Resolve the probatio codec import before the event loop (#183909)

homeassistant/__init__.py

@@ -1,5 +1,6 @@
 """Init file for Home Assistant."""
 
+import probatio
 from probatio import BuildPolicy, set_build_policy
 from probatio.compat import install_as_voluptuous
 
@@ -13,3 +14,11 @@ install_as_voluptuous()
 # a large number of schemas, many of which are never validated in a given run, so
 # lazy building avoids that upfront cost. Only the application may set this policy.
 set_build_policy(BuildPolicy.LAZY)
+
+# Probatio resolves its codec re-exports through a lazy import on first attribute
+# access. Both of these are reached from the event loop, where that import is a
+# blocking call: to_field_list renders every config flow form, to_openapi builds
+# the tool schemas for a conversation turn. Resolve them here instead. It costs
+# about 4 ms and pulls in no voluptuous of its own.
+_ = probatio.to_field_list
+_ = probatio.to_openapi

tests/test_init.py

@@ -0,0 +1,32 @@
+"""Test the Home Assistant package init."""
+
+import subprocess
+import sys
+
+IMPORT_TIMEOUT = 60
+
+
+def test_probatio_codecs_is_imported_before_the_event_loop() -> None:
+    """Importing Home Assistant must leave no lazy codec import for the loop.
+
+    Probatio resolves to_field_list and to_openapi through a lazy import of
+    probatio.codecs on first attribute access, and both are reached from the
+    event loop: the first renders every config flow form, the second builds the
+    tool schemas for a conversation turn.
+
+    A clean interpreter is the only way to see this: anything the test suite
+    imported first would hide it.
+    """
+    result = subprocess.run(
+        [
+            sys.executable,
+            "-c",
+            "import sys, homeassistant; print('probatio.codecs' in sys.modules)",
+        ],
+        capture_output=True,
+        check=True,
+        text=True,
+        timeout=IMPORT_TIMEOUT,
+    )
+
+    assert result.stdout.strip() == "True"