fix map_index is null dynamic task mapping bug

[isaiahiruoha]

closes: #52881

Problem Description

When tasks within nested task groups try to access XCom values from sibling tasks in the same mapped parent task group instance, the system incorrectly uses map_index=null instead of the correct map index, causing XCom lookup failures.

Example DAG Structure

Error Observed (Note: map_index=null)

No XCom value found; defaulting to None.: key="return_value": dag_id="test_dag": task_id="expandable_task_group.inner_task_group.alter_input": run_id="manual__2025-07-04T14:56:47.815002+00:00": map_index=null

Related Code

The issue is in airflow-core/src/airflow/api_fastapi/execution_api/routes/task_instances.py in the _get_upstream_map_indexes function.

The logic only checks the immediate parent task group (upstream_task.task_group) but doesn't consider nested scenarios where tasks are within a regular task group that is itself within a mapped task group.

Proposed Solution

Replace the immediate parent check with a hierarchy-aware check using get_closest_mapped_task_group(). This ensures that both tasks recognize they share the same mapped task group ancestor and use the same map_index.

Testing

• Verified proposed solution resolves issue: map_index = null for passed parameters in tasks in child task group when parent task group is expanded #52881 when tested locally using breeze start-airflow against the latest main branch development environment.
• Added test_nested_mapped_task_group_upstream_indexes to verify that tasks in nested mapped task groups correctly resolve upstream map indexes.

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[o-nikolas]

CC @MarcusCramer91

[o-nikolas]

Changes lgtm, but someone more familiar with task mapping should have a second look.

[Lee-W]

Tested with a few different cases and looks good

[github-actions]

Backport failed to create: v3-0-test. View the failure log Run details

Status	Branch	Result
❌	v3-0-test

You can attempt to backport this manually by running:

cherry_picker f685c61 v3-0-test

This should apply the commit to the v3-0-test branch and leave the commit in conflict state marking
the files that need manual conflict resolution.

After you have resolved the conflicts, you can continue the backport process by running:

cherry_picker --continue