Fix duplicate log reads when resuming from log_pos

[gopidesupavan]

Fix FileTaskHandler._read() so resumed log reads actually honor metadata["log_pos"].

Previously we called islice(...) without reassigning the returned iterator, so no log lines were skipped and resumed reads could return duplicate content from the beginning of the stream.

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[jason810496]

Nice catch! It's a long time bug introduced in 3.0, thanks a lot!

[Copilot]

Pull request overview

Fixes FileTaskHandler._read() so resumed log reads correctly honor metadata["log_pos"], preventing duplicate log content when tailing/resuming from a prior position.

Changes:

• Reassigns the islice(...) result to actually advance the output stream when log_pos is provided.
• Adds a unit test that verifies resumed reads skip previously-returned log lines and return only new content.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
airflow-core/src/airflow/utils/log/file_task_handler.py	Fixes resume/tailing behavior by correctly slicing the stream based on metadata["log_pos"].
airflow-core/tests/unit/utils/test_log_handlers.py	Adds regression coverage ensuring resumed reads don’t return duplicate earlier log lines.

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[kaxil]

lgtm

[kaxil]

CI Test failures look unrelated but worth veriyfing

[gopidesupavan]

CI Test failures look unrelated but worth veriyfing

yeah looks like tests are failing for the lowest dep likely the pydantic==2.12.0 causing issues https://github.com/apache/airflow/actions/runs/23058551095/job/66982917534?pr=63531#step:8:1938

[gopidesupavan]

failures test pr is here #63570

[github-actions]

Backport successfully created: v3-1-test

Note: As of Merging PRs targeted for Airflow 3.X
the committer who merges the PR is responsible for backporting the PRs that are bug fixes (generally speaking) to the maintenance branches.

In matter of doubt please ask in #release-management Slack channel.

Status	Branch	Result
✅	v3-1-test