app/appengine, cmd/genbuilderkey: move key gen from App Engine to new tool

This moves the /key handler to generate build keys to a standalone
tool. The old way has been largely broken for years (when using the
build.golang.org domain) due to internal App Engine changes. We have
to access it via https://build-dot-golang-org.appspot.com/key instead
to get authenticated.

Also, the App Engine go112 runtime doesn't support authenticated
handlers, so more reason to move off App Engine.

This CL is part of a series to move off the the build.golang.org App
Engine app that mirrors the git history into Datastore Entities, which
is full of complication and bugs.

These early steps are about removing a bunch of code from the App
Engine app so the important bits are easy to see and refactor.

Updates golang/go#34744

Change-Id: Iaf8e2bf458b5fea45bf05026d8a6eaf0ead88ec2
Reviewed-on: https://go-review.googlesource.com/c/build/+/208320
Reviewed-by: Bryan C. Mills <[email protected]>
Run-TryBot: Bryan C. Mills <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: bradfitz

app/appengine/app.yaml

@@ -5,7 +5,7 @@ handlers:
   - url: /static
     static_dir: app/appengine/static
     secure: always
-  - url: /(init|buildtest|key|_ah/queue/go/delay)
+  - url: /(init|buildtest|_ah/queue/go/delay)
     script: auto
     login: admin
     secure: always

app/appengine/dash.go

@@ -19,7 +19,6 @@ func main() {
 
 	// admin handlers
 	handleFunc("/init", initHandler)
-	handleFunc("/key", keyHandler)
 
 	// authenticated handlers
 	handleFunc("/building", AuthHandler(buildingHandler))

app/appengine/handler.go

@@ -603,16 +603,6 @@ func AuthHandler(h dashHandler) http.HandlerFunc {
 	}
 }
 
-func keyHandler(w http.ResponseWriter, r *http.Request) {
-	builder := r.FormValue("builder")
-	if builder == "" {
-		logErr(w, r, errors.New("must supply builder in query string"))
-		return
-	}
-	c := contextForRequest(r)
-	fmt.Fprint(w, builderKey(c, builder))
-}
-
 func validHash(hash string) bool {
 	// TODO(adg): correctly validate a hash
 	return hash != ""

cmd/genbuilderkey/genbuilderkey.go

@@ -0,0 +1,50 @@
+// Copyright 2019 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 genbuilderkey binary generates a builder key or gomote user key
+// from the build system's master key.
+package main
+
+import (
+	"bytes"
+	"crypto/hmac"
+	"crypto/md5"
+	"flag"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"log"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"cloud.google.com/go/compute/metadata"
+)
+
+func main() {
+	flag.Parse()
+	if flag.NArg() != 1 {
+		log.Fatalf("expect one argument")
+	}
+	fmt.Println(key(flag.Arg(0)))
+}
+
+func key(principal string) string {
+	h := hmac.New(md5.New, getMasterKey())
+	io.WriteString(h, principal)
+	return fmt.Sprintf("%x", h.Sum(nil))
+}
+
+func getMasterKey() []byte {
+	v, err := metadata.ProjectAttributeValue("builder-master-key")
+	if err == nil {
+		return []byte(strings.TrimSpace(v))
+	}
+	key, err := ioutil.ReadFile(filepath.Join(os.Getenv("HOME"), "keys/gobuilder-master.key"))
+	if err == nil {
+		return bytes.TrimSpace(key)
+	}
+	log.Fatalf("no builder master key found")
+	panic("not reachable")
+}