https://github.com/cihub/seelog/issues/44

gistfile1.txt

package main

import (
	"crypto/rand"
	"fmt"
	log "github.com/cihub/seelog"
	"io"
	"os"
)

const lines = 100000

var chars = []byte("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789")

func main() {
	logger, err := log.LoggerFromConfigAsString(`
		<seelog>
		 <outputs formatid="common">
		   <filter levels="info,warn,error,critical">
		     <file path="/tmp/seelog-44.log" />
		   </filter>
		 </outputs>
		 <formats>
		   <format id="common" format="%Date %Time%t[%LEVEL]%t%Msg%n" />
		 </formats>
		</seelog>
	`)
	if err != nil {
		fmt.Fprintf(os.Stderr, err.Error())
		os.Exit(1)
	}
	defer logger.Flush()

	for i := 0; i < lines; i++ {
		logger.Info(randomString(256))
	}

	fmt.Printf("lines: %d\n", lines)
}

// Shamelessly stolen from github.com/dchest/uniuri
func randomString(length int) string {
	b := make([]byte, length)
	r := make([]byte, length+(length/4)) // storage for random bytes.
	clen := byte(len(chars))
	maxrb := byte(256 - (256 % len(chars)))
	i := 0
	for {
		if _, err := io.ReadFull(rand.Reader, r); err != nil {
			panic("error reading from random source: " + err.Error())
		}
		for _, c := range r {
			if c >= maxrb {
				continue
			}
			b[i] = chars[c%clen]
			i++
			if i == length {
				return string(b)
			}
		}
	}
}

I started four of these processes in quick succession. They must run concurrently. Each finished successfully after logging (or attempting to log) 100000 lines, but only 300007 lines were actually logged (approximately 3/4).

If you replace seelog.LoggerInterface with *os.File, it works as expected.

#!/bin/sh

$GOPATH/bin/example &
$GOPATH/bin/example &
$GOPATH/bin/example &
$GOPATH/bin/example &