Fix CallerReference collision in unique_string() using uuid4

[shreyaskommuri]

Problem

unique_string() in awscli/customizations/cloudfront.py generates the
CallerReference for create-invalidation and create-distribution using
random.randint(1, 1000000) combined with a Unix timestamp at 1-second
precision:

def unique_string(prefix='cli'):
    return '%s-%s-%s' % (prefix, int(time.time()), random.randint(1, 1000000))

Two invocations within the same second share the same timestamp and have a
1-in-1,000,000 chance of producing an identical CallerReference. When
this occurs CloudFront silently treats the second request as a duplicate and
does not create a new invalidation or distribution.

Additionally, Python's random module uses the Mersenne Twister algorithm and
is not cryptographically secure, which is inappropriate for values that are
expected to be unique and unpredictable.

Closes #10281.

Solution

Replace the implementation with uuid.uuid4(), which provides 122 bits of
cryptographically random entropy. This eliminates both the timestamp dependency
and the collision risk entirely. uuid is part of the Python standard library
and requires no new dependencies.

def unique_string(prefix='cli'):
    return '%s-%s' % (prefix, uuid.uuid4())

Changes

• awscli/customizations/cloudfront.py — replace import time/import random
  with import uuid; rewrite unique_string() to use uuid.uuid4()
• tests/unit/customizations/test_cloudfront.py — new unit tests verifying
  the UUID4 format, custom prefix support, and uniqueness across 1 000 calls

Testing

# unit tests
pytest tests/unit/customizations/test_cloudfront.py -v

# existing functional tests still pass
pytest tests/functional/cloudfront/ -v

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Generated by AI tools, and reviewed by shreyaskommuri