Enable Invoking Custom Subcommands with Cross

[Alexhuszagh]

It's often useful to run custom subcommands for another target, such as asm, deb, or others. Since we can't necessarily enumerate every subcommand, there's a few solutions I see:

1. Create an environment variable, forcing the command to run in the image, such as CROSS_FORCE_RUN_IN_IMAGE=1. This name isn't ideal, so we'd probably modify it to something else.
2. If pre-build hooks are a thing and we're fine not supporting custom images, we could write a config file (just a TOML list of known extra subcommands) and parse this if the subcommand isn't known.
3. Get the output of cross --list if the subcommand isn't known, parse it, and if the subcommand is known, run it in the image.

We need to support the following, at the very least:

• cargo-valgrind
• cargo-asm
• cargo-deb
• cargo-auditable
• cargo-bloat

Related to #207, #210, #715.

[Alexhuszagh]

Just because of the lack of updates: I've added a very rough base design for this, found on my subcommands fork.

The general idea is as follows:

1. We have an allowlist of subcommands we recognize. This would be relatively small, but definitely include cargo-asm and cargo-deb. Ones like cargo-valgrind which rely on external dependencies would not be included, since they require pre-build hooks to get to work.
2. If you use the subcommand and it is not installed but is part of the allowlist, it will be installed automatically. This will be installed in a local data directory, so the subcommand will be native for the target architecture (required for these subcommands to work).
3. If the subcommand is not recognized, you can install it via cross-util. This will install it to the same directory as mentioned above. So for example, ~/.local/share/cross-rs/aarch64-unknown-linux-gnu/bin.
4. If the subcommand is used, then this install point will be mounted and the subcommand invoked directly (such as cargo-asm rather than using cargo asm). There might be a better design here, but it might require some wrappers to get it to work with calling cargo directly.
5. Subcommands can also be uninstalled via cross-util.

Now, the specifics. Installed subcommands will be registered in a JSON file in a general data directory (~/.local/share/cross-rs/subcommands.json). The general layout is quite simple:

{
    "aarch64-unknown-linux-gnu": [
        "asm",
        "deb"
    ]
}

This allows us to quickly look up the subcommand and if it's registered easily. We can also have cross-util list all registered subcommands, including for a given target. We can also likely add this to cross --list.

Thoughts? I've tested the general idea with cargo-asm, cargo-deb, and both work well.

[Emilgardis]

There might be a better design here, but it might require some wrappers to get it to work with calling cargo directly.

this is what cargo does, it doesn't set any extra env, only invokes the binary with cargo-tool tool <..args>

2. If you use the subcommand and it is not installed but is part of the allowlist,

I think the automatic installation should be replaced with a suggestion to install the tool with cross-util

[Alexhuszagh]

I think the automatic installation should be replaced with a suggestion to install the tool with cross-util

It would be handled by the same code as cross-util, and only invoked from a small list of known subcommands without external dependencies. Everything else would have to be handled independently.

[Emilgardis]

What i'm thinking is that we should probably preserve the same behaviour as cargo, cargo doesn't install automatically

[DASPRiD]

I assume there was no further progress made on this? Is there maybe a workaround at the moment which would allow running cargo-deb in cross without any direct cross support? Assuming using a custom docker image.

[Alexhuszagh]

I assume there was no further progress made on this? Is there maybe a workaround at the moment which would allow running cargo-deb in cross without any direct cross support? Assuming using a custom docker image.

There's a mostly working PR for this based off of #931 at https://github.com/Alexhuszagh/cross/tree/subcommands. I've still got notes on the implementation status, I'm just waiting for some dust to settle before rebasing (it touches too many things so I'll have to do it entirely manually), and it also requires cargo configs and aliases to be resolved (hence, #931). So there is a decent amount of progress, there's substantial effort right now in ensuring I have the time to ensure it is fully comprehensive and correct, that no more changes are made to the config settings in the meantime (which they currently are), and that another maintainer has time to review a massive PR.

That branch should be almost entirely working IIRC. It only works if runners for the target are available, such as Qemu, which in this case it is, however, it requires a few tricks for pre-installing the commands.

[Shnatsel]

this is what cargo does, it doesn't set any extra env, only invokes the binary with cargo-tool tool <..args>

This is not entirely true - Cargo does set one environment variable, CARGO.