lxd: system bringup appears to be britle

```
2025-03-19T16:42:48.3650957Z 2025-03-19 16:42:48 Preparing google:ubuntu-20.04-64:tests/reboot (mar191639-896695)...
2025-03-19T16:42:48.6455223Z 2025-03-19 16:42:48 Executing google:ubuntu-20.04-64:tests/reboot (mar191639-896695) (1/10)...
2025-03-19T16:44:24.0828409Z 2025-03-19 16:44:24 Error executing google:ubuntu-20.04-64:tests/reboot (mar191639-896695) : + spread -vv -reuse -resend
2025-03-19T16:44:24.3632779Z 2025-03-19 16:44:24 Debug output for google:ubuntu-20.04-64:tests/reboot (mar191639-896695) :
2025-03-19T16:44:24.3634356Z -----
2025-03-19T16:44:24.3635168Z + cat task.out
2025-03-19T16:44:24.3636206Z 2025-03-19 16:42:48 Trying to read /home/test/src/github.com/snapcore/spread/tests/reboot/spread.yaml...
2025-03-19T16:44:24.3637928Z 2025-03-19 16:42:48 Found /home/test/src/github.com/snapcore/spread/tests/reboot/spread.yaml.
2025-03-19T16:44:24.3639186Z 2025-03-19 16:42:48 Loaded project: &spread.Project{
2025-03-19T16:44:24.3639984Z Name: "spread",
2025-03-19T16:44:24.3640480Z Backends: {
2025-03-19T16:44:24.3641160Z "lxd": &spread.Backend{
2025-03-19T16:44:24.3641801Z Name: "lxd",
2025-03-19T16:44:24.3642462Z Type: "lxd",
2025-03-19T16:44:24.3643011Z Key: "",
2025-03-19T16:44:24.3643589Z Allocate: "",
2025-03-19T16:44:24.3644235Z Discard: "",
2025-03-19T16:44:24.3644876Z Memory: 0,
2025-03-19T16:44:24.3645602Z Plan: "",
2025-03-19T16:44:24.3646231Z Location: "",
2025-03-19T16:44:24.3646865Z Storage: 0,
2025-03-19T16:44:24.3648173Z Systems: {
2025-03-19T16:44:24.3648964Z "ubuntu-16.04": &spread.System{
2025-03-19T16:44:24.3649749Z Backend: "lxd",
2025-03-19T16:44:24.3650867Z Name: "ubuntu-16.04",
2025-03-19T16:44:24.3651602Z Image: "ubuntu-16.04",
2025-03-19T16:44:24.3652208Z Kernel: "",
2025-03-19T16:44:24.3652755Z Username: "",
2025-03-19T16:44:24.3653541Z Password: "",
2025-03-19T16:44:24.3654086Z Workers: 1,
2025-03-19T16:44:24.3654661Z Storage: 0,
2025-03-19T16:44:24.3655231Z SecureBoot: false,
2025-03-19T16:44:24.3655793Z Bios: "",
2025-03-19T16:44:24.3656331Z CPUFamily: "",
2025-03-19T16:44:24.3656934Z Plan: "",
2025-03-19T16:44:24.3657657Z Environment: &spread.Environment{
2025-03-19T16:44:24.3658377Z err: nil,
2025-03-19T16:44:24.3659010Z keys: {},
2025-03-19T16:44:24.3660118Z vals: {},
2025-03-19T16:44:24.3660847Z },
2025-03-19T16:44:24.3661315Z Variants: nil,
2025-03-19T16:44:24.3661949Z Priority: spread.OptionalInt{},
2025-03-19T16:44:24.3663571Z Manual: false,
2025-03-19T16:44:24.3664126Z Vm: false,
2025-03-19T16:44:24.3664653Z },
2025-03-19T16:44:24.3665228Z },
2025-03-19T16:44:24.3665735Z Prepare: "",
2025-03-19T16:44:24.3666249Z Restore: "",
2025-03-19T16:44:24.3666785Z Debug: "",
2025-03-19T16:44:24.3724880Z PrepareEach: "",
2025-03-19T16:44:24.3725842Z RestoreEach: "",
2025-03-19T16:44:24.3726482Z DebugEach: "",
2025-03-19T16:44:24.3727071Z Environment: &spread.Environment{
2025-03-19T16:44:24.3727841Z err: nil,
2025-03-19T16:44:24.3728424Z keys: {},
2025-03-19T16:44:24.3729083Z vals: {},
2025-03-19T16:44:24.3729766Z },
2025-03-19T16:44:24.3730308Z Variants: nil,
2025-03-19T16:44:24.3731029Z WarnTimeout: spread.Timeout{},
2025-03-19T16:44:24.3731667Z KillTimeout: spread.Timeout{},
2025-03-19T16:44:24.3732405Z HaltTimeout: spread.Timeout{},
2025-03-19T16:44:24.3733065Z Priority: spread.OptionalInt{},
2025-03-19T16:44:24.3733945Z Manual: false,
2025-03-19T16:44:24.3734692Z Vm: false,
2025-03-19T16:44:24.3735203Z },
2025-03-19T16:44:24.3735699Z },
2025-03-19T16:44:24.3736306Z Environment: &spread.Environment{
2025-03-19T16:44:24.3736889Z err: nil,
2025-03-19T16:44:24.3737476Z keys: {},
2025-03-19T16:44:24.3738002Z vals: {},
2025-03-19T16:44:24.3738442Z },
2025-03-19T16:44:24.3738858Z Repack: "",
2025-03-19T16:44:24.3739563Z Reroot: "",
2025-03-19T16:44:24.3740275Z Prepare: "",
2025-03-19T16:44:24.3741078Z Restore: "",
2025-03-19T16:44:24.3741671Z Debug: "",
2025-03-19T16:44:24.3742661Z PrepareEach: "",
2025-03-19T16:44:24.3743171Z RestoreEach: "",
2025-03-19T16:44:24.3743638Z DebugEach: "",
2025-03-19T16:44:24.3744103Z Suites: {
2025-03-19T16:44:24.3744606Z "checks/": &spread.Suite{
2025-03-19T16:44:24.3745200Z Summary: "Verification tasks.",
2025-03-19T16:44:24.3745824Z Systems: nil,
2025-03-19T16:44:24.3746423Z Backends: nil,
2025-03-19T16:44:24.3746965Z Variants: nil,
2025-03-19T16:44:24.3747726Z Environment: &spread.Environment{
2025-03-19T16:44:24.3748417Z err: nil,
2025-03-19T16:44:24.3748989Z keys: {},
2025-03-19T16:44:24.3749512Z vals: {},
2025-03-19T16:44:24.3750711Z },
2025-03-19T16:44:24.3751163Z Prepare: "",
2025-03-19T16:44:24.3751744Z Restore: "",
2025-03-19T16:44:24.3752406Z Debug: "",
2025-03-19T16:44:24.3753018Z PrepareEach: "",
2025-03-19T16:44:24.3753629Z RestoreEach: "",
2025-03-19T16:44:24.3754523Z DebugEach: "",
2025-03-19T16:44:24.3755153Z Name: "checks/",
2025-03-19T16:44:24.3756044Z Path: "/home/test/src/github.com/snapcore/spread/tests/reboot/checks",
2025-03-19T16:44:24.3757006Z Tasks: {
2025-03-19T16:44:24.3757538Z "main": &spread.Task{
2025-03-19T16:44:24.3758132Z Suite: "checks/",
2025-03-19T16:44:24.3758884Z Summary: "Ensure it works.",
2025-03-19T16:44:24.3759574Z Details: "",
2025-03-19T16:44:24.3760154Z Systems: nil,
2025-03-19T16:44:24.3760877Z Backends: nil,
2025-03-19T16:44:24.3761594Z Variants: nil,
2025-03-19T16:44:24.3762325Z Environment: &spread.Environment{
2025-03-19T16:44:24.3763467Z err: nil,
2025-03-19T16:44:24.3764058Z keys: {},
2025-03-19T16:44:24.3764754Z vals: {},
2025-03-19T16:44:24.3765371Z },
2025-03-19T16:44:24.3765977Z Samples: 1,
2025-03-19T16:44:24.3766740Z Prepare: "",
2025-03-19T16:44:24.3767417Z Restore: "",
2025-03-19T16:44:24.3769639Z Execute: "echo \"REBOOT=$SPREAD_REBOOT\"\ncase $SPREAD_REBOOT in\n0)\n REBOOT\n ;;\n1)\n REBOOT\n ;;\n2)\n REBOOT foo\n ;;\nfoo)\n REBOOT bar\n ;;\nbar)\n echo WORKS\n ;;\n*)\n echo 'Unexpected $SPREAD_REBOOT value:' $SPREAD_REBOOT\n exit 1\n ;;\nesac\n",
2025-03-19T16:44:24.3771723Z Debug: "",
2025-03-19T16:44:24.3772769Z Artifacts: nil,
2025-03-19T16:44:24.3773501Z Name: "checks/main",
2025-03-19T16:44:24.3774622Z Path: "/home/test/src/github.com/snapcore/spread/tests/reboot/checks/main",
2025-03-19T16:44:24.3775587Z WarnTimeout: spread.Timeout{},
2025-03-19T16:44:24.3776405Z KillTimeout: spread.Timeout{},
2025-03-19T16:44:24.3777226Z Priority: spread.OptionalInt{},
2025-03-19T16:44:24.3778101Z Manual: false,
2025-03-19T16:44:24.3778813Z },
2025-03-19T16:44:24.3779330Z },
2025-03-19T16:44:24.3779859Z WarnTimeout: spread.Timeout{},
2025-03-19T16:44:24.3780630Z KillTimeout: spread.Timeout{},
2025-03-19T16:44:24.3781334Z Priority: spread.OptionalInt{},
2025-03-19T16:44:24.3781935Z Manual: false,
2025-03-19T16:44:24.3782733Z },
2025-03-19T16:44:24.3783200Z },
2025-03-19T16:44:24.3783640Z RemotePath: "/home/test",
2025-03-19T16:44:24.3784124Z Include: nil,
2025-03-19T16:44:24.3784720Z Exclude: nil,
2025-03-19T16:44:24.3785263Z Rename: nil,
2025-03-19T16:44:24.3786028Z Path: "/home/test/src/github.com/snapcore/spread/tests/reboot",
2025-03-19T16:44:24.3786925Z WarnTimeout: spread.Timeout{},
2025-03-19T16:44:24.3787634Z KillTimeout: spread.Timeout{},
2025-03-19T16:44:24.3788275Z }
2025-03-19T16:44:24.3788797Z 2025-03-19 16:42:48 Project content is packed for delivery (1.54KB).
2025-03-19T16:44:24.3789843Z 2025-03-19 16:42:48 Starting 1 worker for the following jobs: 1
2025-03-19T16:44:24.3790600Z - lxd:ubuntu-16.04:checks/main
2025-03-19T16:44:24.3791272Z 2025-03-19 16:42:48 Allocating lxd:ubuntu-16.04...
2025-03-19T16:44:24.3792141Z 2025-03-19 16:42:49 Cannot find cached LXD image for lxd:ubuntu-16.04.
2025-03-19T16:44:24.3794366Z 2025-03-19 16:43:18 Waiting for lxd container spread-1-ubuntu-16-04 to have an address...
2025-03-19T16:44:24.3796121Z 2025-03-19 16:43:18 lxd list output: []*spread.lxdServerJSON{
2025-03-19T16:44:24.3796877Z &spread.lxdServerJSON{
2025-03-19T16:44:24.3797491Z Name: "spread-1-ubuntu-16-04",
2025-03-19T16:44:24.3798526Z State: struct { Network map[string]spread.lxdDeviceJSON "json:\"network\"" }{
2025-03-19T16:44:24.3799406Z Network: {
2025-03-19T16:44:24.3799910Z "eth0": {
2025-03-19T16:44:24.3800449Z State: "up",
2025-03-19T16:44:24.3801045Z Addresses: {
2025-03-19T16:44:24.3810697Z {Family:"inet6", Address:"fe80::216:3eff:fe73:4f61"},
2025-03-19T16:44:24.3811467Z },
2025-03-19T16:44:24.3812057Z },
2025-03-19T16:44:24.3812533Z "lo": {
2025-03-19T16:44:24.3813143Z State: "up",
2025-03-19T16:44:24.3813813Z Addresses: {
2025-03-19T16:44:24.3814447Z {Family:"inet", Address:"127.0.0.1"},
2025-03-19T16:44:24.3815392Z {Family:"inet6", Address:"::1"},
2025-03-19T16:44:24.3816025Z },
2025-03-19T16:44:24.3817029Z },
2025-03-19T16:44:24.3817444Z },
2025-03-19T16:44:24.3817832Z },
2025-03-19T16:44:24.3818311Z },
2025-03-19T16:44:24.3818736Z }
2025-03-19T16:44:24.3819268Z 2025-03-19 16:43:19 lxd list output: []*spread.lxdServerJSON{
2025-03-19T16:44:24.3819981Z &spread.lxdServerJSON{
2025-03-19T16:44:24.3820569Z Name: "spread-1-ubuntu-16-04",
2025-03-19T16:44:24.3821419Z State: struct { Network map[string]spread.lxdDeviceJSON "json:\"network\"" }{
2025-03-19T16:44:24.3822165Z Network: {
2025-03-19T16:44:24.3822898Z "lo": {
2025-03-19T16:44:24.3823346Z State: "up",
2025-03-19T16:44:24.3823868Z Addresses: {
2025-03-19T16:44:24.3824481Z {Family:"inet", Address:"127.0.0.1"},
2025-03-19T16:44:24.3825246Z {Family:"inet6", Address:"::1"},
2025-03-19T16:44:24.3825794Z },
2025-03-19T16:44:24.3826264Z },
2025-03-19T16:44:24.3826741Z "eth0": {
2025-03-19T16:44:24.3827274Z State: "up",
2025-03-19T16:44:24.3827865Z Addresses: {
2025-03-19T16:44:24.3828623Z {Family:"inet6", Address:"fe80::216:3eff:fe73:4f61"},
2025-03-19T16:44:24.3829320Z },
2025-03-19T16:44:24.3829768Z },
2025-03-19T16:44:24.3830198Z },
2025-03-19T16:44:24.3830601Z },
2025-03-19T16:44:24.3830983Z },
2025-03-19T16:44:24.3831327Z }
2025-03-19T16:44:24.3831805Z 2025-03-19 16:43:20 lxd list output: []*spread.lxdServerJSON{
2025-03-19T16:44:24.3832477Z &spread.lxdServerJSON{
2025-03-19T16:44:24.3833029Z Name: "spread-1-ubuntu-16-04",
2025-03-19T16:44:24.3834305Z State: struct { Network map[string]spread.lxdDeviceJSON "json:\"network\"" }{
2025-03-19T16:44:24.3835140Z Network: {
2025-03-19T16:44:24.3835607Z "lo": {
2025-03-19T16:44:24.3836090Z State: "up",
2025-03-19T16:44:24.3836623Z Addresses: {
2025-03-19T16:44:24.3837150Z {Family:"inet", Address:"127.0.0.1"},
2025-03-19T16:44:24.3837672Z {Family:"inet6", Address:"::1"},
2025-03-19T16:44:24.3838046Z },
2025-03-19T16:44:24.3838313Z },
2025-03-19T16:44:24.3838592Z "eth0": {
2025-03-19T16:44:24.3838914Z State: "up",
2025-03-19T16:44:24.3839268Z Addresses: {
2025-03-19T16:44:24.3839745Z {Family:"inet6", Address:"fd42:7f57:a74e:c67d:216:3eff:fe73:4f61"},
2025-03-19T16:44:24.3840316Z {Family:"inet6", Address:"fe80::216:3eff:fe73:4f61"},
2025-03-19T16:44:24.3840726Z },
2025-03-19T16:44:24.3841667Z },
2025-03-19T16:44:24.3841931Z },
2025-03-19T16:44:24.3842176Z },
2025-03-19T16:44:24.3842405Z },
2025-03-19T16:44:24.3842630Z }
2025-03-19T16:44:24.3842990Z 2025-03-19 16:43:21 lxd list output: []*spread.lxdServerJSON{
2025-03-19T16:44:24.3843431Z &spread.lxdServerJSON{
2025-03-19T16:44:24.3843785Z Name: "spread-1-ubuntu-16-04",
2025-03-19T16:44:24.3844456Z State: struct { Network map[string]spread.lxdDeviceJSON "json:\"network\"" }{
2025-03-19T16:44:24.3845153Z Network: {
2025-03-19T16:44:24.3845446Z "lo": {
2025-03-19T16:44:24.3845889Z State: "up",
2025-03-19T16:44:24.3846297Z Addresses: {
2025-03-19T16:44:24.3846758Z {Family:"inet", Address:"127.0.0.1"},
2025-03-19T16:44:24.3847276Z {Family:"inet6", Address:"::1"},
2025-03-19T16:44:24.3847706Z },
2025-03-19T16:44:24.3847985Z },
2025-03-19T16:44:24.3848368Z "eth0": {
2025-03-19T16:44:24.3848683Z State: "up",
2025-03-19T16:44:24.3849136Z Addresses: {
2025-03-19T16:44:24.3849807Z {Family:"inet", Address:"10.144.14.120"},
2025-03-19T16:44:24.3850424Z {Family:"inet6", Address:"fd42:7f57:a74e:c67d:216:3eff:fe73:4f61"},
2025-03-19T16:44:24.3851058Z {Family:"inet6", Address:"fe80::216:3eff:fe73:4f61"},
2025-03-19T16:44:24.3851516Z },
2025-03-19T16:44:24.3851842Z },
2025-03-19T16:44:24.3852258Z },
2025-03-19T16:44:24.3852620Z },
2025-03-19T16:44:24.3852897Z },
2025-03-19T16:44:24.3853131Z }
2025-03-19T16:44:24.3853566Z 2025-03-19 16:43:22 Allocated lxd:ubuntu-16.04 (spread-1-ubuntu-16-04).
2025-03-19T16:44:24.3854288Z 2025-03-19 16:43:22 Connecting to lxd:ubuntu-16.04 (spread-1-ubuntu-16-04)...
2025-03-19T16:44:24.3855764Z 2025-03-19 16:43:22 Cannot connect to lxd:ubuntu-16.04 (spread-1-ubuntu-16-04): cannot connect to lxd:ubuntu-16.04 (spread-1-ubuntu-16-04): ssh: handshake failed: read tcp 10.144.14.1:54610->10.144.14.120:22: read: connection reset by peer
2025-03-19T16:44:24.3857975Z 2025-03-19 16:43:27 Cannot connect to lxd:ubuntu-16.04 (spread-1-ubuntu-16-04): cannot connect to lxd:ubuntu-16.04 (spread-1-ubuntu-16-04): dial tcp 10.144.14.120:22: connect: connection refused
2025-03-19T16:44:24.3860042Z 2025-03-19 16:43:30 Cannot connect to lxd:ubuntu-16.04 (spread-1-ubuntu-16-04): cannot connect to lxd:ubuntu-16.04 (spread-1-ubuntu-16-04): dial tcp 10.144.14.120:22: connect: connection refused
2025-03-19T16:44:24.3861964Z 2025-03-19 16:43:38 Cannot connect to lxd:ubuntu-16.04 (spread-1-ubuntu-16-04): cannot connect to lxd:ubuntu-16.04 (spread-1-ubuntu-16-04): dial tcp 10.144.14.120:22: connect: connection refused
2025-03-19T16:44:24.3863888Z 2025-03-19 16:43:46 Cannot connect to lxd:ubuntu-16.04 (spread-1-ubuntu-16-04): cannot connect to lxd:ubuntu-16.04 (spread-1-ubuntu-16-04): dial tcp 10.144.14.120:22: connect: connection refused
2025-03-19T16:44:24.3865363Z 2025-03-19 16:43:54 Cannot connect to lxd:ubuntu-16.04 (spread-1-ubuntu-16-04): cannot connect to lxd:ubuntu-16.04 (spread-1-ubuntu-16-04): dial tcp 10.144.14.120:22: connect: connection refused
2025-03-19T16:44:24.3866808Z 2025-03-19 16:44:02 Cannot connect to lxd:ubuntu-16.04 (spread-1-ubuntu-16-04): cannot connect to lxd:ubuntu-16.04 (spread-1-ubuntu-16-04): dial tcp 10.144.14.120:22: connect: connection refused
2025-03-19T16:44:24.3868249Z 2025-03-19 16:44:10 Cannot connect to lxd:ubuntu-16.04 (spread-1-ubuntu-16-04): cannot connect to lxd:ubuntu-16.04 (spread-1-ubuntu-16-04): dial tcp 10.144.14.120:22: connect: connection refused
2025-03-19T16:44:24.3869976Z 2025-03-19 16:44:18 Cannot connect to lxd:ubuntu-16.04 (spread-1-ubuntu-16-04): cannot connect to lxd:ubuntu-16.04 (spread-1-ubuntu-16-04): dial tcp 10.144.14.120:22: connect: connection refused
2025-03-19T16:44:24.3871812Z 2025-03-19 16:44:22 Discarding lxd:ubuntu-16.04 (spread-1-ubuntu-16-04), cannot connect: cannot connect to lxd:ubuntu-16.04 (spread-1-ubuntu-16-04): dial tcp 10.144.14.120:22: connect: connection refused
2025-03-19T16:44:24.3873336Z 2025-03-19 16:44:24 Worker terminated.
2025-03-19T16:44:24.3874283Z 2025-03-19 16:44:24 Pending jobs after workers returned: 1
2025-03-19T16:44:24.3874729Z - lxd:ubuntu-16.04:checks/main
2025-03-19T16:44:24.3875170Z 2025-03-19 16:44:24 Successful tasks: 0
2025-03-19T16:44:24.3875529Z 2025-03-19 16:44:24 Aborted tasks: 1
2025-03-19T16:44:24.3875922Z error: unsuccessful run
```

Looking at the code there are no attempts made to ensure boot-up of the instance is actually done, we could potentially improve the situation by running `cloud-init status --wait` before returning from `Allocate()`.

I'll propose this as part of #221, as that would also solve my need to wait for the LXD VM agent to start.

0: https://github.com/canonical/spread/actions/runs/13951932405/job/39053459252

Start by reproducing the failure with `spread -vv -reuse -resend` for the lxd:ubuntu-16.04 job, then inspect tests/reboot/spread.yaml and its checks/main task. Determine why container bringup waits without an IPv4 address and make the reboot check complete reliably on LXD.

Written by the indexing model from the issue text.