[CI] mismatch between available backend and dispatch

[sayakpaul]

Currently, the bot checks out from main when a comment is made on a PR:

kernels-community/.github/workflows/pr-comment-build.yaml

Line 20 in da8ff02

ref: ${{ github.event.repository.default_branch }}

Then dispatch.py is called through:

kernels-community/.github/workflows/pr-comment-build.yaml

Line 49 in da8ff02

python3 .github/scripts/pr_comment_kernel_bot.py

kernels-community/.github/scripts/pr_comment_kernel_bot.py

Line 12 in da8ff02

from dispatch import (

We have the following LOC in dispatch.py:

kernels-community/.github/scripts/dispatch.py

Lines 140 to 153 in da8ff02

	def read_backends(kernel_name: str) -> list[str] | None:
	"""Read the backends list from a kernel's build.toml. Returns None if not found."""
	build_toml = Path(kernel_name) / "build.toml"
	if not build_toml.exists():
	return None
	with open(build_toml, "rb") as f:
	config = tomllib.load(f)
	backends = config.get("general", {}).get("backends")
	if backends is None:
	backends = config.get("backends")
	if isinstance(backends, list):
	return backends
	return None

Since it's invoked from main, it means all release workflows will get triggered since backends can be none for kernels that don't exist on main yet:

kernels-community/.github/scripts/dispatch.py

Lines 167 to 170 in da8ff02

	backends = read_backends(kernel_name)
	if backends is None:
	print(f"Could not read backends for {kernel_name}, dispatching all workflows")
	return set(RELEASE_WORKFLOWS)

I think this is what happened here:
#919

The said kernel doesn't declare a "metal" backend but still, a build-mac.yaml was dispatched and it failed here: https://github.com/huggingface/kernels-community/actions/runs/27025228114 because it couldn't find metadata.

My comments are:

• The dispatch should probably be done based on the content of the PR branch (now that we are making it mandatory to trigger builds through the bot after an initial vetting)? cc: @drbh
• The build here succeeded but there was nothing produced: https://github.com/huggingface/kernels-community/actions/runs/27025228114/job/79763455748#step:12:61. Should it count valid? cc: @danieldk

[arnavnaik22]

Hey @sayakpaul! I can take this one. I'll update the checkout step in the PR comment workflow to pull the actual PR branch ref instead of main, so dispatch.py can correctly read the build.toml backends from the contributor's code. Could you assign this to me?