dashboard: Ensure disk I/O happens in the executor (#5789)

* Ensure I/O executor

* safe file writer

* fixes

* more io

* more io

tests/dashboard/util/test_file.py

@@ -0,0 +1,53 @@
+import os
+from pathlib import Path
+from unittest.mock import patch
+
+import py
+import pytest
+
+from esphome.dashboard.util.file import write_file, write_utf8_file
+
+
+def test_write_utf8_file(tmp_path: Path) -> None:
+    write_utf8_file(tmp_path.joinpath("foo.txt"), "foo")
+    assert tmp_path.joinpath("foo.txt").read_text() == "foo"
+
+    with pytest.raises(OSError):
+        write_utf8_file(Path("/not-writable"), "bar")
+
+
+def test_write_file(tmp_path: Path) -> None:
+    write_file(tmp_path.joinpath("foo.txt"), b"foo")
+    assert tmp_path.joinpath("foo.txt").read_text() == "foo"
+
+
+def test_write_utf8_file_fails_at_rename(
+    tmpdir: py.path.local, caplog: pytest.LogCaptureFixture
+) -> None:
+    """Test that if rename fails not not remove, we do not log the failed cleanup."""
+    test_dir = tmpdir.mkdir("files")
+    test_file = Path(test_dir / "test.json")
+
+    with pytest.raises(OSError), patch(
+        "esphome.dashboard.util.file.os.replace", side_effect=OSError
+    ):
+        write_utf8_file(test_file, '{"some":"data"}', False)
+
+    assert not os.path.exists(test_file)
+
+    assert "File replacement cleanup failed" not in caplog.text
+
+
+def test_write_utf8_file_fails_at_rename_and_remove(
+    tmpdir: py.path.local, caplog: pytest.LogCaptureFixture
+) -> None:
+    """Test that if rename and remove both fail, we log the failed cleanup."""
+    test_dir = tmpdir.mkdir("files")
+    test_file = Path(test_dir / "test.json")
+
+    with pytest.raises(OSError), patch(
+        "esphome.dashboard.util.file.os.remove", side_effect=OSError
+    ), patch("esphome.dashboard.util.file.os.replace", side_effect=OSError):
+        write_utf8_file(test_file, '{"some":"data"}', False)
+
+    assert "File replacement cleanup failed" in caplog.text