feat: prefer reading the build toml from current ref

.github/scripts/dispatch.py

@@ -95,12 +95,25 @@ class DispatchPlan:
 # Reading build.toml and selecting workflows
 
 
-def read_backends(kernel_name: str) -> list[str] | None:
-    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)
+def read_backends(kernel_name: str, ref: str = "") -> list[str] | None:
+    # ref reads build.toml from that revision (the PR branch), not the working tree.
+    if ref:
+        try:
+            raw = subprocess.run(
+                ["git", "show", f"{ref}:{kernel_name}/build.toml"],
+                capture_output=True,
+                text=True,
+                check=True,
+            ).stdout
+        except (FileNotFoundError, subprocess.CalledProcessError):
+            return None
+        config = tomllib.loads(raw)
+    else:
+        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")
@@ -109,8 +122,10 @@ def read_backends(kernel_name: str) -> list[str] | None:
     return None
 
 
-def select_workflows(kernel_name: str, *, notes: list[str]) -> set[str]:
-    backends = read_backends(kernel_name)
+def select_workflows(
+    kernel_name: str, *, notes: list[str], metadata_ref: str = ""
+) -> set[str]:
+    backends = read_backends(kernel_name, metadata_ref)
     if backends is None:
         notes.append(
             f"Could not read backends for {kernel_name}, dispatching all workflows"
@@ -232,9 +247,12 @@ def _plan_build_actions(
     head_sha: str,
     target_branch: str,
     upload: bool,
+    metadata_ref: str,
 ) -> None:
-    backends = read_backends(kernel_name) or []
-    workflows = select_workflows(kernel_name, notes=plan.notes)
+    backends = read_backends(kernel_name, metadata_ref) or []
+    workflows = select_workflows(
+        kernel_name, notes=plan.notes, metadata_ref=metadata_ref
+    )
     plan.skipped = sorted(set(WORKFLOWS["build"]) - workflows)
 
     for workflow in sorted(workflows):
@@ -294,6 +312,7 @@ def plan_dispatch(
     upload: bool = True,
     run_security: bool = False,
     security_only: bool = False,
+    metadata_ref: str = "",
 ) -> DispatchPlan:
     want_security = run_security or security_only
     plan = DispatchPlan(kernel_name=kernel_name, head_sha=head_sha)
@@ -306,6 +325,7 @@ def plan_dispatch(
             mode=mode,
             repo_prefix=repo_prefix,
             dispatch_key_prefix=dispatch_key_prefix,
+            metadata_ref=metadata_ref,
             skip_build=skip_build,
             pr_number=pr_number,
             head_sha=head_sha,
@@ -460,6 +480,7 @@ def dispatch(
     upload: bool = True,
     run_security: bool = False,
     security_only: bool = False,
+    metadata_ref: str = "",
 ) -> DispatchResult:
     if not security_only and (not kernel_name or not KERNEL_NAME_RE.match(kernel_name)):
         result = DispatchResult(kernel_name=kernel_name)
@@ -481,6 +502,7 @@ def dispatch(
         upload=upload,
         run_security=run_security,
         security_only=security_only,
+        metadata_ref=metadata_ref,
     )
     if dry_run:
         return _result_from_plan(plan)

.github/scripts/pr_comment_kernel_bot.py

@@ -878,6 +878,8 @@ def main(*, dry_run: bool = False):
             # The audit is per-PR, so request it only once (on the first kernel).
             run_security=run_security and index == 0,
             dry_run=dry_run,
+            # Read backends from the PR commit (dry-run reads the working tree).
+            metadata_ref="" if dry_run else (pr_head_sha or ""),
         )
         emit_dispatch_diagnostics(release_result, dry_run=dry_run)
         for wf, dk in release_result.dispatched:

.github/scripts/test_dispatch.py

@@ -1,5 +1,6 @@
 import io
 import os
+import subprocess
 import sys
 import unittest
 import urllib.error
@@ -68,6 +69,31 @@ def test_security_only_plans_only_security(self):
             self.assertEqual(action.body["inputs"]["head_sha"], "deadbeef")
 
 
+# Metadata read from a git ref (PR branch) instead of the working tree.
+class MetadataRefTest(unittest.TestCase):
+    def test_read_backends_from_ref_uses_git_show(self):
+        run = mock.Mock(return_value=mock.Mock(stdout='backends = ["cuda", "rocm"]\n'))
+        with mock.patch.object(dispatch.subprocess, "run", run):
+            backends = dispatch.read_backends("somekernel", ref="abc123")
+        self.assertEqual(backends, ["cuda", "rocm"])
+        self.assertEqual(
+            run.call_args.args[0], ["git", "show", "abc123:somekernel/build.toml"]
+        )
+
+    def test_read_backends_from_ref_missing_returns_none(self):
+        err = subprocess.CalledProcessError(128, ["git", "show"])
+        with mock.patch.object(dispatch.subprocess, "run", side_effect=err):
+            self.assertIsNone(dispatch.read_backends("missing", ref="abc123"))
+
+    def test_metadata_ref_routes_metal_less_kernel_without_mac_build(self):
+        # Regression: a cuda-only kernel read from the ref must not trigger build-mac.yaml.
+        run = mock.Mock(return_value=mock.Mock(stdout='backends = ["cuda"]\n'))
+        with mock.patch.object(dispatch.subprocess, "run", run):
+            plan = dispatch.plan_dispatch("newkernel", metadata_ref="prsha")
+        builds = _workflows([a for a in plan.actions if a.kind == "build"])
+        self.assertEqual(builds, ["build.yaml"])
+
+
 # Orchestration: kernel-name validation and the dry-run no-I/O contract.
 class DispatchOrchestratorTest(unittest.TestCase):
     def test_invalid_kernel_name_marks_all_builds_failed(self):

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

@@ -19,6 +19,9 @@ jobs:
         with:
           ref: ${{ github.event.repository.default_branch }}
           fetch-depth: 1
+      - name: Fetch PR head for metadata
+        # Make the PR commit available so the bot can read its build.toml (data only).
+        run: git fetch --depth=1 origin "refs/pull/${{ github.event.issue.number }}/head"
       - name: Handle /kernel-bot command
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}