Difficult to override default firecracker command runner when not using jailer

[bduffany]

I tried temporarily disabling jailer to try and debug an issue that I suspect might have been related to running firecracker under jailer.

When not using jailer, the default firecracker command runner sets stdin, stdout, stderr to os.Stdin, os.Stdout, os.Stderr respectively. I didn't want that behavior (especially stdin), because I'm running several VMs at once from the same go binary. So I tried to override it.

However, I discovered that the SDK makes it difficult to override these. The default firecracker command is set here:

firecracker-go-sdk/machine.go

Line 384 in e5e3dea

m.cmd = configureBuilder(defaultFirecrackerVMMCommandBuilder, cfg).Build(ctx)

Note, it uses a private function, configureBuilder:

firecracker-go-sdk/machine.go

Lines 352 to 357 in e5e3dea

	func configureBuilder(builder VMCommandBuilder, cfg Config) VMCommandBuilder {
	return builder.
	WithSocketPath(cfg.SocketPath).
	AddArgs("--id", cfg.VMID).
	AddArgs(seccompArgs(&cfg)...)
	}

At first glance, that function seems small enough to just copy. However, it references this private seccompArgs function, which would also need to be copied:

firecracker-go-sdk/machine.go

Lines 342 to 350 in e5e3dea

	func seccompArgs(cfg *Config) []string {
	var args []string
	if !cfg.Seccomp.Enabled {
	args = append(args, "--no-seccomp")
	} else if len(cfg.Seccomp.Filter) > 0 {
	args = append(args, "--seccomp-filter", cfg.Seccomp.Filter)
	}
	return args
	}

I think maybe a better alternative to WithProcessRunner in this case could be to have a function like WithCommandModifier(defaultBuilder VMCommandBuilder) VMCommandBuilder that allows modifying the default command builder, instead of just the WithProcessRunner(cmd *exec.Command) which requires code-copying from the SDK.