Add beginnings of new cr command

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

+// 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
+}