Simplify plugin registrations

[edolstra]

Motivation

Use Meyer's singleton pattern in our various registration classes, as suggested by @xokdvium (#13138 (comment)).

Context

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[xokdvium]

Diff LGTM

[xokdvium]

@edolstra Formatter is not happy :(

[Mic92]

Looks like this made initialization of classes somehow more lazy so they are no longer initialized in the manual generation.

nix-manual> 5 │ There are [multiple types of Nix stores](./types/index.md) with different capabilities, such as the default one on the [local filesystem](./types/local-store.md) (`/nix/store`) or [binary caches](./types/http-binary-cache-store.md).
nix-manual>   │                                                                                                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File not found: ./types/local-store.md
nix-manual> 
nix-manual> error: File not found: ./types/http-binary-cache-store.md
nix-manual>   ┌─ store/index.md:5:181
nix-manual>   │
nix-manual> 5 │ There are [multiple types of Nix stores](./types/index.md) with different capabilities, such as the default one on the [local filesystem](./types/local-store.md) (`/nix/store`) or [binary caches](./types/http-binary-cache-store.md).
nix-manual>   │                                                                                                                                                                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File not found: ./types/http-binary-cache-store.md
nix-manual> 
nix-manual> error: File not found: ./local-store.md
nix-manual>    ┌─ store/types/index.md:32:11
nix-manual>    │
nix-manual> 32 │ * Use the [local store](./local-store.md) `/nix/store` if `/nix/var/nix`
nix-manual>    │           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File not found: ./local-store.md
nix-manual> 
nix-manual> error: File not found: ./local-daemon-store.md
nix-manual>    ┌─ store/types/index.md:35:61
nix-manual>    │
nix-manual> 35 │   * Otherwise, if `/nix/var/nix/daemon-socket/socket` exists, [connect
nix-manual>    │ ╭─────────────────────────────────────────────────────────────^
nix-manual> 36 │ │   to the Nix daemon listening on that socket](./local-daemon-store.md).
nix-manual>    │ ╰──────────────────────────────────────────────────────────────────────^ File not found: ./local-daemon-store.md
nix-manual> 
nix-manual> error: File not found: ./local-store.md
nix-manual>    ┌─ store/types/index.md:38:37
nix-manual>    │
nix-manual> 38 │ * Otherwise, on Linux only, use the [local chroot store](./local-store.md)
nix-manual>    │                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File not found: ./local-store.md
nix-manual> 
nix-manual> error: File not found: ./local-store.md
nix-manual>    ┌─ store/types/index.md:42:22
nix-manual>    │
nix-manual> 42 │ * Otherwise, use the [local store](./local-store.md) `/nix/store`.
nix-manual>    │                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File not found: ./local-store.md
nix-manual> 
nix-manual> error: File not found: ../../store/types/ssh-store.md
nix-manual>     ┌─ command-ref/nix-store/import.md:460:29
nix-manual>     │
nix-manual> 460 │ > Import the closure into a [remote SSH store](../../store/types/ssh-store.md) using the [`--store`](../../command-ref/conf-file.md#conf-store) option:
nix-manual>     │                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File not found: ../../store/types/ssh-store.md
nix-manual> 
nix-manual> error: File not found: ../store/types/local-store.html
nix-manual>     ┌─ command-ref/conf-file.md:513:127
nix-manual>     │
nix-manual> 513 │   > If you can’t or don’t want to configure `root` to be able to access the remote machine, set [`store`](#conf-store) to any [local store](../store/types/local-store.html), e.g. by passing `--store /tmp` to the command on the local machine.
nix-manual>     │                                                                                                                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File not found: ../store/types/local-store.html

Overall it seems useful if this makes the loading order more deterministic but we need to fix the manual then.

[roberth]

I don't see how this is a simplification, because it still relies on the mutation of globals, and the amount of code seems to be about the same.
Feel free to go ahead though, assuming it's an incremental improvement. (if the other comments can be addressed)

[Ericson2314]

Yes the end state has to be no globals, but from a quick glance this does feel to me like an improvement.

[edolstra]

I don't see how this is a simplification

It's a simplification because it replaces

static Commands * commands;
...
RegisterCommand::Commands * RegisterCommand::commands = nullptr;
...
if (!commands)
    commands = new Commands;

(i.e. 3 different sites and a conditional), with just

static Commands & commands()
{
    static Commands commands;
    return commands;
}

[Mic92]

Yes the end state has to be no globals, but from a quick glance this does feel to me like an improvement.

Afaik it provides more thread-safety during initialization and we have to deal with less raw pointer.

[Mic92]

Just want to add that the merge above was triggered by auto-merge, that I have set up initially. I saw your comments after the fact.

[corngood]

I'm still investigating, but I think this broke nix on cygwin, because it depends on gnu_unique_object symbols to work properly when compiled into multiple shared libs (e.g. nix / libcmd).

[xokdvium]

Yeah, it was an issue with MacOS as well. We need to move the singletons into the libraries. It's sad that non ELF platforms have issues with "vague" linkage.

[corngood]

How did you work around this on MacOS? I was going to attempt moving the data as you suggested.

Edit: do you mean 060c34b ?

The problem I ran into was with RegisterCommand, which could be fixed in a similar way.

[xokdvium]

Yeah, I did suggest moving all the other static variables from headers to .cc files, but seems like we forgot some.

[corngood]

I fixed what I could find in #14551.