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, replacing a symlink, a named pipe or a device such as
/dev/null with a regular file. A path that is the same file as the
tool's standard output or standard error, under any name, is now
written to that stream, so the file it is redirected to keeps its
contents. Any other path is looked at without following a final
symlink: nothing there or a regular file is replaced by rename as
before, a symlink gets the same treatment for what it points at and is
refused if it points at nothing, and anything else is written to
directly, without catching signals. The README says so, and that a
replaced file has mode 0600.

Model: opus-5-5

README.md

@@ -250,14 +250,15 @@ identity's own recipient, plus any given with `--to`, so the same mnemonic can
 always decrypt what it encrypted. Output goes to `-o` or standard output;
 `--armor` writes the text form. Nothing is written except the 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.
+A `-o` path that is the same file as the tool's own standard output or standard
+error, under any name such as `/dev/stdout` or `/dev/fd/2`, is written to that
+stream, as leaving out `-o` writes to standard output; the file the stream is
+redirected to is written as the redirect says and never replaced, so with `>>`
+the output follows what the file already held. Otherwise, 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.
 
 ### `keyfunc age decrypt [-n N] [-o <file>] [<file>]`
 
@@ -306,12 +307,12 @@ 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, `/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, or a path that is the same file as
+standard output or standard error, 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 +141,10 @@ 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 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 or names the same file as that output, and
+// the command's own error output when it names the same file as that.
+// 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,
@@ -161,16 +161,38 @@ func through(
 		return fmt.Errorf("reading the output file: %w", err)
 	}
 
-	switch name {
-	case "", "/dev/stdout":
+	switch {
+	case name == "", same(name, cmd.OutOrStdout()):
 		return work(cmd.OutOrStdout(), src)
-	case "/dev/stderr":
+	case same(name, cmd.ErrOrStderr()):
 		return work(cmd.ErrOrStderr(), src)
 	default:
 		return output(name, src, work)
 	}
 }
 
+// same reports whether the named path, followed to the end, is the
+// file the stream writes to, whatever name it is reached by, such as
+// /dev/stdout or /dev/fd/1 for standard output.
+func same(name string, stream io.Writer) bool {
+	file, ok := stream.(*os.File)
+	if !ok {
+		return false
+	}
+
+	streamInfo, err := file.Stat()
+	if err != nil {
+		return false
+	}
+
+	info, err := os.Stat(name)
+	if err != nil {
+		return false
+	}
+
+	return os.SameFile(info, streamInfo)
+}
+
 // input returns what to read from: the named file, or the command's
 // own input when no file is named. The second result closes a file
 // that was opened and does nothing otherwise.
@@ -210,7 +232,7 @@ 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/fd/3
 		// 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

@@ -151,14 +151,32 @@ func TestANamedPipeAtTheOutputPathIsWrittenToAndStaysAPipe(t *testing.T) {
 	require.Equal(t, "the secret\n", run(t, "age", "decrypt", sealedFile))
 }
 
-func TestDevStdoutAddsToTheFileStandardOutputIsAppendedTo(t *testing.T) {
+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"))
 
-	// What the shell's ">> notes.out" gives the tool as standard output.
+	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)
@@ -168,16 +186,21 @@ func TestDevStdoutAddsToTheFileStandardOutputIsAppendedTo(t *testing.T) {
 
 	//nolint:gosec // this test's own binary as the tool
 	command := exec.CommandContext(
-		t.Context(), os.Args[0], "age", "decrypt", "-o", "/dev/stdout", sealed,
+		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())
+	require.NoError(t, command.Run(), name)
 	require.Equal(t,
-		"what was already there\nthe secret\n", read(t, existing),
+		"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) {