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

README.md

@@ -253,8 +253,11 @@ 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` or `/dev/stdout`, is
+is refused. A named pipe or a device, such as `/dev/null`, is written to
-written to directly.
+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.
 
 ### `keyfunc age decrypt [-n N] [-o <file>] [<file>]`
 
@@ -303,11 +306,12 @@ 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 is written to directly, and the signal
+named pipe or a device at the `-o` path, `/dev/stdout` or `/dev/stderr`, is
-ends the tool there as it ends any other command. While `ssh to` or
+written to directly, and the signal ends the tool there as it ends any other
-`ssh install` has `ssh` or `sftp` running, the signal ends that program instead,
+command. While `ssh to` or `ssh install` has `ssh` or `sftp` running, the signal
-the tool removes its agent socket or working files, and it exits with status 1,
+ends that program instead, the tool removes its agent socket or working files,
-or for `ssh to` with `ssh`'s own status if `ssh` reported one.
+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

@@ -141,7 +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,
@@ -158,11 +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)
+	}
 }
 
 // input returns what to read from: the named file, or the command's
@@ -204,7 +210,7 @@ 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/stdout
+		// 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)

internal/cli/age_test.go

@@ -151,6 +151,34 @@ 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())