Why is there a data race in this Go program?

I'm trying to store log messages in a buffer to access them only when I get an error. A bit like in Smarter log handling, the case for opportunistic logging. In this example I fetch the logs from the buffer each 5 seconds but I get a data race when I run it with go run -race code.go.

I'm using channels to communicate but I'm doing something wrong, obviously.

package main

import (
    "bytes"
    "fmt"
    "io/ioutil"
    "log"
    "time"
)

type LogRequest struct {
    Buffer chan []byte
}

type LogBuffer struct {
    LogInputChan chan []byte
    LogRequests  chan LogRequest
}

func (f LogBuffer) Write(b []byte) (n int, err error) {
    f.LogInputChan <- b
    return len(b), nil
}

func main() {
    var logBuffer LogBuffer
    logBuffer.LogInputChan = make(chan []byte, 100)
    logBuffer.LogRequests = make(chan LogRequest, 100)

    log.SetOutput(logBuffer)

    // store the log messages in a buffer until we ask for it
    go func() {
        buf := new(bytes.Buffer)

        for {
            select {
            // receive log messages
            case logMessage := <-logBuffer.LogInputChan:
                buf.Write(logMessage) // <- data race
            case logRequest := <-logBuffer.LogRequests:
                c, errReadAll := ioutil.ReadAll(buf)
                if errReadAll != nil {
                    panic(errReadAll)
                }
                logRequest.Buffer <- c
            }
        }
    }()

    // log a test message every 1 second
    go func() {
        for i := 0; i < 30; i++ {
            log.Printf("test: %d", i) // <- data race
            time.Sleep(1 * time.Second)
        }
    }()

    // print the log every 5 seconds
    go func() {
        for {
            time.Sleep(5 * time.Second)

            var logRequest LogRequest
            logRequest.Buffer = make(chan []byte, 1)
            logBuffer.LogRequests <- logRequest

            buffer := <-logRequest.Buffer

            fmt.Printf("**** LOG *****\n%s**** END *****\n\n", buffer)
        }
    }()

    time.Sleep(45 * time.Second)
}