[core] Shared slot count factory for codegen sized listener storage (#18057)

tests/unit_tests/test_cpp_helpers.py

@@ -4,6 +4,7 @@ from unittest.mock import Mock
 import pytest
 
 from esphome import const, cpp_helpers as ch
+from esphome.core import CoroPriority, coroutine_with_priority
 from esphome.cpp_helpers import ComponentSourcePool, register_component_source
 
 
@@ -167,3 +168,53 @@ def test_register_component_source_overflow_suppressed_in_testing_mode(
         idx = register_component_source("overflow_component")
     assert idx == 0
     assert "Too many unique component source names" not in caplog.text
+
+
+def _define_value(name: str) -> str | None:
+    for define in ch.CORE.defines:
+        if define.name == name:
+            # Values are codegen expressions (IntLiteral); compare rendered.
+            return str(define.value)
+    return None
+
+
+def test_slot_counter_emits_requested_count() -> None:
+    """Each request bumps the count; the self-scheduled FINAL job emits it."""
+    request = ch.slot_counter("TEST_SLOT_COUNT")
+    request()
+    request()
+    ch.CORE.flush_tasks()
+    assert _define_value("TEST_SLOT_COUNT") == "2"
+
+
+def test_slot_counter_without_requests_emits_nothing() -> None:
+    """No requests, no job, no define — the guarded storage compiles out."""
+    ch.slot_counter("TEST_SLOT_COUNT_UNUSED")
+    ch.CORE.flush_tasks()
+    assert _define_value("TEST_SLOT_COUNT_UNUSED") is None
+
+
+def test_slot_counter_request_from_final_job_still_emits() -> None:
+    """The FIRST request for a define may come from a FINAL job: its emit job
+    is scheduled mid-drain and flush_tasks() loops until the heap is empty.
+    Later requests do not get this guarantee — see the companion test."""
+    request = ch.slot_counter("TEST_SLOT_COUNT_LATE")
+
+    @coroutine_with_priority(CoroPriority.FINAL)
+    async def late_requester() -> None:
+        request()
+
+    ch.CORE.add_job(late_requester)
+    ch.CORE.flush_tasks()
+    assert _define_value("TEST_SLOT_COUNT_LATE") == "1"
+
+
+def test_slot_counter_request_after_emit_raises() -> None:
+    """The boundary of FINAL-time requests: once the define was emitted, a
+    further request would silently undersize the storage, so it fails loudly."""
+    request = ch.slot_counter("TEST_SLOT_COUNT_TOO_LATE")
+    request()
+    ch.CORE.flush_tasks()
+    assert _define_value("TEST_SLOT_COUNT_TOO_LATE") == "1"
+    with pytest.raises(ValueError, match="TEST_SLOT_COUNT_TOO_LATE"):
+        request()