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

age encrypt -o and age decrypt -o always wrote a new file beside the
named path and renamed it over that 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. The README says so, and
that a replaced file has mode 0600.

Model: opus-5-5

README.md

@@ -253,11 +253,8 @@ always decrypt what it encrypted. Output goes to `-o` or standard output;
 A regular file already at the `-o` path is replaced, and the new file has mode
 `0600`. A symlink there is followed, and what it points at is treated the same
 way, so the link keeps pointing where it did; a symlink that points at nothing
-is refused. A named pipe or a device, such as `/dev/null`, is written to
-directly. `-o /dev/stdout` writes to the tool's own standard output, as leaving
-out `-o` does, and `-o /dev/stderr` to its standard error; a file either stream
-is redirected to is written as the redirect says and never replaced, so with
-`>>` the output follows what the file already held.
+is refused. A named pipe or a device, such as `/dev/null` or `/dev/stdout`, is
+written to directly.
 
 ### `keyfunc age decrypt [-n N] [-o <file>] [<file>]`
 
@@ -306,12 +303,11 @@ already at the named path as it was, and exit with status 1. That holds for a
 signal that has reached `keyfunc` when its input ends; a later one leaves the
 whole file in place. Ctrl-C on a pipeline ends the input at the same moment, and
 on Linux `keyfunc` sees the signal first, though no system promises that. A
-named pipe or a device at the `-o` path, `/dev/stdout` or `/dev/stderr`, is
-written to directly, and the signal ends the tool there as it ends any other
-command. While `ssh to` or `ssh install` has `ssh` or `sftp` running, the signal
-ends that program instead, the tool removes its agent socket or working files,
-and it exits with status 1, or for `ssh to` with `ssh`'s own status if `ssh`
-reported one.
+named pipe or a device at the `-o` path is written to directly, and the signal
+ends the tool there as it ends any other command. While `ssh to` or
+`ssh install` has `ssh` or `sftp` running, the signal ends that program instead,
+the tool removes its agent socket or working files, and it exits with status 1,
+or for `ssh to` with `ssh`'s own status if `ssh` reported one.
 
 ## Entrypoints
 

internal/cli/age/age.go

@@ -141,10 +141,7 @@ 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 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.
+// output when it names none.
 func through(
 	cmd *cobra.Command, args []string,
 	work func(io.Writer, io.Reader) error,
@@ -161,14 +158,11 @@ func through(
 		return fmt.Errorf("reading the output file: %w", err)
 	}
 
-	switch name {
-	case "", "/dev/stdout":
+	if name == "" {
 		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
@@ -210,7 +204,7 @@ func output(
 	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
+		// os.Stat follows the link as opening it would. /dev/stdout
 		// needs that: it reaches a pipe or a terminal through a link
 		// that names no path.
 		info, err = os.Stat(name)

internal/cli/age_test.go

@@ -151,34 +151,6 @@ func TestANamedPipeAtTheOutputPathIsWrittenToAndStaysAPipe(t *testing.T) {
 	require.Equal(t, "the secret\n", run(t, "age", "decrypt", sealedFile))
 }
 
-func TestDevStdoutAddsToTheFileStandardOutputIsAppendedTo(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"))
-
-	// What the shell's ">> notes.out" gives the tool as standard output.
-	existing := written(t, "notes.out", "what was already there\n")
-
-	//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", "/dev/stdout", sealed,
-	)
-	command.Env = append(os.Environ(), runAsTool+"=1")
-	command.Stdout = appended
-
-	require.NoError(t, command.Run())
-	require.Equal(t,
-		"what was already there\nthe secret\n", read(t, existing),
-	)
-}
-
 func TestASignalStopsAnEncryptionAndLeavesNoFile(t *testing.T) {
 	t.Setenv(mnemonic.Variable, example())