log/filelogger: move our Windows disk file writing+rotation package here

It's still Windows-only for now but it's easy to de-Windows-ify when needed.

Moving it out of corp repo and into tailscale/tailscale so we can use
it in ipnserver.BabysitProc.

Updates #726

log/filelogger/log.go

@@ -0,0 +1,199 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package filelogger provides localdisk log writing & rotation, primarily for Windows
+// clients. (We get this for free on other platforms.)
+package filelogger
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"log"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"sync"
+	"time"
+
+	"tailscale.com/types/logger"
+)
+
+const (
+	maxSize  = 100 << 20
+	maxFiles = 50
+)
+
+// New returns a logf wrapper that appends to local disk log
+// files on Windows, rotating old log files as needed to stay under
+// file count & byte limits.
+func New(fileBasePrefix, logID string, logf logger.Logf) logger.Logf {
+	if runtime.GOOS != "windows" {
+		panic("not yet supported on any platform except Windows")
+	}
+	if logf == nil {
+		panic("nil logf")
+	}
+	dir := filepath.Join(os.Getenv("LocalAppData"), "Tailscale", "Logs")
+
+	if err := os.MkdirAll(dir, 0700); err != nil {
+		log.Printf("failed to create local log directory; not writing logs to disk: %v", err)
+		return logf
+	}
+	logf("local disk logdir: %v", dir)
+	lfw := &logFileWriter{
+		fileBasePrefix: fileBasePrefix,
+		logID:          logID,
+		dir:            dir,
+		wrappedLogf:    logf,
+	}
+	return lfw.Logf
+}
+
+// logFileWriter is the state for the log writer & rotator.
+type logFileWriter struct {
+	dir            string      // e.g. `C:\Users\FooBarUser\AppData\Local\Tailscale\Logs`
+	logID          string      // hex logID
+	fileBasePrefix string      // e.g. "tailscale-service" or "tailscale-gui"
+	wrappedLogf    logger.Logf // underlying logger to send to
+
+	mu   sync.Mutex   // guards following
+	buf  bytes.Buffer // scratch buffer to avoid allocs
+	fday civilDay     // day that f was opened; zero means no file yet open
+	f    *os.File     // file currently opened for append
+}
+
+// civilDay is a year, month, and day in the local timezone.
+// It's a comparable value type.
+type civilDay struct {
+	year  int
+	month time.Month
+	day   int
+}
+
+func dayOf(t time.Time) civilDay {
+	return civilDay{t.Year(), t.Month(), t.Day()}
+}
+
+func (w *logFileWriter) Logf(format string, a ...interface{}) {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+
+	w.buf.Reset()
+	fmt.Fprintf(&w.buf, format, a...)
+	if w.buf.Len() == 0 {
+		return
+	}
+	out := w.buf.Bytes()
+	w.wrappedLogf("%s", out)
+
+	// Make sure there's a final newline before we write to the log file.
+	if out[len(out)-1] != '\n' {
+		w.buf.WriteByte('\n')
+		out = w.buf.Bytes()
+	}
+
+	w.appendToFileLocked(out)
+}
+
+// out should end in a newline.
+// w.mu must be held.
+func (w *logFileWriter) appendToFileLocked(out []byte) {
+	now := time.Now()
+	day := dayOf(now)
+	if w.fday != day {
+		w.startNewFileLocked()
+	}
+	if w.f != nil {
+		// RFC3339Nano but with a fixed number (3) of nanosecond digits:
+		const formatPre = "2006-01-02T15:04:05"
+		const formatPost = "Z07:00"
+		fmt.Fprintf(w.f, "%s.%03d%s: %s",
+			now.Format(formatPre),
+			now.Nanosecond()/int(time.Millisecond/time.Nanosecond),
+			now.Format(formatPost),
+			out)
+	}
+}
+
+// startNewFileLocked opens a new log file for writing
+// and also cleans up any old files.
+//
+// w.mu must be held.
+func (w *logFileWriter) startNewFileLocked() {
+	var oldName string
+	if w.f != nil {
+		oldName = filepath.Base(w.f.Name())
+		w.f.Close()
+		w.f = nil
+		w.fday = civilDay{}
+	}
+	w.cleanLocked()
+
+	now := time.Now()
+	day := dayOf(now)
+	name := filepath.Join(w.dir, fmt.Sprintf("%s-%04d%02d%02dT%02d%02d%02d-%d.txt",
+		w.fileBasePrefix,
+		day.year,
+		day.month,
+		day.day,
+		now.Hour(),
+		now.Minute(),
+		now.Second(),
+		now.Unix()))
+	var err error
+	w.f, err = os.Create(name)
+	if err != nil {
+		w.wrappedLogf("failed to create log file: %v", err)
+		return
+	}
+	if oldName != "" {
+		fmt.Fprintf(w.f, "(logID %q; continued from log file %s)\n", w.logID, oldName)
+	} else {
+		fmt.Fprintf(w.f, "(logID %q)\n", w.logID)
+	}
+	w.fday = day
+}
+
+// cleanLocked cleans up old log files.
+//
+// w.mu must be held.
+func (w *logFileWriter) cleanLocked() {
+	fis, _ := ioutil.ReadDir(w.dir)
+	prefix := w.fileBasePrefix + "-"
+	fileSize := map[string]int64{}
+	var files []string
+	var sumSize int64
+	for _, fi := range fis {
+		baseName := filepath.Base(fi.Name())
+		if !strings.HasPrefix(baseName, prefix) {
+			continue
+		}
+		size := fi.Size()
+		fileSize[baseName] = size
+		sumSize += size
+		files = append(files, baseName)
+	}
+	if sumSize > maxSize {
+		w.wrappedLogf("cleaning log files; sum byte count %d > %d", sumSize, maxSize)
+	}
+	if len(files) > maxFiles {
+		w.wrappedLogf("cleaning log files; number of files %d > %d", len(files), maxFiles)
+	}
+	for (sumSize > maxSize || len(files) > maxFiles) && len(files) > 0 {
+		target := files[0]
+		files = files[1:]
+
+		targetSize := fileSize[target]
+		targetFull := filepath.Join(w.dir, target)
+		err := os.Remove(targetFull)
+		if err != nil {
+			w.wrappedLogf("error cleaning log file: %v", err)
+		} else {
+			sumSize -= targetSize
+			w.wrappedLogf("cleaned log file %s (size %d); new bytes=%v, files=%v", targetFull, targetSize, sumSize, len(files))
+		}
+	}
+}