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// Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
// vespa logfmt command
// Author: arnej
//go:build !windows
package logfmt
import (
"bufio"
"fmt"
"io"
"os"
"time"
"golang.org/x/sys/unix"
)
const lastLinesSize = 4 * 1024
// an active "tail -f" like object
type unixTail struct {
lines chan Line
lineBuf []byte
curFile *os.File
fn string
reader *bufio.Reader
curStat unix.Stat_t
}
func (t *unixTail) Lines() chan Line { return t.lines }
// API for starting to follow a log file
func FollowFile(fn string) (Tail, error) {
res := unixTail{}
res.fn = fn
res.lineBuf = make([]byte, lastLinesSize)
res.openTail()
res.lines = make(chan Line, 20)
res.lineBuf = res.lineBuf[:0]
go runTailWith(&res)
return &res, nil
}
func (t *unixTail) setFile(f *os.File) {
if t.curFile != nil {
t.curFile.Close()
}
t.curFile = f
if f != nil {
err := unix.Fstat(int(f.Fd()), &t.curStat)
if err != nil {
f.Close()
fmt.Fprintf(os.Stderr, "unexpected failure: %v\n", err)
return
}
t.reader = bufio.NewReaderSize(f, 1024*1024)
} else {
t.reader = nil
}
}
// open log file and seek to the start of a line near the end, if possible.
func (t *unixTail) openTail() {
file, err := os.Open(t.fn)
if err != nil {
return
}
sz, err := file.Seek(0, os.SEEK_END)
if err != nil {
return
}
if sz < lastLinesSize {
sz, err = file.Seek(0, os.SEEK_SET)
if err == nil {
// just read from start of file, all OK
t.setFile(file)
}
return
}
sz, _ = file.Seek(-lastLinesSize, os.SEEK_END)
n, err := file.Read(t.lineBuf)
if err != nil {
return
}
for i := 0; i < n; i++ {
if t.lineBuf[i] == '\n' {
sz, err = file.Seek(sz+int64(i+1), os.SEEK_SET)
if err == nil {
t.setFile(file)
}
return
}
}
}
func (t *unixTail) reopen(cur *unix.Stat_t) {
for cnt := 0; cnt < 100; cnt++ {
file, err := os.Open(t.fn)
if err != nil {
t.setFile(nil)
if cnt == 0 {
fmt.Fprintf(os.Stderr, "%v (waiting for log file to appear)\n", err)
}
time.Sleep(1000 * time.Millisecond)
continue
}
var stat unix.Stat_t
err = unix.Fstat(int(file.Fd()), &stat)
if err != nil {
file.Close()
fmt.Fprintf(os.Stderr, "unexpected failure: %v\n", err)
time.Sleep(5000 * time.Millisecond)
continue
}
if cur != nil && cur.Dev == stat.Dev && cur.Ino == stat.Ino {
// same file, continue following it
file.Close()
return
}
// new file, start following it
t.setFile(file)
return
}
}
// runs as a goroutine
func runTailWith(t *unixTail) {
defer t.setFile(nil)
loop:
for {
for t.curFile == nil {
t.reopen(nil)
}
bytes, err := t.reader.ReadSlice('\n')
t.lineBuf = append(t.lineBuf, bytes...)
if err == bufio.ErrBufferFull {
continue
}
if err == nil {
ll := len(t.lineBuf) - 1
t.lines <- Line{Text: string(t.lineBuf[:ll])}
t.lineBuf = t.lineBuf[:0]
continue
}
if err == io.EOF {
pos, _ := t.curFile.Seek(0, os.SEEK_CUR)
for cnt := 0; cnt < 100; cnt++ {
time.Sleep(10 * time.Millisecond)
sz, _ := t.curFile.Seek(0, os.SEEK_END)
if sz != pos {
if sz < pos {
// truncation case
pos = 0
}
t.curFile.Seek(pos, os.SEEK_SET)
continue loop
}
}
// no change in file size, try reopening
t.reopen(&t.curStat)
} else {
fmt.Fprintf(os.Stderr, "error tailing '%s': %v\n", t.fn, err)
close(t.lines)
return
}
}
}
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