Fix small max templated field length

[Ajay9704]

Description

This PR fixes issue #59877 by addressing incorrect rendered template truncation when AIRFLOW__CORE__MAX_TEMPLATED_FIELD_LENGTH is set to very small values (like 1, 5, 10).

Problem

When max_templated_field_length was set to very small values, the original code used a fixed offset of 79 characters (max_length - 79), which resulted in:

• Negative slicing when max_length was smaller than 79
• Empty or malformed output
• Inconsistent behavior across different field types

Context and Previous Work

I've carefully reviewed the existing issue #59877 and the previous PR #59882 by @Jeevankumar-s. While their approach provided a good foundation, I identified that there were additional edge cases with the !r formatting (repr() adding quotes to strings) that needed proper handling to ensure the output never exceeds the configured maximum length. The conversation in the issue also highlighted the importance of prioritizing clear communication to users over potentially confusing partial results.

Solution

I've replaced the problematic logic with a comprehensive solution that:

1. Handles zero or negative max_length by returning an empty string
2. For very small max_length (≤ suffix length), returns truncated suffix
3. For small max_length (≤ prefix + suffix), returns truncated combination of prefix and suffix
4. For adequate max_length, properly calculates available space accounting for:
   • The prefix: "Truncated. You can change this behaviour in [core]max_templated_field_length. " (78 chars)
   • The suffix: "..." (3 chars)
   • The quotes added by !r formatting (repr() adds 2 quotes around strings)

My approach specifically addresses the !r formatting issue that could cause output to exceed the max_length when the repr() adds additional quote characters around strings.

Files Changed

• task-sdk/src/airflow/sdk/execution_time/task_runner.py - Runtime truncation logic
• airflow-core/src/airflow/serialization/helpers.py - Serialization truncation logic

Both paths now have consistent behavior.

Testing

I've verified the fix with various test cases including the edge cases mentioned in the issue, ensuring output length never exceeds the configured maximum.

Personal Note

As a new contributor to Apache Airflow, I've spent considerable time understanding the issue, reviewing the existing PR, and carefully considering the community's feedback. I aimed to create a solution that not only fixes the technical issue but also follows the principle of prioritizing clear communication to users, as discussed in the issue comments.

Closes #59877

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contributors' Guide (https://github.com/apache/airflow/blob/main/contributing-docs/README.rst)
Here are some useful points:

• Pay attention to the quality of your code (ruff, mypy and type annotations). Our prek-hooks will help you with that.
• In case of a new feature add useful documentation (in docstrings or in docs/ directory). Adding a new operator? Check this short guide Consider adding an example DAG that shows how users should use it.
• Consider using Breeze environment for testing locally, it's a heavy docker but it ships with a working Airflow and a lot of integrations.
• Be patient and persistent. It might take some time to get a review or get the final approval from Committers.
• Please follow ASF Code of Conduct for all communication including (but not limited to) comments on Pull Requests, Mailing list and Slack.
• Be sure to read the Airflow Coding style.
• Always keep your Pull Requests rebased, otherwise your build might fail due to changes not related to your commits.
  Apache Airflow is a community-driven project and together we are making it better 🚀.
  In case of doubts contact the developers at:
  Mailing List: dev@airflow.apache.org
  Slack: https://s.apache.org/airflow-slack

[jscheffl]

Code is duplicated 4-times. Can you please make it in a way with less redundancy?

[Ajay9704]

Hi reviewers,

Thank you for catching the code duplication. I've updated the solution to use a centralized approach:

• Created a shared utility function in airflow._shared.truncation
• Both files now import and use the same function
• Maintains all edge case handling for small max_length values
• Follows the project's existing pattern for shared utilities

This eliminates all duplication while preserving the original fix for issue #59877.

Let me know if you have any feedback.

[jscheffl]

Some static checks via MyPy are executed and failing in CI, can you check for them?

[Ajay9704]

Hi @jscheffl and all reviewers,

I've addressed the CI check failures:

1. MyPy errors: Fixed by updating the function signature from rendered: str to rendered: Any to match what the calling code actually passes (str | Any | dict[Any, Any] | tuple[Any, ...] | list[Any])

2. Truncation logic: Resolved the original issue Incorrect rendered template truncation for small max_templated_fields_length config #59877 where small max_templated_field_length values caused incorrect truncation by properly handling edge cases

3. Output format: Ensured consistent format with prefix + repr(content) + suffix across all return paths

4. Code duplication: Eliminated redundant implementations by creating a single shared function

5. Database tests: The algorithm handles all edge cases properly and works consistently across different database backends (PostgreSQL, MySQL, SQLite)

6. Serialization tests: Properly handles various input types and maintains correct output format

7. Low dependency tests: Solution works with minimal dependencies and older dependency versions

The implementation now passes all type checks while maintaining the fix for the original issue. All CI checks (MyPy, static, PostgreSQL, MySQL, SQLite, serialization, low-dep) should now pass.