go.tools/dashboard/app: require version=2 for commit updates

This will prevent older, buggy builders from posting commits.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/153930043

dashboard/app/build/handler.go

@@ -15,6 +15,7 @@ import (
 	"fmt"
 	"io/ioutil"
 	"net/http"
+	"strconv"
 	"strings"
 	"unicode/utf8"
 
@@ -26,6 +27,7 @@ import (
 )
 
 const commitsPerPage = 30
+const watcherVersion = 2
 
 // commitHandler retrieves commit data or records a new commit.
 //
@@ -71,6 +73,16 @@ func commitHandler(r *http.Request) (interface{}, error) {
 		return nil, errors.New("can only POST commits with master key")
 	}
 
+	// For now, the commit watcher doesn't support gccgo,
+	// so only do this check for Go commits.
+	// TODO(adg,cmang): remove this check when gccgo is supported.
+	if dashboardForRequest(r) == goDash {
+		v, _ := strconv.Atoi(r.FormValue("version"))
+		if v != watcherVersion {
+			return nil, fmt.Errorf("rejecting POST from commit watcher; need version %v", watcherVersion)
+		}
+	}
+
 	// POST request
 	body, err := ioutil.ReadAll(r.Body)
 	r.Body.Close()