Motivation

AWS S3 supports conditional headers (If-Match and If-None-Match) in the CopyObject API to enable atomic copy operations based on the destination object's ETag. LocalStack previously did not support these headers, making it difficult to test applications that rely on conditional copy operations. Fixes #13522

Changes

Implemented support for conditional headers in S3 CopyObject API:

• Added If-Match header support: Allows copying only if the destination object's ETag matches the specified value
• Added If-None-Match header support: Allows copying only if the destination object doesn't exist (using * wildcard)
• Validation logic: Implemented proper error handling for invalid header combinations and unsupported values, matching AWS behavior
• Precondition checks: Leveraged existing helper functions (object_exists_for_precondition_write and verify_object_equality_precondition_write) to validate conditions before copy operation
  The implementation follows the same pattern as the existing

PutObject conditional header logic.

Tests

Added tests in test_s3_api.py:

• test_copy_object_if_none_match: Validates If-None-Match: * behavior
  • Successful copy when destination doesn't exist
  • PreconditionFailed error when destination already exists
• test_copy_object_if_match: Validates If-Match behavior
• Successful copy when destination ETag matches
• PreconditionFailed error when ETag doesn't match
• NoSuchKey error when destination doesn't exist

Related

#13522