fix(auth): fix request session error

[daniel-sanche]

Fixes #16035

Some requests have no session attribute, which resulted in an exception being thrown. This fix does an extra check for the missing field

Thanks @sakshamgoyal-01 for the detailed bug report!

[gemini-code-assist]

Summary of Changes

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This pull request resolves a critical bug where the authentication process would fail if a request object did not possess a 'session' attribute, leading to an AttributeError. The change enhances the robustness of the system by adding a preliminary check for the attribute's existence, ensuring that mTLS setup proceeds smoothly without unexpected exceptions. This improves the overall stability and reliability of authentication flows.

Highlights

• Bug Fix: Addressed an AttributeError that occurred when a request object lacked a session attribute during mTLS setup, preventing crashes in authentication processes.
• Robustness: Implemented a check using hasattr() to safely determine if the session attribute exists on the request object before attempting to access it.
• Testing: Introduced a new regression test case to specifically validate the fix for the AttributeError scenario, ensuring the issue does not reoccur.

Changelog

• packages/google-auth/google/auth/compute_engine/_metadata.py
  • Modified the _prepare_request_for_mds function to include a hasattr check for the session attribute on the request object before attempting to access it, preventing an AttributeError.
• packages/google-auth/tests/compute_engine/test__metadata.py
  • Added a new test case, test__prepare_request_for_mds_mtls_attribute_error, to specifically test the scenario where a request object initially lacks a session attribute, ensuring the fix works as expected.

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[gemini-code-assist]

Code Review

This pull request correctly fixes an AttributeError that occurred when a request object was missing the session attribute. The fix involves adding a check for the attribute's existence before use, and a new regression test has been added to cover this case. I have one suggestion to make the code slightly more concise and Pythonic.

[daniel-sanche]

Thinking about this some more, is this safe? Should we be attaching a session to a request that wasn't designed to hold one? Or should we drop mtls in that situation?

It looks like some tests fail if we drop mtls. But We should confirm the intention before merging this