1 files changed, 216 insertions, 0 deletions
diff --git a/cmd/viewer/viewer.go b/cmd/viewer/viewer.go
new file mode 100644
index 000000000..df6287c26
--- /dev/null
+++ b/cmd/viewer/viewer.go
@@ -0,0 +1,216 @@
+// Copyright (c) 2021 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 a view type.
+//
+// The generated View method provides a readonly view of the struct.
+//
+// This tool makes lots of implicit assumptions about the types you feed it.
+// In particular, it can only write relatively "shallow" View methods.
+// That is, if a type contains another named struct type, viewer assumes that
+// named type will also have a View method.
+package main
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"go/types"
+	"log"
+	"os"
+	"strings"
+
+	"golang.org/x/tools/go/packages"
+	"tailscale.com/util/codegen"
+)
+
+var (
+	flagTypes     = flag.String("type", "", "comma-separated list of types; required")
+	flagOutput    = flag.String("output", "", "output file; required")
+	flagBuildTags = flag.String("tags", "", "compiler build tags to apply")
+)
+
+func main() {
+	log.SetFlags(0)
+	log.SetPrefix("viewer: ")
+	flag.Parse()
+	if len(*flagTypes) == 0 {
+		flag.Usage()
+		os.Exit(2)
+	}
+	typeNames := strings.Split(*flagTypes, ",")
+
+	cfg := &packages.Config{
+		Mode:  packages.NeedTypes | packages.NeedTypesInfo | packages.NeedSyntax | packages.NeedName,
+		Tests: false,
+	}
+	if *flagBuildTags != "" {
+		cfg.BuildFlags = []string{"-tags=" + *flagBuildTags}
+	}
+	pkgs, err := packages.Load(cfg, ".")
+	if err != nil {
+		log.Fatal(err)
+	}
+	if len(pkgs) != 1 {
+		log.Fatalf("wrong number of packages: %d", len(pkgs))
+	}
+	pkg := pkgs[0]
+	buf := new(bytes.Buffer)
+	imports := make(map[string]struct{})
+	namedTypes := codegen.NamedTypes(pkg)
+	for _, typeName := range typeNames {
+		typ, ok := namedTypes[typeName]
+		if !ok {
+			log.Fatalf("could not find type %s", typeName)
+		}
+		gen(buf, imports, typ, pkg.Types)
+	}
+
+	contents := new(bytes.Buffer)
+	fmt.Fprintf(contents, header, *flagTypes, pkg.Name)
+	fmt.Fprintf(contents, "import (\n")
+	for s := range imports {
+		fmt.Fprintf(contents, "\t%q\n", s)
+	}
+	fmt.Fprintf(contents, ")\n\n")
+	contents.Write(buf.Bytes())
+
+	output := *flagOutput
+	if output == "" {
+		flag.Usage()
+		os.Exit(2)
+	}
+	if err := codegen.WriteFormatted(contents.Bytes(), output); err != nil {
+		log.Fatal(err)
+	}
+}
+
+const header = `// Copyright (c) 2021 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.
+
+// Code generated by the following command; DO NOT EDIT.
+//   tailscale.com/cmd/viewer -type %s
+
+package %s
+
+`
+
+func gen(buf *bytes.Buffer, imports map[string]struct{}, typ *types.Named, thisPkg *types.Package) {
+	pkgQual := func(pkg *types.Package) string {
+		if thisPkg == pkg {
+			return ""
+		}
+		imports[pkg.Path()] = struct{}{}
+		return pkg.Name()
+	}
+	importedName := func(t types.Type) string {
+		return types.TypeString(t, pkgQual)
+	}
+
+	t, ok := typ.Underlying().(*types.Struct)
+	if !ok {
+		return
+	}
+
+	name := typ.Obj().Name()
+	viewName := name + "View"
+	fmt.Fprintf(buf, "// View makes a readonly view of %s.\n", name)
+	fmt.Fprintf(buf, "func (src *%s) View() %s {\n", name, viewName)
+	fmt.Fprintf(buf, "	return %s{src}\n", viewName)
+	fmt.Fprintf(buf, "}\n")
+
+	fmt.Fprintf(buf, "// %s is a readonly view of %s.\n", viewName, name)
+	fmt.Fprintf(buf, "type %s struct{ ж *%s }\n", viewName, name)
+	fmt.Fprintf(buf, "func (v %s) Valid() bool { return v.ж != nil }\n", viewName)
+
+	for i := 0; i < t.NumFields(); i++ {
+		fname := t.Field(i).Name()
+		ft := t.Field(i).Type()
+		if !codegen.ContainsPointers(ft) {
+			fmt.Fprintf(buf, "func (v %s) %s() %s { return v.ж.%s }\n", viewName, fname, importedName(ft), fname)
+			continue
+		}
+		if named, _ := ft.(*types.Named); named != nil && !hasBasicUnderlying(ft) {
+			genViewCall(buf, viewName, fname, importedName(ft))
+			continue
+		}
+		switch ft := ft.Underlying().(type) {
+		case *types.Slice:
+			if !codegen.ContainsPointers(ft.Elem()) {
+				// OK to return the slice as-is, since they can't modify the contents.
+				fmt.Fprintf(buf, "func (v %s) %s() %s { return v.ж.%s }\n", viewName, fname, importedName(ft), fname)
+				continue
+			}
+
+			n := importedName(ft.Elem())
+			if ptrTyp, isPtr := ft.Elem().(*types.Pointer); isPtr {
+				n = importedName(ptrTyp.Elem())
+			}
+
+			// Generate slice view.
+			styp := fmt.Sprintf("_%s_%s", viewName, fname)
+			fmt.Fprintf(buf, "type %s []%s\n", styp, importedName(ft.Elem()))
+			fmt.Fprintf(buf, "func (s %s) Len() int { return len(s) }\n", styp)
+			fmt.Fprintf(buf, "func (s %s) At(i int) %sView { return s[i].View() }\n", styp, n)
+
+			fmt.Fprintf(buf, "func (v %s) %s() interface { Len() int; At(int) %sView } {\n", viewName, fname, n)
+			fmt.Fprintf(buf, "	return %s(v.ж.%s)\n", styp, fname)
+			fmt.Fprintf(buf, "}\n")
+		case *types.Pointer:
+			if named, _ := ft.Elem().(*types.Named); named != nil && codegen.ContainsPointers(ft.Elem()) {
+				genViewCall(buf, viewName, fname, importedName(named))
+				continue
+			}
+			if codegen.ContainsPointers(ft.Elem()) {
+				log.Fatalf("unhandled: pointers in pointers (%v)", ft)
+			}
+			n := importedName(ft.Elem())
+			fmt.Fprintf(buf, "func (v %s) %s() *%s {\n", viewName, fname, n)
+			fmt.Fprintf(buf, "	ptr := v.ж.%s\n", fname)
+			fmt.Fprintf(buf, "	if ptr == nil {\n")
+			fmt.Fprintf(buf, "		return nil\n")
+			fmt.Fprintf(buf, "	}\n")
+			fmt.Fprintf(buf, "	cp := *ptr\n")
+			fmt.Fprintf(buf, "	return &cp\n")
+			fmt.Fprintf(buf, "}\n")
+		case *types.Map:
+			// TODO: Generate map view, like the slice view.
+			// We need:
+			//   * Len() int
+			//   * Load(k) v
+			//   * LoadOK(k) (v, bool)
+			//   * Range(func(k, v) bool)
+			//
+			// Note that we need to handle a variety of elem types:
+			// basic types (float64), types with a View method,
+			// slices of the foregoing.
+			//
+			// This may require recursion to handle completely,
+			// or we can follow cloner's lead and just manually
+			// inline one level deep the code generation
+			// that we happen to need right now.
+			// (If we figure out recursion in this context,
+			// we might want to backport to cloner, too.)
+			log.Printf("TODO: Handle %s (%s)", name, ft)
+		default:
+			fmt.Fprintf(buf, `panic("TODO: %s (%T)")`, fname, ft)
+		}
+	}
+
+	buf.Write(codegen.AssertStructUnchanged(t, thisPkg, name, "View", imports))
+}
+
+func genViewCall(buf *bytes.Buffer, viewName, fieldName, importedName string) {
+	fmt.Fprintf(buf, "func (v %s) %s() %sView { return v.ж.%s.View() }\n", viewName, fieldName, importedName, fieldName)
+}
+
+func hasBasicUnderlying(typ types.Type) bool {
+	switch typ.Underlying().(type) {
+	case *types.Slice, *types.Map:
+		return true
+	default:
+		return false
+	}
+}