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watch.go
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watch.go
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package watch
import (
"context"
"errors"
"fmt"
"io"
"os"
"path/filepath"
"github.com/fsnotify/fsnotify"
"golang.org/x/sync/errgroup"
)
var (
ErrFileDeleted = errors.New("watched file has been deleted")
ErrFileOverwritten = errors.New("watched file has been overwritten")
ErrFileRenamed = errors.New("watched file has been renamed")
)
// Watcher defines the interface for trasaction head file watchers.
type Watcher interface {
// Watch starts watching a transaction head file for changer.
Watch(ctx context.Context, name string) error
}
// OffsetGetter defines the interface for transaction head file offset getters.
type OffsetGetter interface {
// GetOffset returns the current transaction head file offset.
GetOffset() int64
}
// Reader defines the interface for transaction head file readers.
type Reader interface {
io.ReadCloser
Watcher
OffsetGetter
}
// Option configures watch readers.
type Option func(*reader)
// StartOffset sets the initial offset for the reader.
func StartOffset(offset int64) Option {
return func(r *reader) {
r.offset = offset
}
}
// SeekEnd points the reader to the end of the contents.
func SeekEnd() Option {
return func(r *reader) {
r.seekEnd = true
}
}
// NewReader creates a new transaction head file reader.
func NewReader(options ...Option) Reader {
r := &reader{
read: make(chan struct{}),
}
for _, apply := range options {
apply(r)
}
return r
}
type reader struct {
offset int64
seekEnd bool
file io.ReadSeekCloser
read chan struct{}
}
func (r *reader) Read(b []byte) (n int, err error) {
// Block until watch is called
if _, ok := <-r.read; !ok {
// Channel closed before the first read
return 0, io.EOF
}
for {
n, err := r.file.Read(b)
if err != nil && err != io.EOF {
return n, err
}
// On successful read increment offset
if n > 0 {
r.offset += int64(n)
return n, nil
}
// When read fails with EOF wait until more content is available
if _, ok := <-r.read; !ok {
// Finish reading when read channel is closed
return 0, io.EOF
}
}
}
func (r *reader) Close() error {
close(r.read)
return nil
}
func (r *reader) GetOffset() int64 {
return r.offset
}
func (r *reader) Watch(ctx context.Context, name string) error {
name, err := filepath.Abs(name)
if err != nil {
return err
}
r.file, err = r.openFile(name)
if err != nil {
return err
}
defer func() {
r.file.Close()
}()
watcher, err := fsnotify.NewWatcher()
if err != nil {
return err
}
defer watcher.Close()
dir := filepath.Dir(name)
if err := watcher.Add(dir); err != nil {
return err
}
g, ctx := errgroup.WithContext(ctx)
g.Go(func() error {
for {
select {
case e := <-watcher.Events:
if e.Name != name {
continue
}
if err := r.handleEvent(e); err != nil {
return err
}
case err := <-watcher.Errors:
if err != nil {
return err
}
case <-ctx.Done():
return nil
}
}
})
return g.Wait()
}
func (r *reader) openFile(name string) (io.ReadSeekCloser, error) {
f, err := os.Open(name)
if err != nil {
return nil, err
}
if r.offset > 0 {
if _, err := f.Seek(r.offset, io.SeekStart); err != nil {
return nil, fmt.Errorf("error seeking file offset %d: %w", r.offset, err)
}
} else if r.seekEnd {
if _, err := f.Seek(0, io.SeekEnd); err != nil {
return nil, fmt.Errorf("error seeking file end: %w", err)
}
}
return f, nil
}
func (r *reader) handleEvent(e fsnotify.Event) error {
if e.Has(fsnotify.Remove) {
if _, err := os.Stat(e.Name); os.IsNotExist(err) {
return ErrFileDeleted
}
return ErrFileOverwritten
}
if e.Has(fsnotify.Rename) {
return ErrFileRenamed
}
// On write keep reading the file
if e.Has(fsnotify.Write) {
info, err := os.Stat(e.Name)
if err != nil {
return err
}
// Reset offset when file is truncated
if size := info.Size(); size <= r.offset {
if _, err := r.file.Seek(0, io.SeekStart); err != nil {
return err
}
r.offset = 0
// Ignore empty truncated file events
if size == 0 {
return nil
}
}
r.read <- struct{}{}
}
return nil
}