Add rustc_hash output

[Kijewski]

I.e. the a8314ef7d part in rustc 1.62.0 (a8314ef7d 2022-06-27).
This makes it possible to use the output e.g. as a cache key.
actions-rs/toolchain has the same output with the same function.

[dtolnay]

Wouldn't version work just as well for a cache key?

[Kijewski]

No, because version contains spaces, and cache keys are space separated, also.

I guess you could replace the spaces by underscores or dashes instead of extracting the git revision, but I don't think there would be much advantage in that.

[dtolnay]

Ah okay. I have not used the actions/cache action but I am checking what you are doing in https://github.com/Kijewski/tzdb/blob/v0.2.7/.github/actions/setup-rust/action.yaml with whitespace separation in restore-keys.

From https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#input-parameters-for-the-cache-action and the following code in actions/cache:

const restoreKeys = utils.getInputAsArray(Inputs.RestoreKeys);

export function getInputAsArray(
    name: string,
    options?: core.InputOptions
): string[] {
    return core
        .getInput(name, options)
        .split("\n")
        .map(s => s.trim())
        .filter(x => x !== "");
}

it sounds like the restore keys are separated by newline, so spaces might not be an issue. A better reason against ${{steps.toolchain-install.outputs.version}} for this is just that it's verbose to read in stdout of the action, along with the following:

Keys have a maximum length of 512 characters, and keys longer than the maximum length will cause the action to fail.

meaning that shorter (than rustc --version) would generally be better for forming part of a cache key, like you are doing.

I think my preference would be to expose something dedicated to the cache key use case rather than exposing the git commit hash that rustc was built from. Perhaps something like this to avoid any fiddly parsing:

$(rustc --version | sha256sum | head -c9)

Another observation: it may be more useful to include the date in the cache key, as opposed to the rustc repo hash. Rationale: look at https://github.com/Kijewski/tzdb/runs/7307104972:

Run actions/cache@v3
  with:
    path:
      ~/.cargo/registry/index/
      ~/.cargo/registry/cache/
      ~/.cargo/git/db/
      target/
    key:
      check-Linux-nightly-38b72154d-ba5a620a25fdb47156afec842d6c821f6d4e75085e970e92a6b5e3d10a6f191c
    restore-keys:
      check-Linux-nightly-38b72154d-
      check-Linux-nightly-

/usr/bin/tar --use-compress-program zstd -d -xf /home/runner/work/_temp/51d7cea1-713d-4ba1-bb18-a142fca7b823/cache.tzst -P -C /home/runner/work/tzdb/tzdb
Cache restored successfully
Cache restored from key: check-Linux-nightly-7b46aa594-8b742841c3337363f21f56f2914fdeb4cba421d49f608d42ee85ae149fe863db

We can tell from this that it loaded some cache older than the one that was requested, because 38b72154d and 7b46aa594 are not the same. How much older?— no idea. If the presence of the old cached target/ is relevant to reproducing some job failure, we would need to install a bunch of nightlies with rustup to hunt for that hash, or try to deduce it from the rust-lang/rust commit history or your repo's CI history. An output like 20220701 would tend to be more helpful to directly show which older toolchain's cache output got loaded.

[Kijewski]

Ah, I was quite sure that any whitespaces are used to split the input, but the code shows it does not.

Making rustc_hash return the short hash would make your action a drop-in replacement for actions-rs, but maybe that is not needed. Using the date in the cache key seems like a good idea! I added a commit that makes the other outputs of rustc --verbose --version available, so the user can decide whether to use the revision, the date or a combination thereof.

[dtolnay]

I added a compact cache key output based on just the date and commit hash. Would this address your use case, or is there still a need to have commit-hash and abbrev-hash and commit-date and release as separate outputs?

rust-toolchain/action.yml

Lines 20 to 22 in 1ce4a73

	cachekey:
	description: A short hash of the rustc version, appropriate for use as a cache key. "20220627a831"
	value: ${{steps.rustc-version.outputs.cachekey}}

rust-toolchain/action.yml

Lines 53 to 55 in 1ce4a73

	DATE=$(rustc +${{inputs.toolchain}} --version --verbose | sed -ne 's/^commit-date: \(20[0-9][0-9]\)-\([01][0-9]\)-\([0-3][0-9]\)$/\1\2\3/p')
	HASH=$(rustc +${{inputs.toolchain}} --version --verbose | sed -ne 's/^commit-hash: //p')
	echo "::set-output name=cachekey::$(echo $DATE$HASH | head -c12)"

[Kijewski]

Thank you! The cachekey output works perfectly fine for me.

I only added the other outputs, because, well, the values exists. I don't have a use for the other outputs, and if someone does, I guess they will open an issue.