feat(auth): fix ClientSideCredentialAccessBoundary race condition

[vverman]

This change addresses a race condition in ClientSideCredentialAccessBoundaryFactory that occurred when multiple concurrent calls were made to generateToken.

The fix involves:

• Waiting on the RefreshTask itself rather than its internal task.
• Using a single listener in RefreshTask to ensure finishRefreshTask completes before the outer future unblocks waiting threads.
• Adding a regression test generateToken_freshInstance_concurrent_noNpe.

[gemini-code-assist]

Code Review

This pull request refactors the RefreshTask in ClientSideCredentialAccessBoundaryFactory to use a single listener, ensuring internal state updates occur before the future completes. It also simplifies the refresh logic and adds a concurrent test case to address a race condition. Feedback was provided to enhance the robustness of error handling in the task listener, specifically by catching Throwable and handling null causes in ExecutionException to avoid potential hangs.

[gemini-code-assist]

The error handling in this listener is less robust than the previous implementation. Specifically:

1. Catching Exception instead of Throwable: If finishRefreshTask or Futures.getDone throws an Error (e.g., OutOfMemoryError or NoClassDefFoundError), it will not be caught. This would cause the listener to terminate abruptly without completing the RefreshTask future, leading to all threads waiting on currentRefreshTask.get() hanging indefinitely.
2. Potential NullPointerException: RefreshTask.this.setException(e.getCause()) will throw a NullPointerException if e.getCause() is null (which is technically possible for an ExecutionException, though rare from Future.get()). Since AbstractFuture.setException does not allow null arguments, this would also cause the listener to crash and the future to hang.

Using Throwable and adding a null check for the cause ensures the future is always completed even in extreme failure scenarios.

            } catch (ExecutionException e) {
              Throwable cause = e.getCause();
              RefreshTask.this.setException(cause != null ? cause : e);
            } catch (Throwable t) {
              RefreshTask.this.setException(t);
            }