accounts/keystore: tighten comment and add writeKeyFile test

accounts/keystore/key.go

@@ -205,9 +205,8 @@ func writeTemporaryKeyFile(file string, content []byte) (string, error) {
 		os.Remove(f.Name())
 		return "", err
 	}
-	// Persist the contents to disk before the caller renames it into place,
+	// Sync before rename so a crash between the write and the rename
-	// so that a crash or power loss between the write and the rename cannot
+	// cannot leave a zero-length keyfile at the final path.
-	// leave a zero-length keyfile at the final path.
 	if err := f.Sync(); err != nil {
 		f.Close()
 		os.Remove(f.Name())

accounts/keystore/key_test.go

@@ -0,0 +1,54 @@
+// Copyright 2026 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package keystore
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestWriteKeyFile(t *testing.T) {
+	t.Parallel()
+	dir := t.TempDir()
+	path := filepath.Join(dir, "keyfile")
+	content := []byte(`{"address":"deadbeef"}`)
+
+	if err := writeKeyFile(path, content); err != nil {
+		t.Fatalf("writeKeyFile: %v", err)
+	}
+	got, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("read back: %v", err)
+	}
+	if !bytes.Equal(got, content) {
+		t.Fatalf("content mismatch: got %q want %q", got, content)
+	}
+	// Verify no temp file leaked into the directory.
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		t.Fatalf("readdir: %v", err)
+	}
+	if len(entries) != 1 || entries[0].Name() != "keyfile" {
+		var names []string
+		for _, e := range entries {
+			names = append(names, e.Name())
+		}
+		t.Fatalf("unexpected directory contents: %v", names)
+	}
+}