Write encrypt and decrypt output only once they succeed (closes #143)

`secret encrypt` and `secret decrypt` emptied the file named by
`--output` before reading any input, so a failure left it empty, and a
file given as both `--input` and `--output` was emptied before it was
read. They now write into a temporary file of mode 0600 next to it, or
next to the file it is a symbolic link to, and rename that over it only
on success, using the new `secret.WriteFileAtomicFunc`, which
`secret.WriteFileAtomic` now calls. Ctrl-C or SIGTERM removes the
temporary file. A device or pipe given as `--output`, such as
`/dev/null`, is written to directly.

Judgement call: the device and pipe case goes beyond the plan.
Judgement call: a link to a file that does not exist yet is replaced.

Model: opus-5-5

README.md

@@ -275,11 +275,12 @@ generates a new Age key.
 Decrypts data using an Age key stored as a secret.
 
 Both write the result to a new file next to `--output` and rename it over
-`--output` only once they have succeeded, so a failed command leaves an existing
-`--output` as it was and creates none, and `--input` and `--output` can be the
-same file. The file has mode 0600, whatever mode the file it replaced had, and
-when `--output` is a symbolic link to a file, the link itself is replaced. A
-device or a pipe, such as `/dev/null`, is written to directly.
+`--output` only once they have succeeded, so a failed or interrupted command
+leaves an existing `--output` as it was and creates none, and `--input` and
+`--output` can be the same file. The file has mode 0600, whatever mode the file
+it replaced had. When `--output` is a symbolic link, the file it points to is
+replaced and the link kept. A device or a pipe, such as `/dev/null`, is written
+to directly.
 
 ## Storage Architecture
 

TODO.md

@@ -23,10 +23,12 @@ https://git.eeqj.de/sneak/secret/milestone/12
   `--output` (https://git.eeqj.de/sneak/secret/issues/143). Both emptied the
   output file before reading any input, so a wrong key left it empty, and a file
   given as both was emptied before it was read. They now write the result into a
-  temporary file next to it and rename that over it once they have succeeded,
-  with the new `secret.WriteFileAtomicFunc`, which `secret.WriteFileAtomic` now
-  calls; the output file has mode 0600. A device or a pipe given as `--output`,
-  such as `/dev/null`, is still written to directly.
+  temporary file next to it, or next to the file it is a symbolic link to, and
+  rename that over it once they have succeeded, with the new
+  `secret.WriteFileAtomicFunc`, which `secret.WriteFileAtomic` now calls; the
+  output file has mode 0600. Ctrl-C or SIGTERM removes the temporary file. A
+  device or a pipe given as `--output`, such as `/dev/null`, is still written to
+  directly.
 - 2026-10-12: A mnemonic typed with extra spaces, tabs or line breaks gives the
   same long-term key as its words separated by single spaces
   (https://git.eeqj.de/sneak/secret/issues/135). The key was derived from the

internal/cli/crypto.go

@@ -5,9 +5,12 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"path/filepath"
+	"sync/atomic"
 
 	"filippo.io/age"
 	"github.com/awnumar/memguard"
+	"github.com/spf13/afero"
 	"github.com/spf13/cobra"
 	"sneak.berlin/go/secret/internal/secret"
 	"sneak.berlin/go/secret/internal/vault"
@@ -281,11 +284,18 @@ func (cli *Instance) Decrypt(secretName, inputFile, outputFile string) error {
 	})
 }
 
+// temporaryOutput is the name of the temporary file writeOutput is filling,
+// until it is renamed into place. The signal handler in Entry removes it,
+// since for decrypt it holds part of the secret.
+//
+//nolint:gochecknoglobals // read by the signal handler, which has no Instance
+var temporaryOutput atomic.Pointer[string]
+
 // writeOutput calls write with stdout when outputFile is empty, and with
 // outputFile itself when it is a device or a pipe, such as /dev/null.
-// Otherwise write fills a new file of mode 0600 that replaces outputFile
-// only once write has succeeded, so a failure leaves outputFile as it was,
-// and outputFile may be the input file too.
+// Otherwise write fills a new file of mode 0600 that replaces outputFile,
+// or the file it is a symbolic link to, only once write has succeeded, so a
+// failure leaves that file as it was, and it may be the input file too.
 func (cli *Instance) writeOutput(
 	outputFile string, write func(output io.Writer) error,
 ) error {
@@ -304,7 +314,24 @@ func (cli *Instance) writeOutput(
 		return write(file)
 	}
 
-	return secret.WriteFileAtomicFunc(cli.fs, outputFile, write)
+	// Replace the file a symbolic link points to and keep the link, as
+	// writing through the link would; /dev/stdout with stdout going to a
+	// file is such a link. A path that does not exist yet is used as given.
+	path, err := filepath.EvalSymlinks(outputFile)
+	if err != nil {
+		path = outputFile
+	}
+
+	err = secret.WriteFileAtomicFunc(cli.fs, path, func(file afero.File) error {
+		name := file.Name()
+		temporaryOutput.Store(&name)
+
+		return write(file)
+	})
+
+	temporaryOutput.Store(nil)
+
+	return err
 }
 
 // isValidAgeSecretKey checks if a string is a valid age secret key by

internal/cli/crypto_test.go

@@ -2,8 +2,10 @@
 package cli
 
 import (
+	"io"
 	"os"
 	"path/filepath"
+	"strings"
 	"syscall"
 	"testing"
 
@@ -44,7 +46,8 @@ func newCryptoTest(t *testing.T) (*Instance, string) {
 // TestFailedCryptoLeavesOutputUnchanged is a regression test for
 // https://git.eeqj.de/sneak/secret/issues/143, where secret encrypt and
 // secret decrypt emptied the file named by --output before they failed.
-// A failed command must leave an existing file as it was and create none.
+// A failed command must leave an existing file as it was and create none,
+// and give the same error as without --output.
 func TestFailedCryptoLeavesOutputUnchanged(t *testing.T) {
 	t.Parallel()
 
@@ -70,8 +73,13 @@ func TestFailedCryptoLeavesOutputUnchanged(t *testing.T) {
 			existing := filepath.Join(dir, "existing.txt")
 			require.NoError(t, afero.WriteFile(cli.fs, existing, []byte("precious"), 0o600))
 
-			require.Error(t, tt.run(cli, dir, existing))
-			require.Error(t, tt.run(cli, dir, filepath.Join(dir, "new.txt")))
+			cli.cmd.SetOut(io.Discard)
+			toStdout := tt.run(cli, dir, "")
+			require.Error(t, toStdout)
+
+			require.EqualError(t, tt.run(cli, dir, existing), toStdout.Error())
+			require.EqualError(t, tt.run(cli, dir, filepath.Join(dir, "new.txt")),
+				toStdout.Error())
 
 			content, err := afero.ReadFile(cli.fs, existing)
 			require.NoError(t, err)
@@ -133,3 +141,51 @@ func TestDecryptToPipe(t *testing.T) {
 	require.Equal(t, os.ModeNamedPipe, info.Mode().Type())
 	require.Equal(t, plainText, string(<-received))
 }
+
+// TestDecryptThroughLink checks that secret decrypt given a symbolic link
+// as --output replaces the file the link points to, in another directory,
+// and keeps the link, as writing through the link would.
+func TestDecryptThroughLink(t *testing.T) {
+	t.Parallel()
+
+	cli, dir := newCryptoTest(t)
+	targetDir := t.TempDir()
+	target := filepath.Join(targetDir, "target.txt")
+	require.NoError(t, afero.WriteFile(cli.fs, target, []byte("old"), 0o600))
+
+	link := filepath.Join(dir, "link")
+	require.NoError(t, os.Symlink(target, link))
+
+	require.NoError(t, cli.Decrypt("key", filepath.Join(dir, "plain.txt.age"), link))
+
+	info, err := os.Lstat(link)
+	require.NoError(t, err)
+	require.Equal(t, os.ModeSymlink, info.Mode().Type())
+
+	content, err := afero.ReadFile(cli.fs, target)
+	require.NoError(t, err)
+	require.Equal(t, plainText, string(content))
+
+	// No temporary file left beside the target
+	entries, err := afero.ReadDir(cli.fs, targetDir)
+	require.NoError(t, err)
+	require.Len(t, entries, 1)
+}
+
+// TestDecryptToLongestName checks that secret decrypt writes an --output
+// whose name is as long as a file name can be, 255 bytes on Linux and
+// macOS: the temporary file's name must not be longer.
+func TestDecryptToLongestName(t *testing.T) {
+	t.Parallel()
+
+	const longestName = 255
+
+	cli, dir := newCryptoTest(t)
+	output := filepath.Join(dir, strings.Repeat("a", longestName))
+
+	require.NoError(t, cli.Decrypt("key", filepath.Join(dir, "plain.txt.age"), output))
+
+	content, err := afero.ReadFile(cli.fs, output)
+	require.NoError(t, err)
+	require.Equal(t, plainText, string(content))
+}

internal/cli/entry_test.go

@@ -8,6 +8,7 @@ import (
 	"path/filepath"
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/awnumar/memguard"
 	"github.com/stretchr/testify/assert"
@@ -105,3 +106,59 @@ func TestInterruptExitsThroughMemguard(t *testing.T) {
 	require.ErrorAs(t, err, &exitErr)
 	assert.Equal(t, 1, exitErr.ExitCode(), "add ended with %v", err)
 }
+
+// Ctrl-C while encrypt or decrypt waits for input must remove the temporary
+// file it is writing next to --output, which for decrypt holds part of the
+// secret.
+func TestInterruptRemovesTemporaryOutput(t *testing.T) {
+	t.Parallel()
+
+	ctx, cancel := context.WithTimeout(t.Context(), commandWait)
+	defer cancel()
+
+	secretPath := secretBinaryPath(t)
+	env := []string{
+		secret.EnvStateDir + "=" + t.TempDir(),
+		secret.EnvMnemonic + "=" + testMnemonic,
+		secret.EnvUnlockPassphrase + "=test-passphrase",
+		"PATH=/usr/bin:/bin",
+	}
+
+	//nolint:gosec // G204: test executes the freshly built secret binary
+	initCmd := exec.CommandContext(ctx, secretPath, "init")
+	initCmd.Env = env
+
+	output, err := initCmd.CombinedOutput()
+	require.NoError(t, err, "init should succeed: %s", output)
+
+	// encrypt stores the new key before it starts writing, so decrypt has it
+	for _, command := range []string{"encrypt", "decrypt"} {
+		outputDir := t.TempDir()
+
+		//nolint:gosec // G204: test executes the freshly built secret binary
+		cmd := exec.CommandContext(ctx, secretPath, command, "key",
+			"--output", filepath.Join(outputDir, "out"))
+		cmd.Env = env
+
+		// Held open and never written, so the command keeps waiting for
+		// input. Wait closes it.
+		_, err := cmd.StdinPipe()
+		require.NoError(t, err)
+		require.NoError(t, cmd.Start())
+
+		require.Eventually(t, func() bool {
+			entries, err := os.ReadDir(outputDir)
+
+			return err == nil && len(entries) > 0
+		}, commandWait, 10*time.Millisecond, "%s made no temporary file", command)
+		require.NoError(t, cmd.Process.Signal(os.Interrupt))
+
+		var exitErr *exec.ExitError
+
+		require.ErrorAs(t, cmd.Wait(), &exitErr)
+
+		entries, err := os.ReadDir(outputDir)
+		require.NoError(t, err)
+		assert.Empty(t, entries, "%s left its temporary file", command)
+	}
+}

internal/cli/root.go

@@ -19,6 +19,8 @@ func Entry() int {
 	// off until the read finishes, so a signal there would leave echo off.
 	// Only a process in the terminal's foreground process group may reset
 	// it: one in the background that tries is stopped instead of exiting.
+	// The function also removes the temporary file that encrypt or decrypt
+	// is writing.
 	terminalState, terminalErr := term.GetState(unix.Stdin)
 
 	memguard.CatchSignal(func(os.Signal) {
@@ -26,6 +28,11 @@ func Entry() int {
 		if terminalErr == nil && err == nil && foreground == unix.Getpgrp() {
 			_ = term.Restore(unix.Stdin, terminalState)
 		}
+
+		name := temporaryOutput.Load()
+		if name != nil {
+			_ = os.Remove(*name)
+		}
 	}, os.Interrupt, unix.SIGTERM)
 
 	defer memguard.Purge()

internal/secret/atomic.go

@@ -3,7 +3,6 @@ package secret
 import (
 	"errors"
 	"fmt"
-	"io"
 	"os"
 	"path/filepath"
 	"strings"
@@ -11,9 +10,8 @@ import (
 	"github.com/spf13/afero"
 )
 
-// tempNamePart is in the name of every temporary file WriteFileAtomicFunc
-// makes, ".NAME.tmp-123", and every temporary directory TempDirFor makes,
-// ".tmp-123".
+// tempNamePart starts the name of every temporary file WriteFileAtomicFunc
+// makes and every temporary directory TempDirFor makes, ".tmp-123".
 const tempNamePart = ".tmp-"
 
 // WriteFileAtomic replaces the file at path with data so that a reader, or
@@ -23,30 +21,38 @@ const tempNamePart = ".tmp-"
 // within one filesystem), is synced to disk, and is renamed over path. The
 // temporary file is removed if any step fails.
 func WriteFileAtomic(fs afero.Fs, path string, data []byte) error {
-	return WriteFileAtomicFunc(fs, path, func(w io.Writer) error {
-		_, err := w.Write(data)
+	return WriteFileAtomicFunc(fs, path, func(file afero.File) error {
+		_, err := file.Write(data)
+		if err != nil {
+			return fmt.Errorf("failed to write %s: %w", path, err)
+		}
 
-		return err
+		return nil
 	})
 }
 
 // WriteFileAtomicFunc is WriteFileAtomic for content that write streams
-// into the temporary file instead of holding it in memory. An error from
-// write fails it as any other step does.
+// into the temporary file, which it is given, instead of holding it in
+// memory. An error from write is returned as it is. The temporary file's
+// name leaves out path's, which may already be as long as a name can be.
 func WriteFileAtomicFunc(
-	fs afero.Fs, path string, write func(w io.Writer) error,
+	fs afero.Fs, path string, write func(file afero.File) error,
 ) error {
-	tmp, err := afero.TempFile(fs, filepath.Dir(path),
-		"."+filepath.Base(path)+tempNamePart+"*")
+	tmp, err := afero.TempFile(fs, filepath.Dir(path), tempNamePart)
 	if err != nil {
 		return fmt.Errorf("failed to create temporary file for %s: %w", path, err)
 	}
 
 	err = write(tmp)
-	if err == nil {
-		err = tmp.Sync()
+	if err != nil {
+		_ = tmp.Close()
+		_ = fs.Remove(tmp.Name())
+
+		return err
 	}
 
+	err = tmp.Sync()
+
 	closeErr := tmp.Close()
 	if err == nil {
 		err = closeErr