age -o keeps a symlink, pipe, device or redirected stream at the path (closes #59)

age encrypt -o and age decrypt -o now look at the -o path before writing. A path that is the same file as the tool's standard output or standard error, under any name, is written to that stream, so a redirected file keeps its contents, inode and mode. A new path or a regular file is written beside it and renamed over it, as before, with the signal handling of #48. A symlink gets the same rule for what it points at, so the link survives; a dangling one is refused. A named pipe or a device is written directly. The README says a replaced file gets mode 0600.

Rule suppressed: gosec G304 on the direct open of the -o path.

Model: opus-5-5

internal/cli/age_test.go

@@ -3,6 +3,7 @@ package cli_test
 import (
 	"errors"
 	"io"
+	"io/fs"
 	"os"
 	"os/exec"
 	"os/signal"
@@ -98,6 +99,110 @@ func TestARefusedDecryptionLeavesTheOutputFileAlone(t *testing.T) {
 	require.Equal(t, "what was already there\n", string(kept))
 }
 
+func TestASymlinkAtTheOutputPathStaysAndItsTargetGetsTheOutput(t *testing.T) {
+	t.Setenv(mnemonic.Variable, example())
+
+	plain := written(t, "notes.txt", "the secret\n")
+	target := written(t, "notes.age", "what was already there\n")
+	link := filepath.Join(t.TempDir(), "notes.age")
+	require.NoError(t, os.Symlink(target, link))
+
+	run(t, "age", "encrypt", "-o", link, plain)
+
+	pointsAt, err := os.Readlink(link)
+	require.NoError(t, err)
+	require.Equal(t, target, pointsAt)
+	require.Equal(t, "the secret\n", run(t, "age", "decrypt", target))
+}
+
+func TestANamedPipeAtTheOutputPathIsWrittenToAndStaysAPipe(t *testing.T) {
+	t.Setenv(mnemonic.Variable, example())
+
+	plain := written(t, "notes.txt", "the secret\n")
+	pipe := filepath.Join(t.TempDir(), "notes.age")
+	require.NoError(t, syscall.Mkfifo(pipe, fileMode))
+
+	// Opening the pipe to read waits until the tool opens it to write.
+	var sealed []byte
+
+	finished := make(chan error, 1)
+
+	go func() {
+		var err error
+
+		sealed, err = os.ReadFile(pipe) //nolint:gosec // the test's own path
+		finished <- err
+	}()
+
+	run(t, "age", "encrypt", "-o", pipe, plain)
+
+	select {
+	case err := <-finished:
+		require.NoError(t, err)
+	case <-time.After(5 * time.Second):
+		t.Fatal("nothing was written to the pipe")
+	}
+
+	info, err := os.Lstat(pipe)
+	require.NoError(t, err)
+	require.Equal(t, fs.ModeNamedPipe, info.Mode().Type())
+
+	sealedFile := written(t, "notes.age", string(sealed))
+	require.Equal(t, "the secret\n", run(t, "age", "decrypt", sealedFile))
+}
+
+func TestANameForStandardOutputAddsToTheFileItIsAppendedTo(t *testing.T) {
+	t.Setenv(mnemonic.Variable, example())
+
+	sealed := filepath.Join(t.TempDir(), "notes.age")
+	run(t, "age", "encrypt", "-o", sealed, written(t, "notes.txt", "the secret\n"))
+
+	for _, name := range []string{"/dev/stdout", "/dev/fd/1"} {
+		appendedThrough(t, name, sealed)
+	}
+}
+
+// appendedThrough decrypts sealed with -o name while the tool's standard
+// output is appended to a file that already has contents, as the shell's
+// ">> notes.out" does, and checks that the file is the same one, with
+// the same mode, and holds its earlier contents and then the output.
+func appendedThrough(t *testing.T, name, sealed string) {
+	t.Helper()
+
+	// A mode of its own, so that a replaced file would show.
+	const ownMode = 0o644
+
+	existing := written(t, "notes.out", "what was already there\n")
+	require.NoError(t, os.Chmod(existing, ownMode))
+
+	before, err := os.Stat(existing)
+	require.NoError(t, err)
+
+	//nolint:gosec // the test made this path itself
+	appended, err := os.OpenFile(existing, os.O_WRONLY|os.O_APPEND, 0)
+	require.NoError(t, err)
+
+	defer func() { _ = appended.Close() }()
+
+	//nolint:gosec // this test's own binary as the tool
+	command := exec.CommandContext(
+		t.Context(), os.Args[0], "age", "decrypt", "-o", name, sealed,
+	)
+
+	command.Env = append(os.Environ(), runAsTool+"=1")
+	command.Stdout = appended
+
+	require.NoError(t, command.Run(), name)
+	require.Equal(t,
+		"what was already there\nthe secret\n", read(t, existing), name,
+	)
+
+	after, err := os.Stat(existing)
+	require.NoError(t, err)
+	require.True(t, os.SameFile(before, after), name)
+	require.Equal(t, os.FileMode(ownMode), after.Mode().Perm(), name)
+}
+
 func TestASignalStopsAnEncryptionAndLeavesNoFile(t *testing.T) {
 	t.Setenv(mnemonic.Variable, example())