使用 io.Pipe 从 exec.command 获取输出以进行 http post

Question

正在阅读使用 io.Pipe 来减少分配（无需将 cmd.Stdout 分配给 bytes.Buffer）。如果我非常了解如何使用 io.Pipe 从 exec.command 获取输出以发出 http mutliform 发布请求，我无法让以下工作和感激。

func main() {
    cmd := exec.Command("convert", "test.png", "-resize" "30x30", "png:-")
    bodyReader, bodyWriter := io.Pipe()
    cmd.Stdout = bodyWriter
    formWriter := multipart.NewWriter(bodyWriter)
    go func() {
        cmd.Run()
        partWriter, err := formWriter.CreateFormFile("file", "file")
        if err != nil {
            fmt.Println("form writer create error")
            return
        }
        _, err = io.Copy(partWriter, bodyReader)
        if err != nil {
            fmt.Println("io copy error")
            return
        }
        formWriter.Close()
        bodyWriter.Close()
    }()

    url := "http://example.com/upload"
    req, _ := http.NewRequest(http.MethodPost, url, bodyReader)
    req.Header.Set("Content-Type", formWriter.FormDataContentType())
    resp, err := http.DefaultClient.Do(req)
    if err != nil {
        fmt.Println("htttp do error")
        return
    }
    fmt.Println(resp.StatusCode)
    respBody, err := ioutil.ReadAll(resp.Body)
    if err != nil {
        fmt.Println("read error")
        return
    }
    fmt.Println("Result:", string(respBody))
}

使用调试工具，它只是挂在resp, err := http.DefaultClient.Do(req)，最终输出如下。

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x1218c3b]

goroutine 1 [running]:
main.main()

Answer 1

通过将命令的标准输出设置为部分编写器来修复。

func main() {
    bodyReader, bodyWriter := io.Pipe()
    formWriter := multipart.NewWriter(bodyWriter)
    go func() (err error) {
        defer func() {
            bodyWriter.CloseWithError(err)
        }()
    
        partWriter, err := formWriter.CreateFormFile("file", "file")
        if err != nil {
            return err
        }
    
        cmd := exec.Command("convert", "test.png", "-resize" "30x30", "png:-")
        cmd.Stdout = partWriter
    
        if err := cmd.Run(); err != nil {
            return err
        }
    
        formWriter.Close()
        return nil
    }()

    ...

}

Run an example on the playground.

额外改进：使用 CloseWithError 将错误从 goroutine 传播到主函数。

Answer 2

您需要为multipart.Writer添加另一个Pipe

func main() {
    cmd := exec.Command("convert", "./test.jpg", "-resize", "30x30", "jpg:-")
    bodyReader, bodyWriter := io.Pipe()
    cmd.Stdout = bodyWriter
    cmd.Stderr = os.Stderr
    formReader, formWriter := io.Pipe()
    multiWriter := multipart.NewWriter(formWriter)

    go func() {
        if err := cmd.Run(); err != nil {
            bodyWriter.CloseWithError(err)
        } else {
            bodyWriter.Close()
        }
    }()

    go func() {
        partWriter, err := multiWriter.CreateFormFile("file", "file")
        if err != nil {
            formWriter.CloseWithError(err)
            return
        }

        _, err = io.Copy(partWriter, bodyReader)
        if err != nil {
            formWriter.CloseWithError(err)
            return
        }

        multiWriter.Close()
        formWriter.Close()
    }()

    url := "http://example.com/upload"
    req, _ := http.NewRequest(http.MethodPost, url, formReader)
    fmt.Println(multiWriter.FormDataContentType())
    req.Header.Set("Content-Type", multiWriter.FormDataContentType())
    resp, err := http.DefaultClient.Do(req)
    if err != nil {
        fmt.Println("htttp do error")
        return
    }
    fmt.Println(resp.StatusCode)
    respBody, err := ioutil.ReadAll(resp.Body)
    if err != nil {
        fmt.Println("read error")
        return
    }
    fmt.Println("Result:", string(respBody))
}