use Cargo.toml & Cargo.lock for sccache build dependency

[Be-ing]

No description provided.

[Swiftb0y]

Suggested change

	sccache = { git = "https://github.com/mozilla/sccache.git", branch = "master" }
	sccache = { git = "https://github.com/mozilla/sccache.git", branch = "master" }
	[lib]
	path = "non-existent" # required so `cargo update` runs

If you add this, you can remove the main.rs file. works on my machine but try it yourself as well please.

[Be-ing]

cool, thanks for this hack

[Be-ing]

This lets cargo update work but I think what we actually need on CI is cargo build.

[Swiftb0y]

Can't say much about that. I don't understand what this PR is supposed to do anyways. I just figured there must be a better way than leaving a dummy main.rs in the repo.

[Be-ing]

The purpose of this PR is to use Cargo.lock as the cache key instead of the ugly hack of a git commit hash which won't work well as we add more Rust dependencies.

[Swiftb0y]

Right, but why do we need to change cargo install --git https://github.com/mozilla/sccache.git --branch master

[Swiftb0y]

Sorry for asking stupid questions. The solution seems overcomplicated to me, but thats usually a sign that I don't know enough about the actual topic.

[Be-ing]

The current solution works for now. But it won't work when we also have other Rust dependencies to cache.

[Be-ing]

It seems that adding a binary as a dependency doesn't install it to $PATH:

-- Could NOT find sccache (missing executable)
-- Support for sccache: OFF

[Holzhaus]

It seems that adding a binary as a dependency doesn't install it to $PATH:

-- Could NOT find sccache (missing executable)
-- Support for sccache: OFF

Cargo installs to the home dir (on Linux: $HOME/.cargo/bin) and adds that path to $PATH. However, GitHub Actions does not preserve environment variables across job steps. You need to mark explicitly what env cars should be preserved.

This is why we do echo FOO=bar > "$GITHUB_ENV" in the other build steps.
https://docs.github.com/en/actions/reference/environment-variables#about-environment-variables

[Be-ing]

cargo install --path . only installs the mixxx executable but not sccache. cargo build builds sccache but not the executable...

[Be-ing]

I think cargo might not support this use case yet but it will be implemented soon... rust-lang/cargo#9096

[Holzhaus]

Can't we define an env var SCCACHE_VERSION, then use the env var in the cache key and use cargo install --git <url> --version %SCCACHE_VERSION% instead of using a lock file and a dummy package?

[uklotzde]

Can't we define an env var SCCACHE_VERSION, then use the env var in the cache key and use cargo install --git <url> --version %SCCACHE_VERSION% instead of using a lock file and a dummy package?

I agree. Referencing the git ref directly instead of using a dummy project might be the way to go in this special case. I didn't think about that.

Maybe obsolete then: Be-ing#55

[Be-ing]

Can't we define an env var SCCACHE_VERSION, then use the env var in the cache key and use cargo install --git --version %SCCACHE_VERSION% instead of using a lock file and a dummy package?

That would be a bit more complicated than the current implementation for no benefit.

Maybe we could just cache C:\Users\runneradmin\.cargo\bin\sccache instead of all of C:\Users\runneradmin\.cargo. That way we could also have separate caches for other Rust binaries (aoide).

[Be-ing]

I started a thread on the Rust forum asking for help.

[Holzhaus]

That would be a bit more complicated than the current implementation for no benefit.

Why? It would be a ~10 line diff, not a >3K line diff.

[Be-ing]

Your proposal has no advantage over simply using the commit hash as the cache key and referencing the Git branch with cargo install.

[Holzhaus]

Your proposal has no advantage over simply using the commit hash as the cache key and referencing the Git branch with cargo install.

That is exactly what I proposed?! Just save the commit hash in an env var to avoid duplication of that value for cache key and and cargo install. That way we don't have the risk of updating one but not the other.

[Be-ing]

There is no duplication of the commit hash currently. cargo install references the branch, not the commit.

[Holzhaus]

We should pin a specific commit. I don't want our CI builds failing because they introduced a breaking change or broke main upstream.

[Be-ing]

Okay then you can open another PR to do that.