Implement Legacy Model Pickle Serialization

[phil-lopreiato]

This is the followup to #3234

It implements the write side, as well as some other bug fixes and cleanup. Also adds tests to assert that we can do a round trip serialize + deserialize and get the original value back.

This includes some of the serialization code from the legacy runtime, which maintains its original license. I made the basic modifications to port it to py3, and left links to where to find the original to make future debugging easier.

I also verified that I could take pickles generated by these unit tests and be able to deserialize them in the legacy app's console. After fixing imports, of course. Which we'll need to add to the py2 code anyway.

import base64
import pickle
import io
import pprint

class ImportFixingUnpickler(pickle.Unpickler):
    def find_class(self, module, name):
        renamed_module = module
        prefix = "backend.common."
        if module.startswith(prefix):
            renamed_module = module[len(prefix):]

        return pickle.Unpickler.find_class(self, renamed_module, name)

b64 = "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"

p = ImportFixingUnpickler(io.BytesIO(base64.b64decode(b64))).load()
pprint.PrettyPrinter(indent=4).pprint(p)

This gives

[   Media(key=Key('Media', 'instagram-profile_titaniumtigers4829', app='tbatv-prod-hrd'), created=datetime.datetime(2020, 12, 20, 20, 3, 17, 962520), details_json=None, foreign_key=u'titaniumtigers4829', media_type_enum=7, private_details_json=None, references=[Key('Team', 'frc4829', app='tbatv-prod-hrd')], updated=datetime.datetime(2020, 12, 20, 20, 3, 17, 962533), year=None)]

[codecov]

Codecov Report

Merging #3271 (8399e84) into py3 (4984844) will decrease coverage by 0.86%.
The diff coverage is 74.27%.

@@            Coverage Diff             @@
##              py3    #3271      +/-   ##
==========================================
- Coverage   95.52%   94.65%   -0.87%     
==========================================
  Files         371      375       +4     
  Lines       16899    17576     +677     
==========================================
+ Hits        16143    16637     +494     
- Misses        756      939     +183     

Impacted Files	Coverage Δ
...legacy_protobuf/legacy_gae_entity_model_encoder.py	58.91% <58.91%> (ø)
...kend/common/legacy_protobuf/legacy_gae_protobuf.py	63.28% <63.28%> (ø)
...on/legacy_protobuf/legacy_gae_entity_pb_encoder.py	81.00% <81.00%> (ø)
...legacy_protobuf/legacy_gae_entity_model_decoder.py	86.58% <86.58%> (ø)
src/backend/common/models/cached_model.py	100.00% <100.00%> (+11.57%)	⬆️
src/backend/common/models/cached_query_result.py	100.00% <100.00%> (ø)
...es/tests/cached_query_result_compatibility_test.py	100.00% <100.00%> (ø)
... and 1 more

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