git.giacomolli.uk - onechat/blob - syncer.go

   1 package main
   2
   3 import (
   4         "context"
   5         "errors"
   6         "fmt"
   7         "os"
   8         "sync"
   9
  10         "golang.org/x/sys/unix"
  11 )
  12
  13 // openSyncer returns the syncer of the file system that holds dir. It syncs
  14 // with syncfs, which writes back only that file system.
  15 func openSyncer(dir string) (*Syncer, error) {
  16         file, err := os.Open(dir)
  17         if err != nil {
  18                 return nil, fmt.Errorf("open chat directory: %w", err)
  19         }
  20
  21         s := newSyncer(func() error {
  22                 return unix.Syncfs(int(file.Fd()))
  23         })
  24         s.file = file
  25
  26         return s, nil
  27 }
  28
  29 // Syncer makes the writes of a chat durable by group commit. Each write goes to
  30 // the operating system at once, so a crash of the process loses nothing. After
  31 // each write, Written starts a background sync of the file system of the chat,
  32 // unless one runs already. Then the next sync starts when the running one ends.
  33 // One sync makes every write before its start durable, so a sync covers many
  34 // writes, and there is never more than one sync at a time. A power loss loses
  35 // at most the writes after the start of the last finished sync.
  36 //
  37 // A failed sync can mean that the kernel dropped written data, so the error
  38 // stays, and each later Flush returns it.
  39 type Syncer struct {
  40         mutex   sync.Mutex
  41         sync    func() error
  42         file    *os.File
  43         written uint64
  44         durable uint64
  45         running bool
  46         done    chan struct{}
  47         failure error
  48         workers sync.WaitGroup
  49 }
  50
  51 // Written tells the syncer that a write went to the operating system, and
  52 // starts a sync when none runs.
  53 func (s *Syncer) Written() {
  54         s.mutex.Lock()
  55         defer s.mutex.Unlock()
  56
  57         s.written++
  58         s.start()
  59 }
  60
  61 // Flush returns when every write before the call is durable.
  62 func (s *Syncer) Flush(ctx context.Context) error {
  63         s.mutex.Lock()
  64         target := s.written
  65         s.mutex.Unlock()
  66
  67         for {
  68                 s.mutex.Lock()
  69                 reached := s.durable >= target
  70                 failure := s.failure
  71                 done := s.done
  72                 s.mutex.Unlock()
  73
  74                 if reached {
  75                         return nil
  76                 }
  77
  78                 if failure != nil {
  79                         return failure
  80                 }
  81
  82                 select {
  83                 case <-done:
  84                 case <-ctx.Done():
  85                         return ctx.Err()
  86                 }
  87         }
  88 }
  89
  90 // Close makes every write durable, waits for the syncs, and closes the
  91 // directory.
  92 func (s *Syncer) Close() error {
  93         err := s.Flush(context.Background())
  94         s.workers.Wait()
  95         if s.file != nil {
  96                 err = errors.Join(err, s.file.Close())
  97                 s.file = nil
  98         }
  99
 100         return err
 101 }
 102
 103 // start starts a sync of the writes so far when none runs and a write is not
 104 // durable. The caller holds the mutex.
 105 func (s *Syncer) start() {
 106         if s.running || s.durable == s.written || s.failure != nil {
 107                 return
 108         }
 109
 110         s.running = true
 111         s.workers.Add(1)
 112         go s.run(s.written)
 113 }
 114
 115 // run syncs once, so the writes up to target become durable, and starts the
 116 // next sync when writes came during this one.
 117 func (s *Syncer) run(target uint64) {
 118         defer s.workers.Done()
 119
 120         err := s.sync()
 121
 122         s.mutex.Lock()
 123         defer s.mutex.Unlock()
 124
 125         s.running = false
 126         if err != nil {
 127                 s.failure = fmt.Errorf("sync chat files: %w", err)
 128         } else {
 129                 s.durable = target
 130         }
 131
 132         close(s.done)
 133         s.done = make(chan struct{})
 134         s.start()
 135 }
 136
 137 // newSyncer returns a syncer that makes the writes durable with sync.
 138 func newSyncer(sync func() error) *Syncer {
 139         return &Syncer{
 140                 sync: sync,
 141                 done: make(chan struct{}),
 142         }
 143 }