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 path 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.

Model: opus-5-5

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)
+	}
+}