goroutine not writing to channel

Question content

I am new to go and I have a problem using the following code

func (h *Handler) GeneratePdfFromHTML(c echo.Context) (err error) {
    req := &createPdfFromHTMLRequest{}
    if err := req.bind(c); err != nil {
        return c.JSON(http.StatusBadRequest, utils.NewError(err))
    }

    rawDecodedText, err := base64.StdEncoding.DecodeString(req.HTML)
    if err != nil {
        return c.JSON(http.StatusInternalServerError, utils.NewError(err))
    }

    buf := make(chan []byte)

    go func() {
        defer close(buf)
        pdf, err := pdf.GenerateFromHTML(string(rawDecodedText))
        if err == nil {
            buf <- pdf
        }
    }()

    if err != nil {
        return c.JSON(http.StatusInternalServerError, utils.NewError(err))
    }

    return c.Blob(http.StatusOK, MIMEApplicationPdf, <-buf)
}

Receive information from generatefromhtml in goroutine pdf, but buf does not receive any value, so the function where this code is located returns a size of 0 byte.

Any help is greatly appreciated. Thanks in advance

Correct answer

This code is synchronous in nature. The handler generates a slice of bytes and should use the c.blob method to return that slice when these bytes are ready.

The posted code starts work in a goroutine, does not wait for the work to complete and returns a null byte slice.

You can solve this problem by removing the goroutine:

data, err := pdf.GenerateFromHTML(string(rawDecodedText))
if err == nil {
    // handle error here
}
return c.Blob(http.StatusOK, MIMEApplicationPdf, data)

The only problem with this code is loading all the data into memory, but this is unavoidable if pdf.generatefromhtml returns []byte. If necessary, you can improve this by updating pdf.generatefromhtml to return io.reader and using c.stream.

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