Add support for interfaces, part 2: list-of-interface (#54)

## Summary:
In this commit I remove one of the limitations of our support for
interfaces, from #52, by adding support for list-of-interface fields.
This was surprisingly complex!  The issue is that, as before, it's the
containing type that has to do all the glue work -- and it's that glue
work that is complicated by list-of-interface fields.

All in all, it's not that much new code, and by far the hard part is
just 20 lines in the UnmarshalJSON template (which come with almost
twice as many lines of comments to explain them).  It may be easiest to
start by reading some of the generated code, and then read the template.

I also added support for such fields with `pointer: true` specified,
such that the type is `[][]...[]*MyInterface`, although I don't know why
you would want that.  This does *not* allow e.g. `*[]*[][]*MyInterface`;
that would require a way to specify it (see #16) but also add some extra
complexity (as we'd have to actually walk the type-unwrap chain
properly, instead of just counting the number of slices and whether
there's a pointer).

Issue: #8

## Test plan:
make check


Author: benjaminjkraft

Reviewers: dnerdy, benjaminjkraft, aberkan, csilvers, MiguelCastillo

Required Reviewers: 

Approved by: dnerdy

Checks: ⌛ Test (1.17), ⌛ Test (1.16), ⌛ Test (1.15), ⌛ Test (1.14), ⌛ Test (1.13), ⌛ Lint, ⌛ Lint, ⌛ Test (1.17), ⌛ Test (1.16), ⌛ Test (1.15), ⌛ Test (1.14), ⌛ Test (1.13)

Pull request URL: #54

generate/generate_test.go

@@ -71,10 +71,6 @@ 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 == "InterfaceListField.graphql" {
-					t.Skip("TODO: enable after fixing " +
-						"https://github.com/Khan/genqlient/issues/8")
 				} else if sourceFilename == "Omitempty.graphql" {
 					t.Skip("TODO: enable after fixing " +
 						"https://github.com/Khan/genqlient/issues/43")

generate/template.go

@@ -4,6 +4,7 @@ import (
 	"io"
 	"path/filepath"
 	"runtime"
+	"strings"
 	"text/template"
 )
 
@@ -12,13 +13,34 @@ var (
 	thisDir               = filepath.Dir(thisFilename)
 )
 
+func repeat(n int, s string) string {
+	var builder strings.Builder
+	for i := 0; i < n; i++ {
+		builder.WriteString(s)
+	}
+	return builder.String()
+}
+
+func intRange(n int) []int {
+	ret := make([]int, n)
+	for i := 0; i < n; i++ {
+		ret[i] = i
+	}
+	return ret
+}
+
+func sub(x, y int) int { return x - y }
+
 // execute executes the given template with the funcs from this generator.
 func (g *generator) execute(tmplRelFilename string, w io.Writer, data interface{}) error {
 	tmpl := g.templateCache[tmplRelFilename]
 	if tmpl == nil {
 		absFilename := filepath.Join(thisDir, tmplRelFilename)
 		funcMap := template.FuncMap{
-			"ref": g.ref,
+			"ref":      g.ref,
+			"repeat":   repeat,
+			"intRange": intRange,
+			"sub":      sub,
 		}
 		var err error
 		tmpl, err = template.New(tmplRelFilename).Funcs(funcMap).ParseFiles(absFilename)

generate/testdata/queries/InterfaceListField.graphql

@@ -1,4 +1,4 @@
-query InterfaceNoFragmentsQuery {
+query InterfaceListField {
   root {
     id
     name
@@ -8,4 +8,14 @@ query InterfaceNoFragmentsQuery {
       name
     }
   }
+  # @genqlient(pointer: true)
+  withPointer: root {
+    id
+    name
+    children {
+      __typename
+      id
+      name
+    }
+  }
 }

generate/testdata/queries/InterfaceListOfListsOfListsField.graphql

@@ -0,0 +1,5 @@
+query InterfaceListOfListOfListsField {
+  listOfListsOfListsOfContent { __typename id name }
+  # @genqlient(pointer: true)
+  withPointer: listOfListsOfListsOfContent { __typename id name }
+}

generate/testdata/queries/InterfaceNoFragments.graphql

@@ -1,4 +1,6 @@
 query InterfaceNoFragmentsQuery {
   root { id name }       # (make sure sibling fields work)
   randomItem { __typename id name }
+  # @genqlient(pointer: true)
+  withPointer: randomItem { __typename id name }
 }

generate/testdata/queries/schema.graphql

@@ -111,6 +111,7 @@ type Query {
   getJunk: Junk
   getComplexJunk: ComplexJunk
   listOfListsOfLists: [[[String!]!]!]!
+  listOfListsOfListsOfContent: [[[Content!]!]!]!
 }
 
 type Mutation {

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

@@ -1,8 +1,8 @@
 {
   "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",
+      "operationName": "InterfaceListField",
+      "query": "\nquery InterfaceListField {\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\twithPointer: root {\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"
     }
   ]