cmd/viewer: add codegen tool for Views

Signed-off-by: Maisem Ali <maisem@tailscale.com>

cmd/viewer/viewer.go

@@ -0,0 +1,316 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Viewer is a tool to automate the creation of "view" wrapper types that
+// provide read-only accessor methods to underlying fields.
+package main
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"go/types"
+	"html/template"
+	"log"
+	"os"
+	"strings"
+
+	"tailscale.com/util/codegen"
+)
+
+const viewTemplateStr = `{{define "common"}}
+// View returns a readonly view of {{.StructName}}.
+func (p *{{.StructName}}) View() {{.ViewName}} {
+	return {{.ViewName}}{ж: p}
+}
+
+// {{.ViewName}} provides a read-only view over {{.StructName}}.
+//
+// Its methods should only be called if ` + "`Valid()`" + ` returns true.
+type {{.ViewName}} struct {
+	// ж is the underlying mutable value, named with a hard-to-type
+	// character that looks pointy like a pointer.
+	// It is named distinctively to make you think of how dangerous it is to escape
+	// to callers. You must not let callers be able to mutate it.
+	ж *{{.StructName}}
+}
+
+// Valid reports whether underlying value is non-nil.
+func (v {{.ViewName}}) Valid() bool { return v.ж != nil }
+
+// AsStruct returns a clone of the underlying value which aliases no memory with
+// the original.
+func (v {{.ViewName}}) AsStruct() *{{.StructName}}{ 
+	if v.ж == nil {
+		return nil
+	}
+	return v.ж.Clone()
+}
+
+func (v {{.ViewName}}) MarshalJSON() ([]byte, error) { return json.Marshal(v.ж) }
+
+func (v *{{.ViewName}}) UnmarshalJSON(b []byte) error {
+	if v.ж != nil {
+		return errors.New("already initialized")
+	}
+	if len(b) == 0 {
+		return nil
+	}
+	var x {{.StructName}}
+	if err := json.Unmarshal(b, &x); err != nil {
+		return err
+	}
+	v.ж=&x
+	return nil
+}
+
+{{end}}
+{{define "valueField"}}func (v {{.ViewName}}) {{.FieldName}}() {{.FieldType}} { return v.ж.{{.FieldName}} }
+{{end}}
+{{define "byteSliceField"}}func (v {{.ViewName}}) {{.FieldName}}() mem.RO { return mem.B(v.ж.{{.FieldName}}) }
+{{end}}
+{{define "ipPrefixSliceField"}}func (v {{.ViewName}}) {{.FieldName}}() views.IPPrefixSlice { return views.IPPrefixSliceOf(v.ж.{{.FieldName}}) }
+{{end}}
+{{define "sliceField"}}func (v {{.ViewName}}) {{.FieldName}}() views.Slice[{{.FieldType}}] { return views.SliceOf(v.ж.{{.FieldName}}) }
+{{end}}
+{{define "viewSliceField"}}func (v {{.ViewName}}) {{.FieldName}}() views.SliceView[{{.FieldType}},{{.FieldViewName}}] { return views.SliceOfViews[{{.FieldType}},{{.FieldViewName}}](v.ж.{{.FieldName}}) }
+{{end}}
+{{define "viewField"}}func (v {{.ViewName}}) {{.FieldName}}() {{.FieldType}}View { return v.ж.{{.FieldName}}.View() }
+{{end}}
+{{define "valuePointerField"}}func (v {{.ViewName}}) {{.FieldName}}() {{.FieldType}} {
+	if v.ж.{{.FieldName}} == nil {
+		return nil
+	}
+	x := *v.ж.{{.FieldName}}
+	return &x
+}
+
+{{end}}
+{{define "mapField"}}
+// Unsupported, panics.
+func(v {{.ViewName}}) {{.FieldName}}() {{.FieldType}} {panic("unsupported")}
+{{end}}
+{{define "unsupportedField"}}func(v {{.ViewName}}) {{.FieldName}}() {{.FieldType}} {panic("unsupported")}
+{{end}}
+{{define "stringFunc"}}func(v {{.ViewName}}) String() string { return v.ж.String() }
+{{end}}
+{{define "equalFunc"}}func(v {{.ViewName}}) Equal(v2 {{.ViewName}}) bool { return v.ж.Equal(v2.ж) }
+{{end}}
+`
+
+var viewTemplate *template.Template
+
+func init() {
+	viewTemplate = template.Must(template.New("view").Parse(viewTemplateStr))
+}
+
+func requiresCloning(t types.Type) (shallow, deep bool, base types.Type) {
+	switch v := t.(type) {
+	case *types.Pointer:
+		_, deep, base = requiresCloning(v.Elem())
+		return true, deep, base
+	case *types.Slice:
+		_, deep, base = requiresCloning(v.Elem())
+		return true, deep, base
+	}
+	p := codegen.ContainsPointers(t)
+	return p, p, t
+}
+
+func genView(buf *bytes.Buffer, it *codegen.ImportTracker, typ *types.Named, thisPkg *types.Package) {
+	t, ok := typ.Underlying().(*types.Struct)
+	if !ok || codegen.IsViewType(t) {
+		return
+	}
+	it.Import("encoding/json")
+	it.Import("errors")
+
+	args := struct {
+		StructName    string
+		ViewName      string
+		FieldName     string
+		FieldType     string
+		FieldViewName string
+	}{
+		StructName: typ.Obj().Name(),
+		ViewName:   typ.Obj().Name() + "View",
+	}
+
+	writeTemplate := func(name string) {
+		if err := viewTemplate.ExecuteTemplate(buf, name, args); err != nil {
+			log.Fatal(err)
+		}
+	}
+	writeTemplate("common")
+	for i := 0; i < t.NumFields(); i++ {
+		f := t.Field(i)
+		fname := f.Name()
+		if !f.Exported() {
+			continue
+		}
+		args.FieldName = fname
+		fieldType := f.Type()
+		if codegen.IsInvalid(fieldType) {
+			continue
+		}
+		if !codegen.ContainsPointers(fieldType) || codegen.IsViewType(fieldType) || codegen.HasNoClone(t.Tag(i)) {
+			args.FieldType = it.QualifiedName(fieldType)
+			writeTemplate("valueField")
+			continue
+		}
+		switch underlying := fieldType.Underlying().(type) {
+		case *types.Slice:
+			slice := underlying
+			elem := slice.Elem()
+			args.FieldType = it.QualifiedName(elem)
+			switch elem.String() {
+			case "byte":
+				it.Import("go4.org/mem")
+				writeTemplate("byteSliceField")
+			case "inet.af/netaddr.IPPrefix":
+				it.Import("tailscale.com/types/views")
+				writeTemplate("ipPrefixSliceField")
+			default:
+				it.Import("tailscale.com/types/views")
+				shallow, deep, base := requiresCloning(elem)
+				if deep {
+					if _, isPtr := elem.(*types.Pointer); isPtr {
+						args.FieldViewName = it.QualifiedName(base) + "View"
+						writeTemplate("viewSliceField")
+					} else {
+						writeTemplate("unsupportedField")
+					}
+					continue
+				} else if shallow {
+					if _, isBasic := base.(*types.Basic); isBasic {
+						writeTemplate("unsupportedField")
+					} else {
+						args.FieldViewName = it.QualifiedName(base) + "View"
+						writeTemplate("viewSliceField")
+					}
+					continue
+				}
+				writeTemplate("sliceField")
+			}
+			continue
+		case *types.Struct:
+			strucT := underlying
+			args.FieldType = it.QualifiedName(fieldType)
+			if codegen.ContainsPointers(strucT) {
+				writeTemplate("viewField")
+				continue
+			}
+			writeTemplate("valueField")
+			continue
+		case *types.Map:
+			// TODO(maisem): support this.
+			// args.FieldType = importedName(ft)
+			// writeTemplate("mapField")
+			continue
+		case *types.Pointer:
+			ptr := underlying
+			_, deep, base := requiresCloning(ptr)
+			if deep {
+				args.FieldType = it.QualifiedName(base)
+				writeTemplate("viewField")
+			} else {
+				args.FieldType = it.QualifiedName(ptr)
+				writeTemplate("valuePointerField")
+			}
+			continue
+		}
+		writeTemplate("unsupportedField")
+	}
+	for i := 0; i < typ.NumMethods(); i++ {
+		f := typ.Method(i)
+		if !f.Exported() {
+			continue
+		}
+		sig, ok := f.Type().(*types.Signature)
+		if !ok {
+			continue
+		}
+
+		switch f.Name() {
+		case "Clone", "View":
+			continue // "AsStruct"
+		case "String":
+			writeTemplate("stringFunc")
+			continue
+		case "Equal":
+			if sig.Results().Len() == 1 && sig.Results().At(0).Type().String() == "bool" {
+				writeTemplate("equalFunc")
+				continue
+			}
+		}
+	}
+	fmt.Fprintf(buf, "\n")
+	buf.Write(codegen.AssertStructUnchanged(t, args.StructName, "View", it))
+}
+
+var (
+	flagTypes     = flag.String("type", "", "comma-separated list of types; required")
+	flagBuildTags = flag.String("tags", "", "compiler build tags to apply")
+	flagCloneFunc = flag.Bool("clonefunc", false, "add a top-level Clone func")
+)
+
+func main() {
+	log.SetFlags(0)
+	log.SetPrefix("viewer: ")
+	flag.Parse()
+	if len(*flagTypes) == 0 {
+		flag.Usage()
+		os.Exit(2)
+	}
+	typeNames := strings.Split(*flagTypes, ",")
+
+	var flagArgs []string
+	flagArgs = append(flagArgs, fmt.Sprintf("-clonefunc=%v", *flagCloneFunc))
+	if *flagTypes != "" {
+		flagArgs = append(flagArgs, "-type="+*flagTypes)
+	}
+	if *flagBuildTags != "" {
+		flagArgs = append(flagArgs, "-tags="+*flagBuildTags)
+	}
+	pkg, namedTypes, err := codegen.LoadTypes(*flagBuildTags, ".")
+	if err != nil {
+		log.Fatal(err)
+	}
+	it := codegen.NewImportTracker(pkg.Types)
+
+	buf := new(bytes.Buffer)
+	fmt.Fprintf(buf, `//go:generate go run tailscale.com/cmd/cloner  %s`, strings.Join(flagArgs, " "))
+	fmt.Fprintln(buf)
+	runCloner := false
+	for _, typeName := range typeNames {
+		typ, ok := namedTypes[typeName]
+		if !ok {
+			log.Fatalf("could not find type %s", typeName)
+		}
+		var hasClone bool
+		for i, n := 0, typ.NumMethods(); i < n; i++ {
+			if typ.Method(i).Name() == "Clone" {
+				hasClone = true
+				break
+			}
+		}
+		if !hasClone {
+			runCloner = true
+		}
+		genView(buf, it, typ, pkg.Types)
+	}
+	out := pkg.Name + "_view.go"
+	if err := codegen.WritePackageFile("tailscale/cmd/viewer", pkg, out, it, buf); err != nil {
+		log.Fatal(err)
+	}
+	if runCloner {
+		// When a new pacakge is added or when existing generated files have
+		// been deleted, we might run into a case where tailscale.com/cmd/cloner
+		// has not run yet. We detect this by verifying that all the structs we
+		// interacted with have had Clone method already generated. If they
+		// haven't we ask the caller to rerun generation again so that those get
+		// generated.
+		log.Printf("%v requires regeneration. Please run go generate again", pkg.Name+"_clone.go")
+	}
+}