[Test] Add @global_cache decorator for cross-process file-based caching, used to reduce the number of EC2 calls to fetch flexible instance types

[gmarciani]

Description of changes

Add @global_cache decorator for cross-process file-based caching, used to reduce the number of EC2 calls to fetch flexible instance types.

Example of usage

The decorator will cache results by arguments. Whatever caller, in whatever process, that calls the function with the same arguments will retrieve the cached result. The results are cached into a random file in /tmp whose access is synchronized by file-locking.

from framework.file_cache import file_cache

@file_cache("my_results")
def my_function(arg1, arg2):
    ...

Tests

ONGOING 2 parallel executions of test_ebs_single will trigger just one request to retrieve the list of flexible instance types, while before this fix it used to trigger one.

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