[Core] Log actor class import failures with their traceback

[xuwwwww]

Description

When an actor class fails to import on a worker, Ray logs the failure at DEBUG level and creates a temporary actor to report it later. In the case reported in #49285, the user saw an async actor startup error without the original ModuleNotFoundError in the worker log.

Log the import failure with its traceback at ERROR level. The temporary actor behavior stays the same. A focused test simulates a missing module and checks both the log and the existing fallback error. The test is registered in Bazel and includes a pytest entrypoint.

Related issues

Fixes #49285.

Additional information

I checked the issue and open PRs on 2026-09-29. The issue is open and unassigned, and I found no open PR for this log path.

Tests run on Windows:

• ..\ray-venv\Scripts\python.exe ..\..\outputs\run-ray-first-pr-test.py — 1 passed after the CI logging fix. This local helper loads the modified function_manager.py over the Ray 2.58.0 binary wheel and sets the ray logger's propagation to False to reproduce the CI condition; it is not part of the PR.
• The test file's __main__ entrypoint — 1 passed.
• Applicable pre-commit hooks passed. The Bash-based docstyle and Buildifier hooks could not run through Windows pre-commit. I ran docstyle with Git Bash and checked BUILD.bazel with Buildifier 8.0.1 on an LF-normalized copy; both passed.
• git diff --check — passed.

AI assistance was used to investigate this issue and prepare the patch and test. The submitter ran the focused test locally.

[gemini-code-assist]

Code Review

This pull request updates the actor class loading logic in function_manager.py to use logger.exception instead of logger.debug when an error occurs, ensuring that the full traceback is captured. Additionally, a new test case has been added to verify that actor import errors are correctly logged. I have no further feedback to provide.

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Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

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