When client sends request to upgrade from http1.1 to http2 and the request contains the payload body, the server fails to upgrade to http2. (RFC 7540 section 3.2)
And observed the following error.
2020/09/17 12:11:10 http: response.Write on hijacked connection from fmt.Fprintf (print.go:205)

GO Versions
go version go1.15.2 linux/amd64

Steps to Reproduce

1. Run the server.
2. Capture the packets using wireshark
3. send upgrade request from curl and include payload body
   sudo curl -vs -o /dev/null --http2 --request GET --data '{"username":"xyz","password":"xyz"}' http://localhost:8090/hello

Expected behavior:
The HTTP2 upgrade should be successful and the server should send the response back to the client.

Code Snippets

package main

import (
        "fmt"
        "net"
        "net/http"
        "os"

        "golang.org/x/net/http2"
        "golang.org/x/net/http2/h2c"
)


func main() {
        H2CUpgrade()
}

func H2CUpgrade() {
        h2s := &http2.Server{}

        handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
                fmt.Fprintf(w, "Hello, %v, http: %v", r.URL.Path, r.TLS == nil)
                return

        })

        server := &http.Server{
                Addr:    "0.0.0.0:8090",
                Handler: h2c.NewHandler(handler, h2s),
        }

        fmt.Printf("Listening [0.0.0.0:8090]...\n")
        server.ListenAndServe()
}