fix(plan): keep planner runtime off fleet workers

.github/actions/plan/action.yml

@@ -27,8 +27,26 @@ runs:
   steps:
     - id: expand
       shell: bash
-      run: >-
-        python3 "${GITHUB_ACTION_PATH}/plan.py"
-        --plan "${{ inputs.plan-path }}"
-        --group "${{ inputs.group }}"
-        --github-output "${GITHUB_OUTPUT}"
+      env:
+        CI_FLEET_PLAN_PATH: ${{ inputs.plan-path }}
+        CI_FLEET_PLAN_GROUP: ${{ inputs.group }}
+      run: |
+        set -Eeuo pipefail
+        tmp=$(mktemp -d)
+        trap 'rm -rf "$tmp"' EXIT
+        cp -- "${GITHUB_ACTION_PATH}/plan.py" "$tmp/plan.py"
+        cp -- "$CI_FLEET_PLAN_PATH" "$tmp/plan.json"
+        tar -C "$tmp" -cf - plan.py plan.json |
+          docker run --rm -i \
+            --network none \
+            --read-only \
+            --cap-drop all \
+            --security-opt no-new-privileges \
+            --pids-limit 64 \
+            --memory 128m \
+            --cpus 0.5 \
+            --user 65534:65534 \
+            --tmpfs /tmp:rw,noexec,nosuid,nodev,size=4m \
+            python:3.12.11-slim-bookworm@sha256:519591d6871b7bc437060736b9f7456b8731f1499a57e22e6c285135ae657bf7 \
+            sh -c 'tar -xf - -C /tmp && python3 /tmp/plan.py --plan /tmp/plan.json --group "$1" --github-output -' \
+            sh "$CI_FLEET_PLAN_GROUP" >>"$GITHUB_OUTPUT"

.github/actions/plan/plan.py

@@ -139,12 +139,18 @@ def main() -> int:
         print(f"ERROR: {exc}", file=sys.stderr)
         return 1
     encoded = json.dumps(matrix, separators=(",", ":"))
-    if args.github_output:
-        with args.github_output.open("a", encoding="utf-8") as output:
-            output.write(f"matrix={encoded}\n")
-            output.write(f"job-count={len(matrix['include'])}\n")
-            output.write(f"estimated-test-minutes={estimated_total}\n")
-    print(encoded)
+    output_lines = (
+        f"matrix={encoded}\n"
+        f"job-count={len(matrix['include'])}\n"
+        f"estimated-test-minutes={estimated_total}\n"
+    )
+    if args.github_output == Path("-"):
+        print(output_lines, end="")
+    else:
+        if args.github_output:
+            with args.github_output.open("a", encoding="utf-8") as output:
+                output.write(output_lines)
+        print(encoded)
     print(
         f"OK: {args.group} expands to {len(matrix['include'])} jobs "
         f"covering approximately {estimated_total} test-minutes",

.github/actions/plan/test_plan.py

@@ -5,6 +5,8 @@
 
 import copy
 import json
+import subprocess
+import sys
 import unittest
 from pathlib import Path
 
@@ -53,6 +55,38 @@ def test_fast_tasks_must_also_run_in_full(self) -> None:
         plan["tasks"][0]["groups"] = ["fast"]
         self.assert_rejected(plan, "whenever it includes fast")
 
+    def test_action_runs_planner_in_pinned_runtime_container(self) -> None:
+        action = (ROOT / ".github/actions/plan/action.yml").read_text(encoding="utf-8")
+        self.assertIn(
+            "python:3.12.11-slim-bookworm@sha256:519591d6871b7bc437060736b9f7456b8731f1499a57e22e6c285135ae657bf7",
+            action,
+        )
+        self.assertIn("--network none", action)
+        self.assertIn("--read-only", action)
+        self.assertIn("--cap-drop all", action)
+        self.assertNotIn('python3 "${GITHUB_ACTION_PATH}/plan.py"', action)
+
+    def test_cli_can_emit_github_outputs_to_stdout(self) -> None:
+        result = subprocess.run(
+            [
+                sys.executable,
+                str(ROOT / ".github/actions/plan/plan.py"),
+                "--plan",
+                str(ROOT / "examples/project/scripts/ci/plan.json"),
+                "--group",
+                "fast",
+                "--github-output",
+                "-",
+            ],
+            check=True,
+            capture_output=True,
+            text=True,
+        )
+        self.assertEqual(result.stdout.count("matrix="), 1)
+        self.assertIn("job-count=6\n", result.stdout)
+        self.assertIn("estimated-test-minutes=20\n", result.stdout)
+        self.assertNotIn("\n{", result.stdout)
+
     def test_matrix_limit_is_enforced(self) -> None:
         plan = sample_plan()
         plan["tasks"][0]["shards"] = 257