handle dataclass field type sentinels

[rmorshea]

Closes: #386

In short, the dataclasses module relies on the identity of certain sentinel values being preserved. To preserve their identity, this PR relies on a minimal set of internal dataclasses APIs to reduce those sentinels to their _FIELD_BASE.name. Then, when reconstituting them, the names key into a _DATACLASSE_FIELD_TYPE_SENTINELS mapping that contains the expected sentinel instances.

The implementation accesses the following private APIs:

• dataclasses._FIELD_BASE
• dataclasses._FIELD
• dataclasses._FIELD_CLASSVAR
• dataclasses._FIELD_INITVAR

[jrbourbeau]

We started running into this over in Dask when sending dataclasses around. @ogrisel @pierreglaser I wonder if this is something you have bandwidth to take a look at? I'm not super familiar with the dataclass logic here, but can confirm it resolves the serialization issue I've seen recently.

[ogrisel]

@jrbourbeau do you have a minimal reproducer that involves dask? That would help me review this PR.

EDIT: hopefully the new test in e15fb62 covers both the original bug report in #386 and your use case with dask.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (0f330b6) 83.28% compared to head (9892fba) 83.44%.

❗ Current head 9892fba differs from pull request most recent head 550f7df. Consider uploading reports for the commit 550f7df to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #513      +/-   ##
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+ Coverage   83.28%   83.44%   +0.16%     
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  Files           4        4              
  Lines         718      725       +7     
  Branches      157      157              
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+ Hits          598      605       +7     
  Misses         95       95              
  Partials       25       25              

Files	Coverage Δ
cloudpickle/cloudpickle_fast.py	91.61% <100.00%> (+0.17%)	⬆️

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[ogrisel]

The dowstream CI config is broken (for unrelated causes) for joblib, dask and ray and would require some maintenance effort.

The PR itself looks good to me: the changes are not invasive and I checked that it actually fixes problem from the original report.

[ogrisel]

Thank you very much @rmorshea!