Add vsock-backed SSH session reminders

Remind users when a VM is still running after 	hanger vm ssh exits instead of silently dropping them back to the host shell.\n\nAttach a Firecracker vsock device to each VM, persist the host vsock path/CID,\nadd a new guest-side banger-vsock-pingd responder to the runtime bundle and both\nimage-build paths, and expose a vm.ping RPC that the CLI and TUI call after SSH\nreturns. Doctor and start/build preflight now validate the helper plus\n/dev/vhost-vsock so the feature fails early and clearly.\n\nValidated with go mod tidy, bash -n customize.sh, git diff --check, make build,\nand GOCACHE=/tmp/banger-gocache go test ./... outside the sandbox because the\ndaemon tests need real Unix/UDP sockets. Rebuild the image/rootfs used for new\nVMs so the guest ping service is present.

internal/firecracker/client.go

@@ -1,17 +1,23 @@
 package firecracker
 
 import (
+	"bufio"
 	"context"
+	"fmt"
 	"io"
 	"log/slog"
 	"os"
 	"os/exec"
 	"strings"
 	"sync"
+	"time"
 
 	sdk "github.com/firecracker-microvm/firecracker-go-sdk"
 	models "github.com/firecracker-microvm/firecracker-go-sdk/client/models"
+	sdkvsock "github.com/firecracker-microvm/firecracker-go-sdk/vsock"
 	"github.com/sirupsen/logrus"
+
+	"banger/internal/vsockping"
 )
 
 type MachineConfig struct {
@@ -25,6 +31,8 @@ type MachineConfig struct {
 	KernelArgs      string
 	Drives          []DriveConfig
 	TapDevice       string
+	VSockPath       string
+	VSockCID        uint32
 	VCPUCount       int
 	MemoryMiB       int
 	Logger          *slog.Logger
@@ -132,6 +140,7 @@ func buildConfig(cfg MachineConfig) sdk.Config {
 				HostDevName: cfg.TapDevice,
 			},
 		}},
+		VsockDevices: buildVsockDevices(cfg),
 		MachineCfg: models.MachineConfiguration{
 			VcpuCount:  sdk.Int64(int64(cfg.VCPUCount)),
 			MemSizeMib: sdk.Int64(int64(cfg.MemoryMiB)),
@@ -141,6 +150,17 @@ func buildConfig(cfg MachineConfig) sdk.Config {
 	}
 }
 
+func buildVsockDevices(cfg MachineConfig) []sdk.VsockDevice {
+	if strings.TrimSpace(cfg.VSockPath) == "" || cfg.VSockCID == 0 {
+		return nil
+	}
+	return []sdk.VsockDevice{{
+		ID:   "vsock",
+		Path: cfg.VSockPath,
+		CID:  cfg.VSockCID,
+	}}
+}
+
 func splitDrives(drives []DriveConfig) (DriveConfig, []DriveConfig) {
 	root := DriveConfig{ID: "rootfs"}
 	var extras []DriveConfig
@@ -192,6 +212,39 @@ func newLogger(base *slog.Logger) *logrus.Entry {
 	return logrus.NewEntry(logger)
 }
 
+func PingVSock(ctx context.Context, logger *slog.Logger, socketPath string) error {
+	conn, err := sdkvsock.DialContext(
+		ctx,
+		socketPath,
+		vsockping.Port,
+		sdkvsock.WithRetryTimeout(3*time.Second),
+		sdkvsock.WithRetryInterval(100*time.Millisecond),
+		sdkvsock.WithLogger(newLogger(logger)),
+	)
+	if err != nil {
+		return err
+	}
+	defer conn.Close()
+
+	if deadline, ok := ctx.Deadline(); ok {
+		_ = conn.SetDeadline(deadline)
+	} else {
+		_ = conn.SetDeadline(time.Now().Add(3 * time.Second))
+	}
+
+	if _, err := io.WriteString(conn, vsockping.RequestLine); err != nil {
+		return err
+	}
+	line, err := bufio.NewReader(conn).ReadString('\n')
+	if err != nil {
+		return err
+	}
+	if strings.TrimSpace(line) != strings.TrimSpace(vsockping.ResponseLine) {
+		return fmt.Errorf("unexpected vsock response %q", strings.TrimSpace(line))
+	}
+	return nil
+}
+
 type slogHook struct {
 	logger *slog.Logger
 }