How to Track Upload Progress in Go HTTP POST Requests

In the development of a ShareX clone for Linux using Go, there's a need to monitor the progress of large file uploads sent via HTTP POST requests. While manually opening a TCP connection and writing HTTP requests in chunks is an option, it's not the most efficient and reliable solution.

ProgressReader: A Custom io.Reader

A more effective approach involves creating a custom io.Reader to wrap around the actual reader. This custom reader, called ProgressReader, has a Reporter function that outputs the upload progress each time the Read method is called, providing real-time monitoring.

Here's an example of how to implement this idea:

type ProgressReader struct {
    io.Reader
    Reporter func(r int64)
}

func (pr *ProgressReader) Read(p []byte) (n int, err error) {
    n, err = pr.Reader.Read(p)
    pr.Reporter(int64(n))
    return
}

This custom reader is then used to wrap the file being uploaded:

file, _ := os.Open("/tmp/blah.go")
total := int64(0)
pr := &ProgressReader{file, func(r int64) {
    total  = r
    if r > 0 {
        fmt.Println("progress", r)
    } else {
        fmt.Println("done", r)
    }
}}

Finally, the data is copied using io.Copy to ioutil.Discard, which acts as a sink for the data while still allowing the progress output to be printed.

io.Copy(ioutil.Discard, pr)

This method provides a simple and reliable way to track upload progress for HTTP POST requests in Go, enabling users to monitor the progress of large file uploads in real-time.