Fixing bad cadwyn migration for upstream map indexes

[amoghrajesh]

Problem

Mapped tasks on old client of version (1.0.1) and new airflow API server (3.0.2) started failing.

Example DAG:

import datetime

from airflow.decorators import dag
from airflow.decorators import task

@dag(start_date=datetime.datetime(2024, 10, 1), schedule=None, catchup=False)
def dynamic_xcom():
    @task
    def task_1():
        print("task_1")

    @task
    def task_2(x):
        print("task_2")
        print(x)
        return x

    @task
    def task_3(y):
        print("task_3")
        print(y)

    t1 = task_1()
    t2 = task_2.expand(x=[0, 1, 2])
    t1 >> t2
    task_3.expand(y=t2)
dynamic_xcom()

Failure:

The issue was in the Cadwyn version downgrade logic that converts API responses for older clients. When _get_upstream_map_indexes correctly returned {"task_2": None}, the downgrade_upstream_map_indexes function was incorrectly converting None to -1 as it seemed semantically right. This conversion was breaking the Task SDK's mapped task expansion logic.

The task SDK's get_task_map_length function expects None values in upstream_map_indexes. However, when None was converted to -1, the function calculated (-1 or 1) * len(resolved_val) = -1 * 3 = -3, which triggered the "cannot expand field mapped to length -3".

Fix and testing

DAG:

import datetime

from airflow.decorators import dag
from airflow.decorators import task

@dag(start_date=datetime.datetime(2024, 10, 1), schedule=None, catchup=False)
def dynamic_xcom():
    @task
    def task_1():
        print("task_1")

    @task
    def task_2(x):
        print("task_2")
        print(x)
        return x

    @task
    def task_3(y):
        print("task_3")
        print(y)

    t1 = task_1()
    t2 = task_2.expand(x=[0, 1, 2])
    t1 >> t2
    task_3.expand(y=t2)
dynamic_xcom()

• Also added a test case that checks for this explictly.
• This fix will ensure that sdk like 1.0.1 can still work with latest airflow API server (3.0.2+)

---

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[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 2bb5d01 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

[kaxil]

Backported: d7825f1