Fix circular import when using XComObjectStorageBackend

[kaxil]

Remove BaseOperator import from version_compat.py to prevent circular import during XCom backend resolution. The import was causing startup failures for scheduler and triggerer when XComObjectStorageBackend was configured as the XCom backend.

Move BaseOperator import directly to file_transfer.py where it's needed, maintaining compatibility with both Airflow 2.x and 3.x while avoiding the circular dependency chain that was introduced in PR #52443.

For use with Airflow 2.x, this is what was causing Circular import:

XCom Backend Resolution
    ↓
airflow.providers.common.io.xcom.backend
    ↓
airflow.providers.common.io.version_compat (imports BaseOperator at module level)
    ↓
airflow.models.BaseOperator (or airflow.sdk.BaseOperator)
    ↓
airflow.models.mappedoperator (BaseOperator imports this)
    ↓
airflow.triggers.base (MappedOperator imports this)
    ↓
airflow.models.taskinstance (triggers.base imports SimpleTaskInstance)
    ↓
airflow.models.xcom (TaskInstance imports XCom)
    ↓
resolve_xcom_backend() (tries to load XCom backend again)
    ↓
CIRCULAR IMPORT! 🔄

Fixes #55803

---

To Reproduce:

❯ docker run -it -v ./dist:/dist -v ~/.cache/uv:/root/.cache/uv -p 8080:8080 apache/airflow:2.11.0 bash

airflow@895deb8d01b2:/opt/airflow$ uv pip install apache-airflow-providers-common-io==1.6.2
Using Python 3.12.10 environment at: /home/airflow/.local
Resolved 136 packages in 961ms
Prepared 1 package in 20ms
Uninstalled 1 package in 40ms
Installed 1 package in 1ms
 - apache-airflow-providers-common-io==1.6.0
 + apache-airflow-providers-common-io==1.6.2
airflow@895deb8d01b2:/opt/airflow$ uvx ipython
Installed 16 packages in 38ms
/tmp/.cache/uv/archive-v0/REzCZt_aez_Mr3pLZsLJ1/lib/python3.12/site-packages/IPython/core/interactiveshell.py:975: UserWarning: Attempting to work in a virtualenv. If you encounter problems, please install IPython inside the virtualenv.
  warn(
Python 3.12.10 (main, May 10 2025, 02:07:43) [GCC 12.2.0]
Type 'copyright', 'credits' or 'license' for more information
IPython 9.5.0 -- An enhanced Interactive Python. Type '?' for help.
Tip: IPython 9.0+ has hooks to integrate AI/LLM completions.

In [1]: import os
   ...: os.environ['AIRFLOW__CORE__XCOM_BACKEND'] = 'airflow.providers.common.io.xcom.backend.XComObjectStorageBackend'
   ...: os.environ['AIRFLOW__COMMON_IO__XCOM_OBJECTSTORAGE_PATH'] = 'file:///tmp/xcom'
   ...: os.environ['AIRFLOW__COMMON_IO__XCOM_OBJECTSTORAGE_THRESHOLD'] = '100'

In [2]: import airflow.models

In [3]: import airflow.models.XCom
---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
Cell In[3], line 1
----> 1 import airflow.models.XCom

ModuleNotFoundError: No module named 'airflow.models.XCom'

In [4]: from airflow.models.xcom import XCom
/home/airflow/.local/lib/python3.12/site-packages/airflow/metrics/base_stats_logger.py:22 RemovedInAirflow3Warning: Timer and timing metrics publish in seconds were deprecated. It is enabled by default from Airflow 3 onwards. Enable timer_unit_consistency to publish all the timer and timing metrics in milliseconds.
[2025-09-17T20:52:50.610+0000] {configuration.py:1348} WARNING - cannot import name 'XCom' from partially initialized module 'airflow.models.xcom' (most likely due to a circular import) (/home/airflow/.local/lib/python3.12/site-packages/airflow/models/xcom.py)
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
File ~/.local/lib/python3.12/site-packages/airflow/configuration.py:1346, in AirflowConfigParser.getimport(self, section, key, **kwargs)
   1345 try:
-> 1346     return import_string(full_qualified_path)
   1347 except ImportError as e:

File ~/.local/lib/python3.12/site-packages/airflow/utils/module_loading.py:39, in import_string(dotted_path)
     37     raise ImportError(f"{dotted_path} doesn't look like a module path")
---> 39 module = import_module(module_path)
     41 try:

File /usr/local/lib/python3.12/importlib/__init__.py:90, in import_module(name, package)
     89         level += 1
---> 90 return _bootstrap._gcd_import(name[level:], package, level)

File <frozen importlib._bootstrap>:1387, in _gcd_import(name, package, level)

File <frozen importlib._bootstrap>:1360, in _find_and_load(name, import_)

File <frozen importlib._bootstrap>:1331, in _find_and_load_unlocked(name, import_)

File <frozen importlib._bootstrap>:935, in _load_unlocked(spec)

File <frozen importlib._bootstrap_external>:999, in exec_module(self, module)

File <frozen importlib._bootstrap>:488, in _call_with_frames_removed(f, *args, **kwds)

File ~/.local/lib/python3.12/site-packages/airflow/providers/common/io/xcom/backend.py:30
     29 from airflow.configuration import conf
---> 30 from airflow.providers.common.io.version_compat import AIRFLOW_V_3_0_PLUS
     31 from airflow.utils.json import XComDecoder, XComEncoder

File ~/.local/lib/python3.12/site-packages/airflow/providers/common/io/version_compat.py:40
     39 else:
---> 40     from airflow.models import BaseOperator
     42 __all__ = [
     43     "AIRFLOW_V_3_0_PLUS",
     44     "BaseOperator",
     45 ]

File ~/.local/lib/python3.12/site-packages/airflow/models/__init__.py:79, in __getattr__(name)
     77     from airflow.utils.module_loading import import_string
---> 79     val = import_string(f"{path}.{name}")
     80 else:

File ~/.local/lib/python3.12/site-packages/airflow/utils/module_loading.py:39, in import_string(dotted_path)
     37     raise ImportError(f"{dotted_path} doesn't look like a module path")
---> 39 module = import_module(module_path)
     41 try:

File /usr/local/lib/python3.12/importlib/__init__.py:90, in import_module(name, package)
     89         level += 1
---> 90 return _bootstrap._gcd_import(name[level:], package, level)

File ~/.local/lib/python3.12/site-packages/airflow/models/baseoperator.py:83
     82 from airflow.models.base import _sentinel
---> 83 from airflow.models.mappedoperator import OperatorPartial, validate_mapping_kwargs
     84 from airflow.models.param import ParamsDict

File ~/.local/lib/python3.12/site-packages/airflow/models/mappedoperator.py:54
     53 from airflow.ti_deps.deps.mapped_task_expanded import MappedTaskIsExpanded
---> 54 from airflow.triggers.base import StartTriggerArgs
     55 from airflow.typing_compat import Literal

File ~/.local/lib/python3.12/site-packages/airflow/triggers/base.py:27
     26 from airflow.callbacks.database_callback_sink import DatabaseCallbackSink
---> 27 from airflow.models.taskinstance import SimpleTaskInstance
     28 from airflow.utils.log.logging_mixin import LoggingMixin

File ~/.local/lib/python3.12/site-packages/airflow/models/taskinstance.py:104
    103 from airflow.models.taskreschedule import TaskReschedule
--> 104 from airflow.models.xcom import LazyXComSelectSequence, XCom
    105 from airflow.plugins_manager import integrate_macros_plugins

ImportError: cannot import name 'XCom' from partially initialized module 'airflow.models.xcom' (most likely due to a circular import) (/home/airflow/.local/lib/python3.12/site-packages/airflow/models/xcom.py)

During handling of the above exception, another exception occurred:

AirflowConfigException                    Traceback (most recent call last)
Cell In[4], line 1
----> 1 from airflow.models.xcom import XCom

File ~/.local/lib/python3.12/site-packages/airflow/models/xcom.py:815
    813     XCom = BaseXCom  # Hack to avoid Mypy "Variable 'XCom' is not valid as a type".
    814 else:
--> 815     XCom = resolve_xcom_backend()

File ~/.local/lib/python3.12/site-packages/airflow/models/xcom.py:798, in resolve_xcom_backend()
    791 def resolve_xcom_backend() -> type[BaseXCom]:
    792     """
    793     Resolve custom XCom class.
    794
    795     Confirm that custom XCom class extends the BaseXCom.
    796     Compare the function signature of the custom XCom serialize_value to the base XCom serialize_value.
    797     """
--> 798     clazz = conf.getimport("core", "xcom_backend", fallback=f"airflow.models.xcom.{BaseXCom.__name__}")
    799     if not clazz:
    800         return BaseXCom

File ~/.local/lib/python3.12/site-packages/airflow/configuration.py:1349, in AirflowConfigParser.getimport(self, section, key, **kwargs)
   1347 except ImportError as e:
   1348     log.warning(e)
-> 1349     raise AirflowConfigException(
   1350         f'The object could not be loaded. Please check "{key}" key in "{section}" section. '
   1351         f'Current value: "{full_qualified_path}".'
   1352     )

AirflowConfigException: The object could not be loaded. Please check "xcom_backend" key in "core" section. Current value: "airflow.providers.common.io.xcom.backend.XComObjectStorageBackend".

---

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[amoghrajesh]

Damn!