avoid leaving hanging object references in helper functions

[mattsb42-aws]

The encrypt and decrypt helper functions in the top-level aws_encryption_sdk module currently return the header object from the EncryptionStream objects. To avoid any possibility of those objects not being released for garbage collection because of active reference, we should copy the header object and return the copy.

[abdulsmapara]

@mattsb42-aws, I am interested in solving this issue.
Does the return value of encrypt function at https://github.com/aws/aws-encryption-sdk-python/blob/master/src/aws_encryption_sdk/internal/crypto/encryption.py#L76
and decrypt function at https://github.com/aws/aws-encryption-sdk-python/blob/master/src/aws_encryption_sdk/internal/crypto/encryption.py#L135 need to be modified ?

[mattsb42-aws]

Hi @abdulsmapara, thanks for the interest!

This is referring to the top-level single-step interfaces for encrypt[1] and decrypt[2]. The concern is that the reference to the attribute on the stream object will keep the stream object around in memory. This is not a concern for the streaming interface because in that case the caller has direct control of the stream object.

Poking around at this a bit today, actually the only thing that we would need to do here is to use copy.deepcopy to copy the header and keyring trace objects from the managed stream before passing them to CryptoResult. It looks like attrs is already handling all the complicated bits. :)

[1]

aws-encryption-sdk-python/src/aws_encryption_sdk/__init__.py

Lines 84 to 87 in 82e2156

	with StreamEncryptor(**kwargs) as encryptor:
	ciphertext = encryptor.read()
	return CryptoResult(result=ciphertext, header=encryptor.header, keyring_trace=encryptor.keyring_trace)

[2]

aws-encryption-sdk-python/src/aws_encryption_sdk/__init__.py

Lines 143 to 146 in 82e2156

	with StreamDecryptor(**kwargs) as decryptor:
	plaintext = decryptor.read()
	return CryptoResult(result=plaintext, header=decryptor.header, keyring_trace=decryptor.keyring_trace)

[abdulsmapara]

I think this PR should complete the task !