Circular Subarray Maximum Sum

Your task is to write a function that accepts a list of integers representing a circular array, and returns the maximum sum obtainable by any subarray. The array is circular, meaning that the end of the array wraps around to the beginning. The subarray must contain at least one element.

Examples:

1. For the array [8, -1, 3, 4], the maximum subarray sum is 15 (considering the subarray [3, 4, 8]).
2. For the array [-4, 5, 1, 0], the maximum subarray sum is 6 (considering the subarray [5, 1, 0]).

Implement a solution that is efficient even for large arrays. Consider all edge cases and be mindful of how to handle scenarios where all numbers may be negative.