[core] Fix wait_until hanging when used in on_boot automations (#11869)

esphome/core/base_automation.h

@@ -412,7 +412,12 @@ template<typename... Ts> class WaitUntilAction : public Action<Ts...>, public Co
 
   void setup() override {
     // Start with loop disabled - only enable when there's work to do
-    this->disable_loop();
+    // IMPORTANT: Only disable if num_running_ is 0, otherwise play_complex() was already
+    // called before our setup() (e.g., from on_boot trigger at same priority level)
+    // and we must not undo its enable_loop() call
+    if (this->num_running_ == 0) {
+      this->disable_loop();
+    }
   }
 
   void play_complex(const Ts &...x) override {

tests/integration/fixtures/wait_until_on_boot.yaml

@@ -0,0 +1,47 @@
+# Test for wait_until in on_boot automation
+# Reproduces bug where wait_until in on_boot would hang forever
+# because WaitUntilAction::setup() would disable_loop() after
+# play_complex() had already enabled it.
+
+esphome:
+  name: wait-until-on-boot
+  on_boot:
+    then:
+      - logger.log: "on_boot: Starting wait_until test"
+      - globals.set:
+          id: on_boot_started
+          value: 'true'
+      - wait_until:
+          condition:
+            lambda: return id(test_flag);
+          timeout: 5s
+      - logger.log: "on_boot: wait_until completed successfully"
+
+host:
+
+logger:
+  level: DEBUG
+
+globals:
+  - id: on_boot_started
+    type: bool
+    initial_value: 'false'
+  - id: test_flag
+    type: bool
+    initial_value: 'false'
+
+api:
+  actions:
+    - action: set_test_flag
+      then:
+        - globals.set:
+            id: test_flag
+            value: 'true'
+    - action: check_on_boot_started
+      then:
+        - lambda: |-
+            if (id(on_boot_started)) {
+              ESP_LOGI("test", "on_boot has started");
+            } else {
+              ESP_LOGI("test", "on_boot has NOT started");
+            }

tests/integration/test_wait_until_on_boot.py

@@ -0,0 +1,91 @@
+"""Integration test for wait_until in on_boot automation.
+
+This test validates that wait_until works correctly when triggered from on_boot,
+which runs at the same setup priority as WaitUntilAction itself. This was broken
+before the fix because WaitUntilAction::setup() would unconditionally disable_loop(),
+even if play_complex() had already been called and enabled the loop.
+
+The bug: on_boot fires during StartupTrigger::setup(), which calls WaitUntilAction::play_complex()
+before WaitUntilAction::setup() has run. Then when WaitUntilAction::setup() runs, it calls
+disable_loop(), undoing the enable_loop() from play_complex(), causing wait_until to hang forever.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import re
+
+import pytest
+
+from .types import APIClientConnectedFactory, RunCompiledFunction
+
+
+@pytest.mark.asyncio
+async def test_wait_until_on_boot(
+    yaml_config: str,
+    run_compiled: RunCompiledFunction,
+    api_client_connected: APIClientConnectedFactory,
+) -> None:
+    """Test that wait_until works in on_boot automation with a condition that becomes true later."""
+    loop = asyncio.get_running_loop()
+
+    on_boot_started = False
+    on_boot_completed = False
+
+    on_boot_started_pattern = re.compile(r"on_boot: Starting wait_until test")
+    on_boot_complete_pattern = re.compile(r"on_boot: wait_until completed successfully")
+
+    on_boot_started_future = loop.create_future()
+    on_boot_complete_future = loop.create_future()
+
+    def check_output(line: str) -> None:
+        """Check log output for test progress."""
+        nonlocal on_boot_started, on_boot_completed
+
+        if on_boot_started_pattern.search(line):
+            on_boot_started = True
+            if not on_boot_started_future.done():
+                on_boot_started_future.set_result(True)
+
+        if on_boot_complete_pattern.search(line):
+            on_boot_completed = True
+            if not on_boot_complete_future.done():
+                on_boot_complete_future.set_result(True)
+
+    async with (
+        run_compiled(yaml_config, line_callback=check_output),
+        api_client_connected() as client,
+    ):
+        # Wait for on_boot to start
+        await asyncio.wait_for(on_boot_started_future, timeout=10.0)
+        assert on_boot_started, "on_boot did not start"
+
+        # At this point, on_boot is blocked in wait_until waiting for test_flag to become true
+        # If the bug exists, wait_until's loop is disabled and it will never complete
+        # even after we set the flag
+
+        # Give a moment for setup to complete
+        await asyncio.sleep(0.5)
+
+        # Now set the flag that wait_until is waiting for
+        _, services = await client.list_entities_services()
+        set_flag_service = next(
+            (s for s in services if s.name == "set_test_flag"), None
+        )
+        assert set_flag_service is not None, "set_test_flag service not found"
+
+        client.execute_service(set_flag_service, {})
+
+        # If the fix works, wait_until's loop() will check the condition and proceed
+        # If the bug exists, wait_until is stuck with disabled loop and will timeout
+        try:
+            await asyncio.wait_for(on_boot_complete_future, timeout=2.0)
+            assert on_boot_completed, (
+                "on_boot wait_until did not complete after flag was set"
+            )
+        except TimeoutError:
+            pytest.fail(
+                "wait_until in on_boot did not complete within 2s after condition became true. "
+                "This indicates the bug where WaitUntilAction::setup() disables the loop "
+                "after play_complex() has already enabled it."
+            )