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 // Syncer makes the writes of a chat durable by group commit. Each write goes to
  14 // the operating system at once, so a crash of the process loses nothing. After
  15 // each write, Written starts a background sync of the file system of the chat,
  16 // unless one runs already. Then the next sync starts when the running one ends.
  17 // One sync makes every write before its start durable, so a sync covers many
  18 // writes, and there is never more than one sync at a time. A power loss loses
  19 // at most the writes after the start of the last finished sync.
  20 //
  21 // A failed sync can mean that the kernel dropped written data, so the error
  22 // stays, and each later Flush returns it.
  23 type Syncer struct {
  24         mutex   sync.Mutex
  25         sync    func() error
  26         file    *os.File
  27         written uint64
  28         durable uint64
  29         running bool
  30         done    chan struct{}
  31         failure error
  32         workers sync.WaitGroup
  33 }
  34
  35 // openSyncer returns the syncer of the file system that holds dir. It syncs
  36 // with syncfs, which writes back only that file system.
  37 func openSyncer(dir string) (*Syncer, error) {
  38         file, err := os.Open(dir)
  39         if err != nil {
  40                 return nil, fmt.Errorf("open chat directory: %w", err)
  41         }
  42
  43         s := newSyncer(func() error {
  44                 return unix.Syncfs(int(file.Fd()))
  45         })
  46         s.file = file
  47
  48         return s, nil
  49 }
  50
  51 // newSyncer returns a syncer that makes the writes durable with sync.
  52 func newSyncer(sync func() error) *Syncer {
  53         return &Syncer{
  54                 sync: sync,
  55                 done: make(chan struct{}),
  56         }
  57 }
  58
  59 // Written tells the syncer that a write went to the operating system, and
  60 // starts a sync when none runs.
  61 func (s *Syncer) Written() {
  62         s.mutex.Lock()
  63         defer s.mutex.Unlock()
  64
  65         s.written++
  66         s.start()
  67 }
  68
  69 // Flush returns when every write before the call is durable.
  70 func (s *Syncer) Flush(ctx context.Context) error {
  71         s.mutex.Lock()
  72
  73         target := s.written
  74         s.mutex.Unlock()
  75
  76         for {
  77                 s.mutex.Lock()
  78
  79                 reached := s.durable >= target
  80                 failure := s.failure
  81                 done := s.done
  82                 s.mutex.Unlock()
  83
  84                 if reached {
  85                         return nil
  86                 }
  87
  88                 if failure != nil {
  89                         return failure
  90                 }
  91
  92                 select {
  93                 case <-done:
  94                 case <-ctx.Done():
  95                         return ctx.Err()
  96                 }
  97         }
  98 }
  99
 100 // Close makes every write durable, waits for the syncs, and closes the
 101 // directory.
 102 func (s *Syncer) Close() error {
 103         err := s.Flush(context.Background())
 104         s.workers.Wait()
 105
 106         if s.file != nil {
 107                 err = errors.Join(err, s.file.Close())
 108                 s.file = nil
 109         }
 110
 111         return err
 112 }
 113
 114 // start starts a sync of the writes so far when none runs and a write is not
 115 // durable. The caller holds the mutex.
 116 func (s *Syncer) start() {
 117         if s.running || s.durable == s.written || s.failure != nil {
 118                 return
 119         }
 120
 121         s.running = true
 122         s.workers.Add(1)
 123
 124         go s.run(s.written)
 125 }
 126
 127 // run syncs once, so the writes up to target become durable, and starts the
 128 // next sync when writes came during this one.
 129 func (s *Syncer) run(target uint64) {
 130         defer s.workers.Done()
 131
 132         err := s.sync()
 133
 134         s.mutex.Lock()
 135         defer s.mutex.Unlock()
 136
 137         s.running = false
 138         if err != nil {
 139                 s.failure = fmt.Errorf("sync chat files: %w", err)
 140         } else {
 141                 s.durable = target
 142         }
 143
 144         close(s.done)
 145
 146         s.done = make(chan struct{})
 147         s.start()
 148 }