postprocessd: init at 0.3.0

[Luflosi]

Description of changes

Add postprocessd, a queueing Megapixels post-processor.
Also patch Megapixels to be able to use the new postprocessor if it is installed globally. See the long comment in the code for a more detailed description and a discussion of tradeoffs.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[donovanglover]

Seems reasonable just wondering if the comment should be simplified and/or in the commit message instead.

[Luflosi]

Seems reasonable just wondering if the comment should be simplified and/or in the commit message instead.

Yeah, the comment is pretty long. But I felt using these global paths goes against the NixOS philosophy and is therefore controversial, so I wanted to explain everything with as much detail as possible. Feel free to tell me any concrete suggestions you may have on how to simplify it.

At first I had this text in the commit message but then I moved it into this comment because I didn't like how the code contained a patch without any explanation and users would have to search the Git history just to find an explanation why this patch exists and what it does.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/prs-already-reviewed/2617/1955

[Luflosi]

I fixed a grammatical error in the comment.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/prs-already-reviewed/2617/1974

[MatthewCroughan]

Let's rebase this on master to see if it still passes CI, and if so I believe we should merge this.

[Luflosi]

Rebased.

[Luflosi]

Everything except the Darwin tests ran and this program is only meant for Linux.

[Luflosi]

Thanks!