fix(ci): bind backend candidates to exact main dispatch

.gitea/workflows/backend-build.yml

@@ -11,6 +11,7 @@ on:
         type: string
 
 env:
+  BUILD_LANE: candidate
   CARGO_TERM_COLOR: never
   PYTHONDONTWRITEBYTECODE: '1'
   EXPECTED_BUILD_SHA: ${{ inputs.expected_sha }}
@@ -29,6 +30,12 @@ jobs:
       RUSTUP_HOME: ${{ runner.temp }}/rustup-backend-candidate
       CARGO_HOME: ${{ runner.temp }}/cargo-backend-candidate
     steps:
+      - name: Require exact main dispatch before checkout
+        run: |
+          test "$GITHUB_EVENT_NAME" = workflow_dispatch || { echo 'Candidate event must be workflow_dispatch.' >&2; exit 1; }
+          test "$GITHUB_REF" = refs/heads/main || { echo 'Candidate dispatch must select main.' >&2; exit 1; }
+          [[ "$EXPECTED_BUILD_SHA" =~ ^[0-9a-f]{40}$ ]] || { echo 'Candidate expected_sha must be a full lowercase commit.' >&2; exit 1; }
+          test "$GITHUB_SHA" = "$EXPECTED_BUILD_SHA" || { echo 'Dispatch main moved from the reviewed expected_sha.' >&2; exit 1; }
       - uses: actions/checkout@v4
         with:
           fetch-depth: 0

.gitea/workflows/ci.yml

@@ -9,6 +9,7 @@ on:
     branches: [main]
 
 env:
+  BUILD_LANE: ci
   CARGO_TERM_COLOR: never
   PYTHONDONTWRITEBYTECODE: '1'
   EXPECTED_BUILD_SHA: ${{ github.event.pull_request.head.sha || github.sha }}

tools/check-build-source.py

@@ -1,6 +1,8 @@
 #!/usr/bin/env python3
 """Admit one reviewed canonical source or its exact Builds merge wrapper."""
+import json
 import os
+from pathlib import Path
 import re
 import subprocess
 import sys
@@ -11,7 +13,11 @@ class Refusal(ValueError):
 
 
 def git(*args):
-    return subprocess.check_output(["git", *args], text=True, stderr=subprocess.PIPE).strip()
+    try:
+        return subprocess.check_output(["git", *args], text=True, stderr=subprocess.PIPE).strip()
+    except subprocess.CalledProcessError as error:
+        cause = " ".join((error.stderr or "Git returned no diagnostic").split())[:600]
+        raise Refusal(f"Git evidence failed (exit {error.returncode}): {cause}") from None
 
 
 def commit(value, label):
@@ -48,16 +54,36 @@ def admit(source, expected_head=None, require_wrapper=False):
     return {"source_commit": source, "build_commit": head, "source_tree": tree}
 
 
-def main():
-    if os.environ.get("GITHUB_REPOSITORY") != "Builds/smirk-backend-core":
+def build_context(environment):
+    if environment.get("GITHUB_REPOSITORY") != "Builds/smirk-backend-core":
         raise Refusal("this workflow is admitted only on Builds/smirk-backend-core")
-    event = os.environ.get("GITHUB_EVENT_NAME")
-    expected = os.environ.get("EXPECTED_BUILD_SHA")
-    if not expected:
-        raise Refusal("this build requires the exact reviewed event or dispatch head")
-    if event not in {"push", "pull_request", "workflow_dispatch"}:
-        raise Refusal("this build event is not admitted")
-    wrapper = event != "pull_request"
+    event = environment.get("GITHUB_EVENT_NAME")
+    lane = environment.get("BUILD_LANE")
+    expected = commit(environment.get("EXPECTED_BUILD_SHA"), "expected event or dispatch SHA")
+    if lane == "candidate":
+        if event != "workflow_dispatch":
+            raise Refusal("candidate builds require workflow_dispatch")
+    elif lane == "ci":
+        if event not in {"push", "pull_request"}:
+            raise Refusal("fundless CI requires push or pull_request")
+    else:
+        raise Refusal("build lane is not admitted")
+    if event == "pull_request":
+        try:
+            payload = json.loads(Path(environment.get("GITHUB_EVENT_PATH", "")).read_text())
+            head = payload["pull_request"]["head"]["sha"]
+            base = payload["pull_request"]["base"]["ref"]
+        except (OSError, ValueError, KeyError, TypeError) as error:
+            raise Refusal(f"pull-request event evidence is unavailable: {type(error).__name__}") from None
+        if head != expected or base != "main":
+            raise Refusal("CI checkout does not match the exact main-targeted pull-request event head")
+    elif environment.get("GITHUB_REF") != "refs/heads/main" or environment.get("GITHUB_SHA") != expected:
+        raise Refusal("main event head must equal the reviewed full expected_sha")
+    return expected, event != "pull_request"
+
+
+def main():
+    expected, wrapper = build_context(os.environ)
     canonical = source_for_checkout(wrapper)
     result = admit(canonical, expected, require_wrapper=wrapper)
     print(f"Admitted canonical source {result['source_commit']} at Builds {result['build_commit']}")
@@ -68,5 +94,3 @@ if __name__ == "__main__":
         main()
     except Refusal as error:
         sys.exit(f"Build source refused: {error}")
-    except subprocess.CalledProcessError:
-        sys.exit("Build source refused: Git could not resolve the reviewed source or checkout")

tools/stage-backend-candidate.py

@@ -40,8 +40,10 @@ def main():
     parser.add_argument("--output", type=Path, required=True)
     args = parser.parse_args()
     os.chdir(ROOT)
-    admitted = source.admit(source.source_for_checkout(True),
-                            os.environ.get("EXPECTED_BUILD_SHA", ""), require_wrapper=True)
+    expected, wrapper = source.build_context(os.environ)
+    if not wrapper or os.environ.get("BUILD_LANE") != "candidate":
+        raise source.Refusal("artifact staging requires the admitted candidate dispatch lane")
+    admitted = source.admit(source.source_for_checkout(True), expected, require_wrapper=True)
     binary = ROOT / "target/x86_64-unknown-linux-gnu/release/smirk-backend-core"
     required = elf_requirements(binary)
     rustc = subprocess.check_output(["rustc", "--version"], text=True).strip()

tools/tests/test_build_admission.py

@@ -101,6 +101,53 @@ class Admission(unittest.TestCase):
                 source.main()
 
 
+class EventBoundary(unittest.TestCase):
+    def setUp(self):
+        self.head = "a" * 40
+        self.environment = {"GITHUB_REPOSITORY": "Builds/smirk-backend-core",
+                            "BUILD_LANE": "candidate", "GITHUB_EVENT_NAME": "workflow_dispatch",
+                            "GITHUB_REF": "refs/heads/main", "GITHUB_SHA": self.head,
+                            "EXPECTED_BUILD_SHA": self.head}
+
+    def test_exact_dispatch_event_admitted(self):
+        self.assertEqual(source.build_context(self.environment), (self.head, True))
+
+    def test_bad_ref_event_sha_or_malformed_input_refused(self):
+        for key, value in (("GITHUB_REF", "refs/heads/topic"), ("GITHUB_EVENT_NAME", "push"),
+                           ("GITHUB_SHA", "b" * 40), ("EXPECTED_BUILD_SHA", "main"),
+                           ("EXPECTED_BUILD_SHA", "a" * 39), ("BUILD_LANE", "unknown")):
+            with self.subTest(key=key, value=value), self.assertRaises(source.Refusal):
+                source.build_context({**self.environment, key: value})
+
+    def test_push_requires_main_and_exact_event_sha(self):
+        env = {**self.environment, "BUILD_LANE": "ci", "GITHUB_EVENT_NAME": "push"}
+        self.assertEqual(source.build_context(env), (self.head, True))
+        with self.assertRaises(source.Refusal):
+            source.build_context({**env, "GITHUB_REF": "refs/heads/topic"})
+
+    def test_pr_binds_actual_event_head_and_target(self):
+        import json
+        with tempfile.TemporaryDirectory() as directory:
+            event = Path(directory) / "event.json"
+            event.write_text(json.dumps({"pull_request": {"head": {"sha": self.head}, "base": {"ref": "main"}}}))
+            env = {**self.environment, "BUILD_LANE": "ci", "GITHUB_EVENT_NAME": "pull_request",
+                   "GITHUB_EVENT_PATH": str(event)}
+            self.assertEqual(source.build_context(env), (self.head, False))
+            with self.assertRaises(source.Refusal):
+                source.build_context({**env, "EXPECTED_BUILD_SHA": "b" * 40})
+            event.write_text(json.dumps({"pull_request": {"head": {"sha": self.head}, "base": {"ref": "topic"}}}))
+            with self.assertRaises(source.Refusal):
+                source.build_context(env)
+
+    def test_git_refusal_reports_bounded_underlying_cause(self):
+        failure = subprocess.CalledProcessError(128, ["git"], stderr="fatal: object unavailable " + "x" * 2000)
+        with patch.object(subprocess, "check_output", side_effect=failure):
+            with self.assertRaises(source.Refusal) as caught:
+                source.git("rev-parse", "HEAD")
+        self.assertIn("object unavailable", str(caught.exception))
+        self.assertLess(len(str(caught.exception)), 700)
+
+
 class BinaryCompatibility(unittest.TestCase):
     def setUp(self):
         self.temp = tempfile.TemporaryDirectory(prefix="smirk-elf-fixture-")