceLocation": "testdata/queries/InterfaceListField.graphql"
+    }
+  ]
+}