Check if another VM is running on machine start

Only one VM can be up at a time. If another VM is running, or the current VM is running, error out on a podman machine start

[NO TESTS NEEDED]

Signed-off-by: Ashley Cui <acui@redhat.com>

cmd/podman/machine/start.go

@@ -7,6 +7,7 @@ import (
 	"github.com/containers/podman/v3/pkg/domain/entities"
 	"github.com/containers/podman/v3/pkg/machine"
 	"github.com/containers/podman/v3/pkg/machine/qemu"
+	"github.com/pkg/errors"
 	"github.com/spf13/cobra"
 )
 
@@ -40,6 +41,18 @@ func start(cmd *cobra.Command, args []string) error {
 	if len(args) > 0 && len(args[0]) > 0 {
 		vmName = args[0]
 	}
+
+	// We only have qemu VM's for now
+	active, activeName, err := qemu.CheckActiveVM()
+	if err != nil {
+		return err
+	}
+	if active {
+		if vmName == activeName {
+			return errors.Wrapf(machine.ErrVMAlreadyRunning, "cannot start VM %s", vmName)
+		}
+		return errors.Wrapf(machine.ErrMultipleActiveVM, "cannot start VM %s. VM %s is currently running", vmName, activeName)
+	}
 	switch vmType {
 	default:
 		vm, err = qemu.LoadVMByName(vmName)

docs/source/markdown/podman-machine-start.1.md

@@ -14,6 +14,9 @@ Podman on MacOS requires a virtual machine. This is because containers are Linux
 containers do not run on any other OS because containers' core functionality are
 tied to the Linux kernel.
 
+Only one Podman managed VM can be active at a time. If a VM is already running,
+`podman machine start` will return an error.
+
 **podman machine start** starts a Linux virtual machine where containers are run.
 
 ## OPTIONS

pkg/machine/config.go

@@ -30,6 +30,8 @@ var (
 	DefaultIgnitionUserName                      = "core"
 	ErrNoSuchVM                                  = errors.New("VM does not exist")
 	ErrVMAlreadyExists                           = errors.New("VM already exists")
+	ErrVMAlreadyRunning                          = errors.New("VM already running")
+	ErrMultipleActiveVM                          = errors.New("only one VM can be active at a time")
 )
 
 type Download struct {

pkg/machine/qemu/machine.go

@@ -519,3 +519,17 @@ func IsValidVMName(name string) (bool, error) {
 	}
 	return false, nil
 }
+
+// CheckActiveVM checks if there is a VM already running
+func CheckActiveVM() (bool, string, error) {
+	vms, err := GetVMInfos()
+	if err != nil {
+		return false, "", errors.Wrap(err, "error checking VM active")
+	}
+	for _, vm := range vms {
+		if vm.Running {
+			return true, vm.Name, nil
+		}
+	}
+	return false, "", nil
+}