Use secure temporary config files

config/user_config.go

@@ -8,6 +8,7 @@ import (
 	"io/ioutil"
 	"net"
 	"os"
+	"path/filepath"
 	"strconv"
 	"strings"
 	"sync"
@@ -61,15 +62,24 @@ func (c *Config) saveConfigLocked() error {
 	if err != nil {
 		return err
 	}
-	tmp := c.fn + ".tmp"
-	if err := ioutil.WriteFile(tmp, data, 0600); err != nil {
+	file, err := os.CreateTemp(filepath.Dir(c.fn), filepath.Base(c.fn)+".tmp-")
+	if err != nil {
 		return err
 	}
-	if err := os.Rename(tmp, c.fn); err != nil {
-		_ = os.Remove(tmp)
+	tmp := file.Name()
+	defer os.Remove(tmp)
+	if err := file.Chmod(0600); err != nil {
+		file.Close()
 		return err
 	}
-	return nil
+	if _, err := file.Write(data); err != nil {
+		file.Close()
+		return err
+	}
+	if err := file.Close(); err != nil {
+		return err
+	}
+	return os.Rename(tmp, c.fn)
 }
 
 func key(k uiterm.Key) *uiterm.Key {

config/user_config_save_test.go

@@ -1,6 +1,7 @@
 package config
 
 import (
+	"os"
 	"path/filepath"
 	"sync"
 	"testing"
@@ -16,6 +17,25 @@ func TestSaveConfigReturnsWriteError(t *testing.T) {
 	}
 }
 
+func TestSaveConfigDoesNotUsePredictableTemporaryPath(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "barnard.toml")
+	legacyTemp := path + ".tmp"
+	if err := os.WriteFile(legacyTemp, []byte("sentinel"), 0600); err != nil {
+		t.Fatal(err)
+	}
+	cfg := NewConfig(&path)
+	if err := cfg.SaveConfig(); err != nil {
+		t.Fatal(err)
+	}
+	contents, err := os.ReadFile(legacyTemp)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(contents) != "sentinel" {
+		t.Fatalf("predictable temporary file was modified: %q", contents)
+	}
+}
+
 func TestConcurrentConfigurationUpdatesAndWrites(t *testing.T) {
 	path := filepath.Join(t.TempDir(), "barnard.toml")
 	cfg := NewConfig(&path)