A client called `run_in_transaction` be stuck if Spanner is down

[jiayun-zou]

Determine this is the right repository

• I determined this is the correct repository in which to report this bug.

Summary of the issue

Context
When the Spanner is unavailable. If we call run_in_transaction, the Spanner client will stuck forever.

We believe the get_session should in the try catch block. Otherwise, the self._local.transaction_running = True will be forever True.

Expected Behavior:
The client will retry when the Spanner DB is back.

Actual Behavior:
The client stuck forever.

API client name and version

No response

Reproduction steps: code

No response

Reproduction steps: supporting files

• Try to create a session when Spanner is unavailable.
• The client will be stuck forever.

Reproduction steps: actual results

No response

Reproduction steps: expected results

No response

OS & version + platform

No response

Python environment

No response

Python dependencies

No response

Additional context

No response