dashboard: Fix file writes on Windows (#6013)

esphome/dashboard/util/file.py

@@ -30,6 +30,7 @@ def write_file(
     """
 
     tmp_filename = ""
+    missing_fchmod = False
     try:
         # Modern versions of Python tempfile create this file with mode 0o600
         with tempfile.NamedTemporaryFile(
@@ -38,8 +39,15 @@ def write_file(
             fdesc.write(utf8_data)
             tmp_filename = fdesc.name
             if not private:
-                os.fchmod(fdesc.fileno(), 0o644)
+                try:
+                    os.fchmod(fdesc.fileno(), 0o644)
+                except AttributeError:
+                    # os.fchmod is not available on Windows
+                    missing_fchmod = True
+
         os.replace(tmp_filename, filename)
+        if missing_fchmod:
+            os.chmod(filename, 0o644)
     finally:
         if os.path.exists(tmp_filename):
             try:

tests/dashboard/util/test_file.py

@@ -13,7 +13,7 @@ def test_write_utf8_file(tmp_path: Path) -> None:
     assert tmp_path.joinpath("foo.txt").read_text() == "foo"
 
     with pytest.raises(OSError):
-        write_utf8_file(Path("/not-writable"), "bar")
+        write_utf8_file(Path("/dev/not-writable"), "bar")
 
 
 def test_write_file(tmp_path: Path) -> None: