fix(firecracker): make vsock field a pointer so omitempty drops it without vAccel

pkg/unikontainers/hypervisors/firecracker.go

@@ -73,7 +73,7 @@ type FirecrackerConfig struct {
 	Machine FirecrackerMachine    `json:"machine-config"`
 	Drives  []FirecrackerDrive    `json:"drives"`
 	NetIfs  []FirecrackerNet      `json:"network-interfaces,omitempty"`
-	VSock   FirecrackerVSockDev   `json:"vsock,omitempty"`
+	VSock   *FirecrackerVSockDev  `json:"vsock,omitempty"`
 }
 
 func (fc *Firecracker) Signal(pid int, signal unix.Signal) error {
@@ -175,9 +175,9 @@ func (fc *Firecracker) BuildExecCmd(args types.ExecArgs, ukernel types.Unikernel
 		InitrdPath: initrdPath,
 	}
 
-	var FCVSockDev FirecrackerVSockDev
+	var FCVSockDev *FirecrackerVSockDev
 	if args.VAccelType == "vsock" {
-		FCVSockDev = FirecrackerVSockDev{
+		FCVSockDev = &FirecrackerVSockDev{
 			GuestCID: args.VSockDevID,
 			UDSPath:  args.VSockDevPath + "/vaccel.sock",
 			VSockID:  "root",

pkg/unikontainers/hypervisors/firecracker_test.go

@@ -0,0 +1,97 @@
+// Copyright (c) 2023-2026, Nubificus LTD
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package hypervisors
+
+import (
+	"encoding/json"
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/urunc-dev/urunc/pkg/unikontainers/types"
+)
+
+const testFCBinary = "/usr/bin/firecracker"
+
+// configPathFromArgs pulls the JSON config path out of the args returned by
+// BuildExecCmd (the element right after "--config-file").
+func configPathFromArgs(t *testing.T, args []string) string {
+	t.Helper()
+	for i, a := range args {
+		if a == "--config-file" && i+1 < len(args) {
+			return args[i+1]
+		}
+	}
+	t.Fatalf("no --config-file argument in %v", args)
+	return ""
+}
+
+// TestFirecrackerBuildExecCmdVSock verifies that the generated Firecracker
+// config only contains a "vsock" device when vAccel/vsock is requested. The
+// field is tagged omitempty, so without vAccel the key must be absent.
+func TestFirecrackerBuildExecCmdVSock(t *testing.T) {
+	// Not parallel: BuildExecCmd writes to a fixed /tmp path.
+	tests := []struct {
+		name      string
+		args      types.ExecArgs
+		wantVSock bool
+	}{
+		{
+			name: "no vAccel omits the vsock device",
+			args: types.ExecArgs{
+				UnikernelPath: testKernelPath,
+				Command:       testCommand,
+			},
+			wantVSock: false,
+		},
+		{
+			name: "vAccel vsock includes the vsock device",
+			args: types.ExecArgs{
+				UnikernelPath: testKernelPath,
+				Command:       testCommand,
+				VAccelType:    "vsock",
+				VSockDevID:    42,
+				VSockDevPath:  "/run/vaccel",
+			},
+			wantVSock: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			fc := &Firecracker{binary: FirecrackerBinary, binaryPath: testFCBinary}
+			out, err := fc.BuildExecCmd(tt.args, &fakeUnikernel{})
+			assert.NoError(t, err)
+
+			cfgPath := configPathFromArgs(t, out)
+			t.Cleanup(func() { _ = os.Remove(cfgPath) })
+
+			data, err := os.ReadFile(cfgPath)
+			assert.NoError(t, err)
+
+			var cfg map[string]json.RawMessage
+			assert.NoError(t, json.Unmarshal(data, &cfg))
+
+			_, present := cfg["vsock"]
+			assert.Equal(t, tt.wantVSock, present,
+				"unexpected vsock presence in generated config: %s", string(data))
+
+			if tt.wantVSock {
+				assert.Contains(t, string(cfg["vsock"]), `"guest_cid":42`,
+					"vsock device should carry the configured guest CID")
+			}
+		})
+	}
+}