// 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 glog

import (
	"errors"
	"flag"
	"fmt"
	"os"
	"os/user"
	"path"
	"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

// If non-empty, overrides the choice of directory in which to write logs.
// See createLogDirs for the full list of possible destinations.
var log_dir = flag.String("log_dir", "", "If non-empty, write log files in this directory")

func createLogDirs() {
	candidates := []string{
		*log_dir,

		// Explicitly-supplied temp dirs
		os.Getenv("TMPDIR"),
		os.Getenv("TMP"),

		// If all else fails
		"/tmp",
	}
	for _, dir := range candidates {
		if len(dir) == 0 {
			continue
		}
		if !filepath.IsAbs(dir) {
			dir = "/" + dir
		}
		logDirs = append(logDirs, dir)
	}
}

var (
	pid      = os.Getpid()
	program  = path.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
	}
}

// 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.
func create(tag string, t time.Time) (f *os.File, filename string, err error) {
	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 := path.Join(dir, name)
		f, err := os.Create(fname)
		if err == nil {
			symlink := path.Join(dir, link)
			os.Remove(symlink)         // ignore err
			os.Symlink(fname, symlink) // ignore err
			return f, fname, nil
		}
		lastErr = err
	}
	return nil, "", fmt.Errorf("log: cannot create log: %v", lastErr)
}