Use cached files on network errors in external_files (#14055)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jesserockz <3060199+jesserockz@users.noreply.github.com>

esphome/external_files.py

@@ -55,10 +55,12 @@ def has_remote_file_changed(url: str, local_file_path: Path) -> bool:
             _LOGGER.debug("has_remote_file_changed: File modified")
             return True
         except requests.exceptions.RequestException as e:
-            raise cv.Invalid(
-                f"Could not check if {url} has changed, please check if file exists "
-                f"({e})"
+            _LOGGER.warning(
+                "Could not check if %s has changed due to network error (%s), using cached file",
+                url,
+                e,
             )
+            return False
 
     _LOGGER.debug("has_remote_file_changed: File doesn't exists at %s", local_file_path)
     return True
@@ -98,6 +100,13 @@ def download_content(url: str, path: Path, timeout=NETWORK_TIMEOUT) -> bytes:
         )
         req.raise_for_status()
     except requests.exceptions.RequestException as e:
+        if path.exists():
+            _LOGGER.warning(
+                "Could not download from %s due to network error (%s), using cached file",
+                url,
+                e,
+            )
+            return path.read_bytes()
         raise cv.Invalid(f"Could not download from {url}: {e}")
 
     path.parent.mkdir(parents=True, exist_ok=True)

tests/unit_tests/test_external_files.py

@@ -144,16 +144,16 @@ def test_has_remote_file_changed_no_local_file(setup_core: Path) -> None:
 def test_has_remote_file_changed_network_error(
     mock_head: MagicMock, setup_core: Path
 ) -> None:
-    """Test has_remote_file_changed handles network errors gracefully."""
+    """Test has_remote_file_changed returns False on network error when file is cached."""
     test_file = setup_core / "cached.txt"
     test_file.write_text("cached content")
 
     mock_head.side_effect = requests.exceptions.RequestException("Network error")
 
     url = "https://example.com/file.txt"
+    result = external_files.has_remote_file_changed(url, test_file)
 
-    with pytest.raises(Invalid, match="Could not check if.*Network error"):
-        external_files.has_remote_file_changed(url, test_file)
+    assert result is False
 
 
 @patch("esphome.external_files.requests.head")
@@ -198,3 +198,41 @@ def test_is_file_recent_handles_float_seconds(setup_core: Path) -> None:
     result = external_files.is_file_recent(test_file, refresh)
 
     assert result is True
+
+
+@patch("esphome.external_files.requests.get")
+@patch("esphome.external_files.has_remote_file_changed")
+def test_download_content_with_network_error_uses_cache(
+    mock_has_changed: MagicMock, mock_get: MagicMock, setup_core: Path
+) -> None:
+    """Test download_content uses cached file when network fails."""
+    test_file = setup_core / "cached.txt"
+    cached_content = b"cached content"
+    test_file.write_bytes(cached_content)
+
+    # Simulate file has changed, so it tries to download
+    mock_has_changed.return_value = True
+    mock_get.side_effect = requests.exceptions.RequestException("Network error")
+
+    url = "https://example.com/file.txt"
+    result = external_files.download_content(url, test_file)
+
+    assert result == cached_content
+
+
+@patch("esphome.external_files.requests.get")
+@patch("esphome.external_files.has_remote_file_changed")
+def test_download_content_with_network_error_no_cache_fails(
+    mock_has_changed: MagicMock, mock_get: MagicMock, setup_core: Path
+) -> None:
+    """Test download_content raises error when network fails and no cache exists."""
+    test_file = setup_core / "nonexistent.txt"
+
+    # Simulate file has changed (doesn't exist), so it tries to download
+    mock_has_changed.return_value = True
+    mock_get.side_effect = requests.exceptions.RequestException("Network error")
+
+    url = "https://example.com/file.txt"
+
+    with pytest.raises(Invalid, match="Could not download from.*Network error"):
+        external_files.download_content(url, test_file)