如何为Go子进程的stdout输出添加前缀？

是否可以为子进程的stdout添加前缀？已知stdout属于io.Writer类型且不可变。我曾考虑为每个子进程添加logrus钩子来在日志中显示PID，但仍想了解是否有修改stdout的方法。

以下测试代码启动3个子进程，将它们的stdout和stderr重定向到主程序的stdout和stderr：

package main

import (
    "os"
    "os/exec"
    "testing"
    "time"

    "github.com/sirupsen/logrus"
)

func Crasher(t *testing.T) {
    logrus.Info("huhaha")
    // This runs test with lots of logs.
}

func startProcess(pid int) {
    cmd := exec.Command(os.Args[0], "-test.run=^TestCrasher$")
    cmd.Env = append(os.Environ(), "BE_CRASHER=1")
    cmd.Stdout = os.Stdout
    cmd.Stderr = os.Stderr

    logrus.Infof("Starting process with pid %d", pid)
    _ = cmd.Run()
}

func TestCrasher(t *testing.T) {
    if os.Getenv("BE_CRASHER") == "1" {
        Crasher(t)
        return
    }

    for pid := 0; pid < 3; pid++ {
        go startProcess(pid)
    }
    time.Sleep(5 * time.Second)
}

当前输出：

=== RUN   TestCrasher
time="2009-11-10T23:00:00Z" level=info msg="Starting process with pid 0"
time="2009-11-10T23:00:00Z" level=info msg="Starting process with pid 2"
time="2009-11-10T23:00:00Z" level=info msg="Starting process with pid 1"
time="2009-11-10T23:00:00Z" level=info msg=huhaha
time="2009-11-10T23:00:00Z" level=info msg=huhaha
time="2009-11-10T23:00:00Z" level=info msg=huhaha
PASS
PASS
PASS
--- PASS: TestCrasher (5.00s)
PASS

期望输出：

time="2009-11-10T23:00:00Z" level=info msg="Starting process with pid 0"
time="2009-11-10T23:00:00Z" level=info msg="Starting process with pid 2"
time="2009-11-10T23:00:00Z" level=info msg="Starting process with pid 1"
pid0 time="2009-11-10T23:00:00Z" level=info msg=huhaha
pid1 time="2009-11-10T23:00:00Z" level=info msg=huhaha
pid2 time="2009-11-10T23:00:00Z" level=info msg=huhaha

可以通过实现自定义io.Writer给子进程输出添加前缀，无需修改原始stdout（确实不可变），只需将自定义Writer作为子进程的Stdout/Stderr，写入时自动添加前缀。

实现步骤 #

1. 定义带前缀的Writer结构体，包含目标Writer和前缀字符串
2. 实现Write方法，按行分割输入内容，给每行添加前缀后写入目标Writer
3. 启动子进程时，将cmd.Stdout和cmd.Stderr替换为自定义Writer

修改后的代码 #

package main

import (
    "bufio"
    "io"
    "os"
    "os/exec"
    "testing"
    "time"

    "github.com/sirupsen/logrus"
)

// PrefixWriter 给输出添加前缀的自定义Writer
type PrefixWriter struct {
    prefix string
    writer io.Writer
}

func NewPrefixWriter(prefix string, w io.Writer) *PrefixWriter {
    return &PrefixWriter{
        prefix: prefix,
        writer: w,
    }
}

// Write 实现io.Writer接口，处理字节流并添加前缀
func (pw *PrefixWriter) Write(data []byte) (n int, err error) {
    scanner := bufio.NewScanner(io.NewReader(data))
    totalWritten := 0

    for scanner.Scan() {
        line := scanner.Text()
        output := []byte(pw.prefix + " " + line + "\n")
        
        written, writeErr := pw.writer.Write(output)
        if writeErr != nil {
            return totalWritten, writeErr
        }
        totalWritten += written
    }

    return len(data), scanner.Err()
}

func Crasher(t *testing.T) {
    logrus.Info("huhaha")
    // This runs test with lots of logs.
}

func startProcess(pid int) {
    cmd := exec.Command(os.Args[0], "-test.run=^TestCrasher$")
    cmd.Env = append(os.Environ(), "BE_CRASHER=1")
    
    // 使用自定义PrefixWriter包装os.Stdout和os.Stderr
    prefix := "pid" + string(rune(pid+'0'))
    cmd.Stdout = NewPrefixWriter(prefix, os.Stdout)
    cmd.Stderr = NewPrefixWriter(prefix, os.Stderr)

    logrus.Infof("Starting process with pid %d", pid)
    _ = cmd.Run()
}

func TestCrasher(t *testing.T) {
    if os.Getenv("BE_CRASHER") == "1" {
        Crasher(t)
        return
    }

    for pid := 0; pid < 3; pid++ {
        go startProcess(pid)
    }
    time.Sleep(5 * time.Second)
}

说明 #

• 自定义PrefixWriter会按行处理子进程输出，每行前自动添加指定前缀，再写入原始stdout
• 无需修改子进程代码，完全在主进程层面处理输出，避免了给每个子进程配置logrus钩子的复杂度
• 输出会符合期望格式，每个子进程的日志行前都会带上对应的pid标识

内容的提问来源于stack exchange，提问作者subtleseeker