fix(examples/system/ota/otatool): fix serial port contention in pytest

The otatool pytest calls dut.serial.close() and then immediately
launches otatool_example.py as a subprocess that re-opens the same
serial port via esptool. This fails intermittently because:

1. pytest-embedded's QueueFeederThread still holds a reference to
   the serial port file descriptor when close() returns, causing
   'argument must be an int, or have a fileno() method' error.

2. The OS has not fully released the serial port by the time the
   subprocess tries to open it.

Add a delay after serial close and retry logic for the subprocess
to handle transient serial port contention.

Made-with: Cursor

examples/system/ota/otatool/pytest_otatool.py

@@ -1,8 +1,10 @@
-# SPDX-FileCopyrightText: 2022-2025 Espressif Systems (Shanghai) CO LTD
+# SPDX-FileCopyrightText: 2022-2026 Espressif Systems (Shanghai) CO LTD
 # SPDX-License-Identifier: Unlicense OR CC0-1.0
+import logging
 import os
 import subprocess
 import sys
+import time
 
 import pytest
 from pytest_embedded import Dut
@@ -29,6 +31,10 @@ def test_otatool_example(dut: Dut) -> None:
     # Close connection to DUT
     dut.serial.close()
 
+    # Allow the OS to fully release the serial port. pytest-embedded's
+    # QueueFeederThread may still hold the port FD when close() returns.
+    time.sleep(2)
+
     script_path = os.path.join(str(os.getenv('IDF_PATH')), 'examples', 'system', 'ota', 'otatool', 'otatool_example.py')
     binary_path = ''
 
@@ -36,4 +42,20 @@ def test_otatool_example(dut: Dut) -> None:
         if 'otatool.bin' in flash_file[1]:
             binary_path = flash_file[1]
             break
-    subprocess.check_call([sys.executable, script_path, '--binary', binary_path])
+
+    # Retry the subprocess to handle transient serial port contention.
+    # The otatool_example.py subprocess opens the serial port independently
+    # via esptool, and may fail if pytest-embedded's QueueFeederThread has
+    # not fully released the port file descriptor yet.
+    last_err = None
+    for attempt in range(3):
+        try:
+            subprocess.check_call([sys.executable, script_path, '--binary', binary_path])
+            return
+        except subprocess.CalledProcessError as e:
+            last_err = e
+            logging.warning('otatool subprocess attempt %d/3 failed: %s', attempt + 1, e)
+            time.sleep(3)
+
+    assert last_err is not None
+    raise last_err