version: add internal/genv to generate version files

Change-Id: I309ae46ddcd494166c72dcf1e94f7d2590576a9a
Reviewed-on: https://go-review.googlesource.com/92935
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: andybons

version/internal/genv/main.go

@@ -0,0 +1,82 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The genv command generates version-specific go command source files.
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"html/template"
+	"io"
+	"os"
+	"path/filepath"
+	"time"
+)
+
+func usage() {
+	fmt.Fprintln(os.Stderr, "usage: genv <version>...")
+	os.Exit(2)
+}
+
+func main() {
+	if len(os.Args) == 1 {
+		usage()
+	}
+	for _, version := range os.Args[1:] {
+		var buf bytes.Buffer
+		if err := mainTmpl.Execute(&buf, struct {
+			Year    int
+			Version string
+		}{time.Now().Year(), version}); err != nil {
+			failf("mainTmpl.execute: %v", err)
+		}
+		path := filepath.Join(os.Getenv("GOPATH"), "src/golang.org/x/build/version", version, version+".go")
+		if err := os.Mkdir(filepath.Dir(path), 0755); err != nil {
+			failf("os.Mkdir: %v", err)
+		}
+		f, err := os.Create(path)
+		if err != nil {
+			failf("os.Create: %v", err)
+		}
+		defer f.Close()
+		if _, err := io.Copy(f, &buf); err != nil {
+			failf("could not write to file: %v", err)
+		}
+	}
+}
+
+func failf(format string, args ...interface{}) {
+	if len(format) == 0 || format[len(format)-1] != '\n' {
+		format += "\n"
+	}
+	fmt.Fprintf(os.Stderr, format, args...)
+	os.Exit(1)
+}
+
+var mainTmpl = template.Must(template.New("main").Parse(`// Copyright {{.Year}} The Go Authors. All rights reserved.
+	// Use of this source code is governed by a BSD-style
+	// license that can be found in the LICENSE file.
+
+	// The {{.Version}} command runs the go command from {{.Version}}.
+	//
+	// To install, run:
+	//
+	//     $ go get golang.org/x/build/version/{{.Version}}
+	//     $ {{.Version}} download
+	//
+	// And then use the {{.Version}} command as if it were your normal go
+	// command.
+	//
+	// See the release notes at https://golang.org/doc/{{.Version}}
+	//
+	// File bugs at https://golang.org/issues/new
+	package main
+
+	import "golang.org/x/build/version"
+
+	func main() {
+		version.Run("{{.Version}}")
+	}
+`))