5 files changed, 133 insertions, 0 deletions
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..472a802
--- /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 || wasm
+
+// 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")
+}
diff --git a/src/os/exec/internal_test.go b/src/os/exec/internal_test.go
new file mode 100644
index 0000000..68d517f
--- /dev/null
+++ b/src/os/exec/internal_test.go
@@ -0,0 +1,61 @@
+// Copyright 2015 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 exec
+
+import (
+	"io"
+	"testing"
+)
+
+func TestPrefixSuffixSaver(t *testing.T) {
+	tests := []struct {
+		N      int
+		writes []string
+		want   string
+	}{
+		{
+			N:      2,
+			writes: nil,
+			want:   "",
+		},
+		{
+			N:      2,
+			writes: []string{"a"},
+			want:   "a",
+		},
+		{
+			N:      2,
+			writes: []string{"abc", "d"},
+			want:   "abcd",
+		},
+		{
+			N:      2,
+			writes: []string{"abc", "d", "e"},
+			want:   "ab\n... omitting 1 bytes ...\nde",
+		},
+		{
+			N:      2,
+			writes: []string{"ab______________________yz"},
+			want:   "ab\n... omitting 22 bytes ...\nyz",
+		},
+		{
+			N:      2,
+			writes: []string{"ab_______________________y", "z"},
+			want:   "ab\n... omitting 23 bytes ...\nyz",
+		},
+	}
+	for i, tt := range tests {
+		w := &prefixSuffixSaver{N: tt.N}
+		for _, s := range tt.writes {
+			n, err := io.WriteString(w, s)
+			if err != nil || n != len(s) {
+				t.Errorf("%d. WriteString(%q) = %v, %v; want %v, %v", i, s, n, err, len(s), nil)
+			}
+		}
+		if got := string(w.Bytes()); got != tt.want {
+			t.Errorf("%d. Bytes = %q; want %q", i, got, tt.want)
+		}
+	}
+}