[modbus] Reject send_wait_time and turnaround_time above 65535 ms (#20160)

esphome/components/modbus/__init__.py

@@ -52,6 +52,12 @@ CONF_TURNAROUND_TIME = "turnaround_time"
 
 MODBUS_ROLES = ["client", "server"]
 
+# The client hub takes these times as 16-bit milliseconds; a Modbus timeout is far shorter anyway.
+_HUB_TIME_PERIOD = cv.All(
+    cv.positive_time_period_milliseconds,
+    cv.Range(max=cv.TimePeriod(milliseconds=65535)),
+)
+
 
 # The write (mutating) function codes, matching modbus::helpers::is_function_code_write(). 0x17
 # (read/write multiple) is included: it mutates, so the hub treats it as a write despite its read half.
@@ -273,12 +279,8 @@ CONFIG_SCHEMA = cv.typed_schema(
             {
                 cv.GenerateID(): cv.declare_id(ModbusClient),
                 cv.Optional(CONF_FLOW_CONTROL_PIN): pins.gpio_output_pin_schema,
-                cv.Optional(
-                    CONF_SEND_WAIT_TIME, default="2000ms"
-                ): cv.positive_time_period_milliseconds,
-                cv.Optional(
-                    CONF_TURNAROUND_TIME, default="600ms"
-                ): cv.positive_time_period_milliseconds,
+                cv.Optional(CONF_SEND_WAIT_TIME, default="2000ms"): _HUB_TIME_PERIOD,
+                cv.Optional(CONF_TURNAROUND_TIME, default="600ms"): _HUB_TIME_PERIOD,
             }
         )
         .extend(cv.COMPONENT_SCHEMA)

tests/component_tests/modbus/test_modbus.py

@@ -4,7 +4,11 @@ import pytest
 
 from esphome import config_validation as cv
 from esphome.components import modbus
-from esphome.components.modbus import CONF_MODBUS_ID, _validate_server_address
+from esphome.components.modbus import (
+    _HUB_TIME_PERIOD,
+    CONF_MODBUS_ID,
+    _validate_server_address,
+)
 from esphome.const import CONF_ADDRESS
 
 
@@ -36,3 +40,13 @@ def test_client_schema_still_accepts_address_zero() -> None:
     # A client may address 0: writes are broadcast, and reads are allowed with allow_broadcast_read.
     schema = modbus.modbus_device_schema(0x01)
     assert schema({CONF_MODBUS_ID: "hub", CONF_ADDRESS: 0})[CONF_ADDRESS] == 0
+
+
+def test_hub_time_accepts_up_to_65535_ms() -> None:
+    assert _HUB_TIME_PERIOD("65535ms").total_milliseconds == 65535
+
+
+def test_hub_time_rejects_values_the_hub_would_truncate() -> None:
+    # The setters take 16-bit milliseconds: 70 s would silently become 4464 ms.
+    with pytest.raises(cv.Invalid):
+        _HUB_TIME_PERIOD("70s")