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

* Ensure I/O executor

* safe file writer

* fixes

* more io

* more io

esphome/dashboard/util/file.py

@@ -0,0 +1,55 @@
+import logging
+import os
+import tempfile
+from pathlib import Path
+
+_LOGGER = logging.getLogger(__name__)
+
+
+def write_utf8_file(
+    filename: Path,
+    utf8_str: str,
+    private: bool = False,
+) -> None:
+    """Write a file and rename it into place.
+
+    Writes all or nothing.
+    """
+    write_file(filename, utf8_str.encode("utf-8"), private)
+
+
+# from https://github.com/home-assistant/core/blob/dev/homeassistant/util/file.py
+def write_file(
+    filename: Path,
+    utf8_data: bytes,
+    private: bool = False,
+) -> None:
+    """Write a file and rename it into place.
+
+    Writes all or nothing.
+    """
+
+    tmp_filename = ""
+    try:
+        # Modern versions of Python tempfile create this file with mode 0o600
+        with tempfile.NamedTemporaryFile(
+            mode="wb", dir=os.path.dirname(filename), delete=False
+        ) as fdesc:
+            fdesc.write(utf8_data)
+            tmp_filename = fdesc.name
+            if not private:
+                os.fchmod(fdesc.fileno(), 0o644)
+        os.replace(tmp_filename, filename)
+    finally:
+        if os.path.exists(tmp_filename):
+            try:
+                os.remove(tmp_filename)
+            except OSError as err:
+                # If we are cleaning up then something else went wrong, so
+                # we should suppress likely follow-on errors in the cleanup
+                _LOGGER.error(
+                    "File replacement cleanup failed for %s while saving %s: %s",
+                    tmp_filename,
+                    filename,
+                    err,
+                )