os: io.Copy(net.Conn, os.Stdin) on MacOS terminate immediately without waiting for input [1.23 backport]

[gopherbot]

@ianlancetaylor requested issue #70763 to be considered for backport to the next 1.23 minor release.

@gopherbot Please open backport to 1.23

This is a simple program that broke with some sendfile work in the Go 1.23 release. Earlier releases are OK.

[gopherbot]

Change https://go.dev/cl/635397 mentions this issue: [release-branch.go1.23] net: don't use sendfile for irregular files; handle EOPNOTSUPP/ENOTSUP

[gopherbot]

Closed by merging CL 635397 (commit 1576793) to release-branch.go1.23.

[wjkoh]

For the record, in Go 1.24, io.Copy(w, os.Stdin) returns immediately when run inside a unit test. See the following example code.

package main

import (
	"bytes"
	"io"
	"os"
	"testing"
)

func TestCopyStdin(t *testing.T) {
	var b bytes.Buffer
        // NOTE: The io.Copy call returns immediately.
	_, err := io.Copy(&b, os.Stdin)
	if err != nil {
		t.Fatal(err)
	}
	t.Log(b.String())
}