Add beginnings of new cr command

go/src/infra/tools/cr/cmd/firstrun/common.go

Index: go/src/infra/tools/cr/cmd/firstrun/common.go
diff --git a/go/src/infra/tools/cr/cmd/firstrun/common.go b/go/src/infra/tools/cr/cmd/firstrun/common.go
new file mode 100644
index 0000000000000000000000000000000000000000..5422ba9a3f637dd517cd6b7cfcaf7e143234509b
--- /dev/null
+++ b/go/src/infra/tools/cr/cmd/firstrun/common.go
@@ -0,0 +1,53 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// OS-agnostic helper functions which are called from run().
+
+package firstrun
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+
+	"infra/tools/cr/lib/terminal"
+)
+
+var getenv = os.Getenv
+var stat = os.Stat
+
+// firstrunCheckNotInstalled is a sanity check to make sure that the user really
+// wants to do firstrun again, if it looks like cr is already installed.
+func firstrunCheckNotInstalled() (string, error) {
+	terminal.Debug("Checking to make sure cr isn't already installed...")
+	// Look for paths on $PATH that end in /cr/bin.
+	pathenv := getenv("PATH")
+	if pathenv == "" {
+		return "", errors.New("no $PATH variable found in the environment")
+	}
+	var possibleDirs []string
+	for _, elem := range filepath.SplitList(pathenv) {
+		head, bindir := filepath.Split(elem)
+		crdir := filepath.Base(head)
+		if crdir == "cr" && bindir == "bin" {
+			possibleDirs = append(possibleDirs, elem)
+		}
+	}
+	// Check to see if an executable called 'cr' is in those paths.
+	var statErr error
+	for _, dir := range possibleDirs {
+		info, err := stat(filepath.Join(dir, executableName))
+		if os.IsNotExist(err) {
+			continue
+		} else if err != nil {
+			statErr = err
+			continue
+		}
+		if info.Mode()&0111 != 0 {
+			return filepath.Join(dir, executableName), nil
+		}
+	}
+	// We didn't find an executable; if we ran into an error, return that.
+	return "", statErr
+}