__init__

uvm/firecracker/config.py

@@ -0,0 +1,94 @@
+"""Build and persist Firecracker configuration for one VM."""
+
+from __future__ import annotations
+
+import json
+import math
+import os
+import tempfile
+from collections.abc import Mapping
+from ipaddress import IPv4Address, IPv4Network
+from pathlib import Path
+from typing import Any
+
+from ..config import Settings
+from ..domain import VmRecord
+
+
+def vcpu_count(cpu: float) -> int:
+    """Firecracker accepts whole vCPUs; fractional capacity rounds up for now."""
+
+    return max(1, math.ceil(cpu))
+
+
+def boot_ip_argument(
+    guest_ip: IPv4Address,
+    gateway: IPv4Address,
+    network: IPv4Network,
+) -> str:
+    return f"ip={guest_ip}::{gateway}:{network.netmask}::eth0:off"
+
+
+def build_config(
+    settings: Settings,
+    vm: VmRecord,
+    kernel: Path,
+    rootfs: Path,
+) -> dict[str, Any]:
+    boot_args = " ".join(
+        (
+            "console=ttyS0",
+            "reboot=k",
+            "panic=1",
+            "pci=off",
+            boot_ip_argument(IPv4Address(vm.guest_ip), settings.gateway, settings.network),
+        )
+    )
+    return {
+        "boot-source": {
+            "kernel_image_path": str(kernel),
+            "boot_args": boot_args,
+        },
+        "drives": [
+            {
+                "drive_id": "rootfs",
+                "path_on_host": str(rootfs),
+                "is_root_device": True,
+                "is_read_only": False,
+            }
+        ],
+        "machine-config": {
+            "vcpu_count": vcpu_count(vm.cpu),
+            "mem_size_mib": vm.ram_mib,
+        },
+        "network-interfaces": [
+            {
+                "iface_id": "eth0",
+                "guest_mac": vm.mac,
+                "host_dev_name": vm.tap,
+            }
+        ],
+    }
+
+
+def write_config(path: Path, config: Mapping[str, Any]) -> None:
+    """Atomically publish a rendered Firecracker configuration file."""
+
+    path.parent.mkdir(parents=True, exist_ok=True)
+    descriptor, temporary_name = tempfile.mkstemp(
+        prefix=f".{path.name}.",
+        suffix=".tmp",
+        dir=path.parent,
+    )
+    try:
+        with os.fdopen(descriptor, "w", encoding="utf-8") as temporary_file:
+            json.dump(config, temporary_file, indent=2)
+            temporary_file.write("\n")
+            temporary_file.flush()
+            os.fsync(temporary_file.fileno())
+        os.replace(temporary_name, path)
+    finally:
+        try:
+            os.unlink(temporary_name)
+        except FileNotFoundError:
+            pass