os: don't refer directly to Sysfd in epipecheck

Instead record in the File whether it is stdout/stderr. This avoids a
race between a call to epipecheck and closing the file.

Fixes #21994

Change-Id: Ic3d552ffa83402136276bcb5029ec3e6691042c2
Reviewed-on: https://go-review.googlesource.com/65750
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/os/file_unix.go

@@ -49,6 +49,7 @@ type file struct {
 	name        string
 	dirinfo     *dirInfo // nil unless directory being read
 	nonblock    bool     // whether we set nonblocking mode
+	stdoutOrErr bool     // whether this is stdout or stderr
 }
 
 // Fd returns the integer Unix file descriptor referencing the open file.
@@ -91,6 +92,7 @@ func newFile(fd uintptr, name string, pollable bool) *File {
 			ZeroReadIsEOF: true,
 		},
 		name:        name,
+		stdoutOrErr: fdi == 1 || fdi == 2,
 	}}
 
 	// Don't try to use kqueue with regular files on FreeBSD.
@@ -130,7 +132,7 @@ type dirInfo struct {
 // output or standard error. See the SIGPIPE docs in os/signal, and
 // issue 11845.
 func epipecheck(file *File, e error) {
-	if e == syscall.EPIPE && (file.pfd.Sysfd == 1 || file.pfd.Sysfd == 2) {
+	if e == syscall.EPIPE && file.stdoutOrErr {
 		sigpipe()
 	}
 }