Improve pickling of HttpResponse instances

[zatro]

When pickling a HTTPResponse, rather than pickling all of its attributes just store what is needed for the creation of a new HttpResponse with the correct content, status code and headers.

See related discussion on ticket #23895 and its PR.

[Mariusz Felisiak]

Thanks, it's worth adding that pickling HttpResponse with CBV crashes:

>>> import pickle
>>> response = self.client.get('/cbv_view/')
>>> pickle.dumps(response)
...
AttributeError: Can't pickle ....

[tigicion]

​https://github.com/django/django/pull/14728

[Mariusz Felisiak]

I agree with ​Keryn's comment.

[Keryn Knight]

Tentatively setting the flag back; I've left feedback and proposed a first sketch of how adapting SimpleTemplateResponse and TemplateResponse might go.

[Anvesh Mishra]

Submitted the ​PR with all the suggested changes according to Keryn and Felix's comments on ​PR-14728

[Mariusz Felisiak <felisiak.mariusz@…>]

In d7f5bfd:

Fixed #32969 -- Fixed pickling HttpResponse and subclasses.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 173034b0:

Refs #34482 -- Reverted "Fixed #32969 -- Fixed pickling HttpResponse and subclasses."

This reverts commit d7f5bfd241666c0a76e90208da1e9ef81aec44db.

Thanks Márton Salomváry for the report.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 791407f:

[4.2.x] Refs #34482 -- Reverted "Fixed #32969 -- Fixed pickling HttpResponse and subclasses."

This reverts commit d7f5bfd241666c0a76e90208da1e9ef81aec44db.

Thanks Márton Salomváry for the report.

Backport of 173034b00589c083793d495e8b07e35be2cb1cf0 from main

[H. White]

PR, ​https://github.com/django/django/pull/21289

Several classes associated with HTTP handling have attributes that reference objects that can't be pickled. Previous fixes have tried removing the attributes that won't pickle, but since the attributes weren't replaced, copied or unpickled objects were missing them.

This PR adds __getstate__ and __setstate__ methods to these classes, allowing instances to serialize and deserialize properly. I see that this approach was discussed for #23895 (​https://github.com/django/django/pull/14664), and rejected because the urlconf could have changed after the object was pickled. However, the unpicklable objects are only present in responses created by the test client, and the reason for their inclusion is documented.