net: fix sendfile regression with io.Copy on macOS

Since CL 472475, io.Copy can no longer use sendfile on macOS for copying
files to a socket due to a too strict type assertion. This CL fixes the
issue by checking for the necessary interfaces instead of the concrete
os.File type in sendfile_unix_alt.go.

Fixes #66988

Change-Id: Ia0dd190f6575016a191c34a935132907147c8e10
Reviewed-on: https://go-review.googlesource.com/c/go/+/581035
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Damien Neil <[email protected]>

Author: philwo

src/net/sendfile_unix_alt.go

@@ -9,7 +9,8 @@ package net
 import (
 	"internal/poll"
 	"io"
-	"os"
+	"io/fs"
+	"syscall"
 )
 
 // sendFile copies the contents of r to c using the sendfile
@@ -34,7 +35,11 @@ func sendFile(c *netFD, r io.Reader) (written int64, err error, handled bool) {
 			return 0, nil, true
 		}
 	}
-	f, ok := r.(*os.File)
+	f, ok := r.(interface {
+		fs.File
+		io.Seeker
+		syscall.Conn
+	})
 	if !ok {
 		return 0, nil, false
 	}