5 files changed, 90 insertions, 0 deletions
diff --git a/src/os/exec/internal/fdtest/exists_js.go b/src/os/exec/internal/fdtest/exists_js.go
new file mode 100644
index 0000000..a7ce33c
--- /dev/null
+++ b/src/os/exec/internal/fdtest/exists_js.go
@@ -0,0 +1,18 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build js
+
+package fdtest
+
+import (
+	"syscall"
+)
+
+// Exists returns true if fd is a valid file descriptor.
+func Exists(fd uintptr) bool {
+	var s syscall.Stat_t
+	err := syscall.Fstat(int(fd), &s)
+	return err != syscall.EBADF
+}
diff --git a/src/os/exec/internal/fdtest/exists_plan9.go b/src/os/exec/internal/fdtest/exists_plan9.go
new file mode 100644
index 0000000..8886e06
--- /dev/null
+++ b/src/os/exec/internal/fdtest/exists_plan9.go
@@ -0,0 +1,20 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build plan9
+
+package fdtest
+
+import (
+	"syscall"
+)
+
+const errBadFd = syscall.ErrorString("fd out of range or not open")
+
+// Exists returns true if fd is a valid file descriptor.
+func Exists(fd uintptr) bool {
+	var buf [1]byte
+	_, err := syscall.Fstat(int(fd), buf[:])
+	return err != errBadFd
+}
diff --git a/src/os/exec/internal/fdtest/exists_test.go b/src/os/exec/internal/fdtest/exists_test.go
new file mode 100644
index 0000000..a02dddf
--- /dev/null
+++ b/src/os/exec/internal/fdtest/exists_test.go
@@ -0,0 +1,21 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fdtest
+
+import (
+	"os"
+	"runtime"
+	"testing"
+)
+
+func TestExists(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("Exists not implemented for windows")
+	}
+
+	if !Exists(os.Stdout.Fd()) {
+		t.Errorf("Exists(%d) got false want true", os.Stdout.Fd())
+	}
+}
diff --git a/src/os/exec/internal/fdtest/exists_unix.go b/src/os/exec/internal/fdtest/exists_unix.go
new file mode 100644
index 0000000..265cb69
--- /dev/null
+++ b/src/os/exec/internal/fdtest/exists_unix.go
@@ -0,0 +1,19 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix
+
+// Package fdtest provides test helpers for working with file descriptors across exec.
+package fdtest
+
+import (
+	"syscall"
+)
+
+// Exists returns true if fd is a valid file descriptor.
+func Exists(fd uintptr) bool {
+	var s syscall.Stat_t
+	err := syscall.Fstat(int(fd), &s)
+	return err != syscall.EBADF
+}
diff --git a/src/os/exec/internal/fdtest/exists_windows.go b/src/os/exec/internal/fdtest/exists_windows.go
new file mode 100644
index 0000000..72b8ccf
--- /dev/null
+++ b/src/os/exec/internal/fdtest/exists_windows.go
@@ -0,0 +1,12 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build windows
+
+package fdtest
+
+// Exists is not implemented on windows and panics.
+func Exists(fd uintptr) bool {
+	panic("unimplemented")
+}