使用golang写入文件字节进行优化

Question

I have the following program that prints out information about each certificate in a .pem file:

package main

import (
    "crypto/x509"
    "encoding/pem"
    "io/ioutil"
    "log"
    "os"
    "strconv"
)

func main() {
    //for dev purposes set to 256
    const SignatureLength int = 256

    certPEMBlock, err := ioutil.ReadFile("testsign.crt")
    if err != nil {
        log.Fatal(err)
    }

    var blocks [][]byte
    for {
        var certDERBlock *pem.Block
        certDERBlock, certPEMBlock = pem.Decode(certPEMBlock)
        if certDERBlock == nil {
            break
        }

        if certDERBlock.Type == "CERTIFICATE" {
            blocks = append(blocks, certDERBlock.Bytes)
        }
    } //end for

    //OPEN FILE TO APPEND CERT INFORMATION INTO
    f, err := os.OpenFile("appendMe.txt", os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0644)
    if err != nil {
        log.Fatal(err)
    }

    for _, block := range blocks {
        cert, err := x509.ParseCertificate(block)
        if err != nil {
            log.Println(err)
            continue
        }

        //APPEND CERT INFO TO FILE
        //VERSION
        if _, err := f.Write([]byte(strconv.Itoa(cert.Version))); err != nil {
            log.Fatal(err)
        }
        //KEY ID
        if _, err := f.Write(cert.SubjectKeyId); err != nil {
            log.Fatal(err)
        }
        //SIGNATURE LENGTH
        if _, err := f.Write([]byte(strconv.Itoa(SignatureLength))); err != nil {
            log.Fatal(err)
        }
        //COMMON NAME
        if _, err := f.Write([]byte(cert.Subject.CommonName)); err != nil {
            log.Fatal(err)
        }

    } //end for

    //CLOSE THE FILE
    if err := f.Close(); err != nil {
        log.Fatal(err)
    }
}

it works, but notice that each line separately writes to the file. That seems a bit wasteful, but I am not clear about options in Go...do I create an array...do I slice it from the cert...another for loop...

QUESTION

Instead of f.Write() on each line, what is the proper or alternative approach in Go?

Instead of using the cert.*, should I be capturing that data into a struct or array?

Follow up

let's say the last write or any write fails, does Go roll back changes or does last write not get written to the file? This needs to be an all or nothing write.

(this is me learning as I do, thanks for your help)

Answer 1

The easiest way to write to a buffer in memory is using the bytes package, which provides a Buffer type. Since Buffer implements the Writer interface, you can update your Write calls to use the Buffer instead, and then use the Buffer.WriteTo method to write the accumulated data out to the file. Here's what that looks like, with the loops combined also:

package main

import (
    "bytes"
    "crypto/x509"
    "encoding/pem"
    "io/ioutil"
    "log"
    "os"
    "strconv"
)

func main() {
    //for dev purposes set to 256
    const SignatureLength int = 256

    certPEMBlock, err := ioutil.ReadFile("testsign.crt")
    if err != nil {
        log.Fatal(err)
    }

    //OPEN FILE TO APPEND CERT INFORMATION INTO
    f, err := os.OpenFile("appendMe.txt", os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0644)
    if err != nil {
        log.Fatal(err)
    }

    var buf bytes.Buffer
    for {
        var certDERBlock *pem.Block
        certDERBlock, certPEMBlock = pem.Decode(certPEMBlock)
        if certDERBlock == nil {
            break
        }

        if certDERBlock.Type == "CERTIFICATE" {
            cert, err := x509.ParseCertificate(certDERBlock.Bytes)
            if err != nil {
                log.Println(err)
                continue
            }

            //APPEND CERT INFO TO FILE
            //VERSION
            if _, err := buf.Write([]byte(strconv.Itoa(cert.Version))); err != nil {
                log.Fatal(err)
            }
            //KEY ID
            if _, err := buf.Write(cert.SubjectKeyId); err != nil {
                log.Fatal(err)
            }
            //SIGNATURE LENGTH
            if _, err := buf.Write([]byte(strconv.Itoa(SignatureLength))); err != nil {
                log.Fatal(err)
            }
            //COMMON NAME
            if _, err := buf.Write([]byte(cert.Subject.CommonName)); err != nil {
                log.Fatal(err)
            }
        }
    } //end for

    // write data accumulated in buf out to f
    buf.WriteTo(f)

    //CLOSE THE FILE
    if err := f.Close(); err != nil {
        log.Fatal(err)
    }
}