Proposal: use context to control ExecTask timeout

[LucasRoesler]

While reviewing #89 I thought about how to set a timeout on the rollout commands. Go provides a CommandContext that is designed to allow you to cancel a command via a context cancel. See https://golang.org/pkg/os/exec/#CommandContext

The provided context is used to kill the process (by calling os.Process.Kill) if the context becomes done before the command completes on its own.

We should update func (et ExecTask) Execute() (ExecResult, error) to accept a context so that the caller can set a timeout and explicitly control the max length of a command.

Expected Behaviour

Methods like certManagerReady() should look like

func certManagerReady() {
	ctx, cancel := context.WithTimeout(context.Background, 30*time.Second)
	defer cancel()
	task := execute.ExecTask{
		Command: "./scripts/get-cert-manager.sh",
		Shell:   true,
	}

	res, err := task.Execute(ctx)
	fmt.Println("cert-manager", res.ExitCode, res.Stdout, res.Stderr, err)
}

Possible Solution

1. Update the ExecTask to accept a context

func (et ExecTask) Execute(ctx context.Context) (ExecResult, error)

2. If the ctx is nil, then just use contetxt.Background()
3. use exec.CommandContext instead of exec.Command
4. update all calls to Execute to send at least a nil context

[alexellis]

I thought about how to set a timeout on the rollout commands

^ this is part of kubectl

      --timeout=0s: The length of time to wait before ending watch, zero means never. Any other values should contain a
corresponding time unit (e.g. 1s, 2m, 3h).