text: // Go support for leveled logs, analogous to https://code.google.com/p/google-glog/
  //
  // Copyright 2013 Google Inc. All Rights Reserved.
  //
  // Licensed under the Apache License, Version 2.0 (the "License");
  // you may not use this file except in compliance with the License.
  // You may obtain a copy of the License at
  //
  //     http://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the License is distributed on an "AS IS" BASIS,
  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  // See the License for the specific language governing permissions and
  // limitations under the License.
  
  // File I/O for logs.
  
  package klog
  
  import (
  	"errors"
  	"fmt"
  	"os"
  	"os/user"
  	"path/filepath"
  	"strings"
  	"sync"
  	"time"
  )
  
  // MaxSize is the maximum size of a log file in bytes.
  var MaxSize uint64 = 1024 * 1024 * 1800
  
  // logDirs lists the candidate directories for new log files.
  var logDirs []string
  
  func createLogDirs() {
  	if logging.logDir != "" {
  		logDirs = append(logDirs, logging.logDir)
  	}
  	logDirs = append(logDirs, os.TempDir())
  }
  
  var (
  	pid      = os.Getpid()
  	program  = filepath.Base(os.Args[0])
  	host     = "unknownhost"
  	userName = "unknownuser"
  )
  
  func init() {
  	h, err := os.Hostname()
  	if err == nil {
  		host = shortHostname(h)
  	}
  
  	current, err := user.Current()
  	if err == nil {
  		userName = current.Username
  	}
  
  	// Sanitize userName since it may contain filepath separators on Windows.
  	userName = strings.Replace(userName, `\`, "_", -1)
  }
  
  // shortHostname returns its argument, truncating at the first period.
  // For instance, given "www.google.com" it returns "www".
  func shortHostname(hostname string) string {
  	if i := strings.Index(hostname, "."); i >= 0 {
  		return hostname[:i]
  	}
  	return hostname
  }
  
  // logName returns a new log file name containing tag, with start time t, and
  // the name for the symlink for tag.
  func logName(tag string, t time.Time) (name, link string) {
  	name = fmt.Sprintf("%s.%s.%s.log.%s.%04d%02d%02d-%02d%02d%02d.%d",
  		program,
  		host,
  		userName,
  		tag,
  		t.Year(),
  		t.Month(),
  		t.Day(),
  		t.Hour(),
  		t.Minute(),
  		t.Second(),
  		pid)
  	return name, program + "." + tag
  }
  
  var onceLogDirs sync.Once
  
  // create creates a new log file and returns the file and its filename, which
  // contains tag ("INFO", "FATAL", etc.) and t.  If the file is created
  // successfully, create also attempts to update the symlink for that tag, ignoring
  // errors.
  // The startup argument indicates whether this is the initial startup of klog.
  // If startup is true, existing files are opened for appending instead of truncated.
  func create(tag string, t time.Time, startup bool) (f *os.File, filename string, err error) {
  	if logging.logFile != "" {
  		f, err := openOrCreate(logging.logFile, startup)
  		if err == nil {
  			return f, logging.logFile, nil
  		}
  		return nil, "", fmt.Errorf("log: unable to create log: %v", err)
  	}
  	onceLogDirs.Do(createLogDirs)
  	if len(logDirs) == 0 {
  		return nil, "", errors.New("log: no log dirs")
  	}
  	name, link := logName(tag, t)
  	var lastErr error
  	for _, dir := range logDirs {
  		fname := filepath.Join(dir, name)
  		f, err := openOrCreate(fname, startup)
  		if err == nil {
  			symlink := filepath.Join(dir, link)
  			os.Remove(symlink)        // ignore err
  			os.Symlink(name, symlink) // ignore err
  			return f, fname, nil
  		}
  		lastErr = err
  	}
  	return nil, "", fmt.Errorf("log: cannot create log: %v", lastErr)
  }
  
  // The startup argument indicates whether this is the initial startup of klog.
  // If startup is true, existing files are opened for appending instead of truncated.
  func openOrCreate(name string, startup bool) (*os.File, error) {
  	if startup {
  		f, err := os.OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_APPEND, 0666)
  		return f, err
  	}
  	f, err := os.Create(name)
  	return f, err
  }
  