dcparser: Updated out-of-panda dc_sort_inheritance_by_file variable to match PRC default

[ksmit799]

Issue description

Using dcparser out-of-panda with multiple .DC files results in a compatibility error as Panda sorts inheritance by file by default. In particular, field numbers do not match.

Solution description

See dcClass.cxx PRC variable for dc_sort_inheritance_by_file (defaults to true)

Checklist

I have done my best to ensure that…

• …I have familiarized myself with the CONTRIBUTING.md file
• …this change follows the coding style and design patterns of the codebase
• …I own the intellectual property rights to this code
• …the intent of this change is clearly explained
• …existing uses of the Panda3D API are not broken
• …the changed code is adequately covered by the test suite, where possible.

[rdb]

The description of the variable says this is a "temporary hack", and the past tense "sorted" suggests that the intended new behaviour is for it to be disabled, not enabled? See 14374ee for the relevant change. A cursory look at this change suggests that not sorting is the "new" behaviour.

This is a temporary hack. This should be true if you are using version 1.42 of the otp_server.exe binary, which sorted inherited fields based on the order of the classes within the DC file, rather than based on the order in which the references are made within the class.

Perhaps @drwrose might remember?

[darktohka]

This is a tricky issue. In my opinion, we should get rid of dc_sort_inheritance_by_file in the first place. Unfortunately, there is a mismatch between the behavior of dc_sort_inheritance_by_file and the most commonly used OTP implementation's, Astron's behavior.

Here's a collection of "cases" that the setting has an effect on:

1. https://github.com/panda3d/panda3d/blob/master/direct/src/dcparser/dcClass.cxx#L505

In Panda3D, unnamed fields are only inherited when dc_sort_inheritance_by_file is set to false.

In Astron, unnamed fields are always inherited (they aren't even checked): https://github.com/Astron/Astron/blob/master/src/dclass/dc/Class.cpp#L46

In fact... none of my production DC files have any unnamed fields! (How do you even create an unnamed field?)

2. In both Panda3D and Astron, inherited fields are sorted: https://github.com/panda3d/panda3d/blob/master/direct/src/dcparser/dcClass.cxx#L545 and https://github.com/Astron/Astron/blob/master/src/dclass/dc/Class.cpp#L173. In Astron, this sorting behaviour is achieved by always inserting the inserted field into the correct position, rather than sorting it at the end.

3. In Panda3D, if dc_sort_inheritance_by_file is set to false, field indices are not even set properly... what?

https://github.com/panda3d/panda3d/blob/master/direct/src/dcparser/dcClass.cxx#L608

I believe this is erroneous behavior. If dc_sort_inheritance_by_file is not set, then... fields don't even have an index set.

4. In Panda3D, dc_sort_inheritance_by_file influences the DC hash. If dc_sort_inheritance_by_file is set to False, it generates a different hash than Astron does, which always hashes as if dc_sort_inheritance_by_file was set:

https://github.com/panda3d/panda3d/blob/master/direct/src/dcparser/dcFile.cxx#L432
https://github.com/Astron/Astron/blob/master/src/dclass/file/hash_legacy.cpp#L62

In my opinion, we should remove dc_sort_inheritance_by_file. otp_server.exe is definitely no longer being used, it's not available anywhere on the internet, and if it is, nobody has complained about it not working due to the PRC default anymore.

Case 1: I believe unnamed fields should always be inherited. In fact, we shouldn't even check if it's unnamed or not. This is consistent with Astron.

Case 2: I think it's fine to keep the fields sorted. If we remove this functionality, we would also have to update Astron.

Case 3: I think we should fix this and this should definitely not rely on the variable.

Case 4: DC hash should be always generated as if dc_sort_inheritance_by_file is set (i.e. hash with 1 rather than 2)

[rdb]

I'll trust your judgement on this, feel free to file PRs or open issues with the suggested changes.