Test the ESP8266 flash topic cutoff in UTF-8 bytes

tests/unit_tests/components/test_mqtt_topic_source.py

@@ -0,0 +1,41 @@
+"""Tests for choosing where an mqtt trigger topic is stored."""
+
+import pytest
+
+from esphome.components.mqtt import MQTT_DEFAULT_TOPIC_MAX_LEN, _topic_source
+from esphome.const import (
+    KEY_CORE,
+    KEY_TARGET_PLATFORM,
+    PLATFORM_ESP32,
+    PLATFORM_ESP8266,
+)
+from esphome.core import CORE
+
+# "é" is two bytes in UTF-8, so these topics are one character shorter than their byte length
+LIMIT = MQTT_DEFAULT_TOPIC_MAX_LEN
+
+
+@pytest.mark.parametrize(
+    ("topic", "platform", "source"),
+    [
+        pytest.param("t" * (LIMIT - 1), PLATFORM_ESP8266, "progmem_topic", id="fits"),
+        pytest.param("t" * LIMIT, PLATFORM_ESP8266, "static_topic", id="too_long"),
+        pytest.param(
+            "é" + "t" * (LIMIT - 3), PLATFORM_ESP8266, "progmem_topic", id="utf8_fits"
+        ),
+        pytest.param(
+            "é" + "t" * (LIMIT - 2),
+            PLATFORM_ESP8266,
+            "static_topic",
+            id="utf8_bytes_too_long",
+        ),
+        pytest.param("t" * (LIMIT - 1), PLATFORM_ESP32, "static_topic", id="esp32"),
+        pytest.param("é" * 10, PLATFORM_ESP32, "static_topic", id="esp32_utf8"),
+    ],
+)
+def test_topic_source(
+    monkeypatch: pytest.MonkeyPatch, topic: str, platform: str, source: str
+) -> None:
+    """ESP8266 keeps a topic in flash only when its UTF-8 bytes fit the copy buffer."""
+    monkeypatch.setitem(CORE.data, KEY_CORE, {KEY_TARGET_PLATFORM: platform})
+    assert str(_topic_source(topic)).startswith(f"mqtt::{source}(")