age -o follows a symlink and writes a pipe or device directly (closes #59)

age encrypt -o and age decrypt -o always renamed a new file over the
named path, which replaced a symlink, a named pipe or a device such as
/dev/null with a regular file and made -o /dev/stdout fail. The path is
now looked at without following a final symlink: a missing path or a
regular file is replaced by rename as before, a symlink gets the same
treatment for what it points at, and anything else is written to
directly, without catching signals. A symlink that points at nothing is
refused. -o /dev/stdout and -o /dev/stderr write to the tool's own
streams, so a file they are redirected to is never replaced. The README
says so, and that a replaced file has mode 0600.

Model: opus-5-5

internal/cli/age/age.go

@@ -7,6 +7,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"os"
 	"os/signal"
 	"path/filepath"
@@ -140,7 +141,10 @@ func runDecrypt(cmd *cobra.Command, args []string) error {
 
 // through opens the input the arguments ask for and hands it to the
 // work, with the file --output names to write to, or the command's own
-// output when it names none.
+// output when it names none or names /dev/stdout, and the command's own
+// error output when it names /dev/stderr. Those two are the streams the
+// tool already has, so whatever they are redirected to is written as
+// the redirect says, never replaced.
 func through(
 	cmd *cobra.Command, args []string,
 	work func(io.Writer, io.Reader) error,
@@ -157,11 +161,14 @@ func through(
 		return fmt.Errorf("reading the output file: %w", err)
 	}
 
-	if name == "" {
+	switch name {
+	case "", "/dev/stdout":
 		return work(cmd.OutOrStdout(), src)
+	case "/dev/stderr":
+		return work(cmd.ErrOrStderr(), src)
+	default:
+		return output(name, src, work)
 	}
-
-	return output(name, src, work)
 }
 
 // input returns what to read from: the named file, or the command's
@@ -180,7 +187,78 @@ func input(cmd *cobra.Command, args []string) (io.Reader, func(), error) {
 	return file, func() { _ = file.Close() }, nil
 }
 
-// output has the work write a new file beside the named one, and puts
+// output has the work write to the named path, going by what is there
+// without following a final symlink:
+//
+//   - nothing, or a regular file: replace writes a new file beside it
+//     and renames that over it;
+//   - a symlink: the same for what it points at, so that the link keeps
+//     pointing where it did; one that points at nothing is refused;
+//   - anything else, such as a named pipe or a device like /dev/null:
+//     direct writes to it, since a rename would put a regular file in
+//     its place.
+func output(
+	name string, src io.Reader, work func(io.Writer, io.Reader) error,
+) error {
+	info, err := os.Lstat(name)
+
+	switch {
+	case errors.Is(err, fs.ErrNotExist):
+		return replace(name, src, work)
+	case err != nil:
+		return fmt.Errorf("looking at %s: %w", name, err)
+	case info.Mode().IsRegular():
+		return replace(name, src, work)
+	case info.Mode().Type() == fs.ModeSymlink:
+		// os.Stat follows the link as opening it would. /dev/fd/1
+		// needs that: it reaches a pipe or a terminal through a link
+		// that names no path.
+		info, err = os.Stat(name)
+		if err != nil {
+			return fmt.Errorf("following %s: %w", name, err)
+		}
+
+		if !info.Mode().IsRegular() {
+			return direct(name, src, work)
+		}
+
+		target, err := filepath.EvalSymlinks(name)
+		if err != nil {
+			return fmt.Errorf("following %s: %w", name, err)
+		}
+
+		return replace(target, src, work)
+	default:
+		return direct(name, src, work)
+	}
+}
+
+// direct has the work write straight to the named path, which is there
+// and is not a regular file. No signal is caught, so one ends the tool
+// as it ends any other command.
+func direct(
+	name string, src io.Reader, work func(io.Writer, io.Reader) error,
+) error {
+	file, err := os.OpenFile(name, os.O_WRONLY, 0) //nolint:gosec // the -o path
+	if err != nil {
+		return fmt.Errorf("opening %s: %w", name, err)
+	}
+
+	failed := work(file, src)
+	closeErr := file.Close()
+
+	if failed != nil {
+		return failed
+	}
+
+	if closeErr != nil {
+		return fmt.Errorf("finishing %s: %w", name, closeErr)
+	}
+
+	return nil
+}
+
+// replace has the work write a new file beside the named one, and puts
 // the new file in the named file's place only when the work succeeded,
 // so a file that is already there survives a run that failed.
 //
@@ -189,7 +267,7 @@ func input(cmd *cobra.Command, args []string) (io.Reader, func(), error) {
 // new file is removed and ErrInterrupted returned, at once if the work
 // is still running, without waiting for it, since it may be blocked
 // reading its input.
-func output(
+func replace(
 	name string, src io.Reader, work func(io.Writer, io.Reader) error,
 ) error {
 	// received is registered before the context, so it gets every