Optimize serving of default Go branch by using local GOROOT.

Previously, we would always serve code in GOROOT via the Go repository.
However, it's slightly faster to use files on disk in /usr/local/go,
which can be done when the requested version matches the version
specified in /usr/local/go/VERSION file.

localgoroot.go

@@ -0,0 +1,17 @@
+package main
+
+import (
+	"go/build"
+	"io/ioutil"
+	"path/filepath"
+)
+
+var LocalGoVersion string
+
+func localGoVersion() (string, error) {
+	version, err := ioutil.ReadFile(filepath.Join(build.Default.GOROOT, "VERSION"))
+	if err != nil {
+		return "", err
+	}
+	return string(version), nil
+}

main.go

@@ -106,6 +106,15 @@ func main() {
 		vs = &localVCSStore{dir: *vcsStoreDirFlag}
 	}
 
+	if *remoteGorootFlag {
+		var err error
+		LocalGoVersion, err = localGoVersion()
+		if err != nil {
+			log.Fatalln("no local Go version available:", err)
+		}
+		fmt.Printf("using local Go version %q\n", LocalGoVersion)
+	}
+
 	http.HandleFunc("/", codeHandler)
 	http.Handle("/favicon.ico", http.NotFoundHandler())
 	http.HandleFunc("/robots.txt", func(w http.ResponseWriter, req *http.Request) {
@@ -606,12 +615,15 @@ func tryRemoteGoroot(importPath, rev string) (
 		return nil, nil, "", "", err
 	}
 
-	// TODO: Use local Go version as default.
-	defaultBranch = "go1.8.3"
+	// Use local Go version as default.
+	defaultBranch = LocalGoVersion
 
+	var local bool
 	if rev != "" {
+		local = rev == LocalGoVersion
 		commitId, err = repo.ResolveRevision(rev)
 	} else {
+		local = true
 		commitId, err = repo.ResolveTag(defaultBranch)
 	}
 	if err != nil {
@@ -634,10 +646,31 @@ func tryRemoteGoroot(importPath, rev string) (
 		fmt.Println("tryRemote: worked on first try")
 	}
 
+	if local {
+		repo = repoWithLocal{
+			Repository:     repo,
+			localGoVersion: commitId,
+		}
+	}
+
 	rs := repoSpec{vcsType: "git", cloneURL: "https://go.googlesource.com/go"} // TODO: Avoid having to return a pointer. It's not optional in this context.
 	return repo, &rs, commitId, defaultBranch, nil
 }
 
+type repoWithLocal struct {
+	vcs.Repository
+	localGoVersion vcs.CommitID
+}
+
+func (r repoWithLocal) FileSystem(at vcs.CommitID) (vfs.FileSystem, error) {
+	if at == r.localGoVersion {
+		fmt.Println("using LocalGoVersion:", at)
+		return vfs.OS(build.Default.GOROOT), nil
+	}
+	fmt.Println("using vcs repo", at)
+	return r.Repository.FileSystem(at)
+}
+
 func tryLocalGopath(importPath, rev string) (
 	repo vcs.Repository,
 	repoImportPath string,