Expose firecracker errors more clearly

[samuelkarp]

Right now, if firecracker has an error, we show a pretty cryptic message like ctr: Put http://localhost/machine-config: dial unix /tmp/firecracker.sock: connect: no such file or directory: unknown. I've been using the following wrapper script around the firecracker binary to save its output and get a better handle at diagnosing what went wrong, but we need to do a better job of making this accessible.

workaround wrapper script

$ cat /usr/local/bin/firecracker-wrapper 
#!/bin/sh

exec > /tmp/firecracker.log
exec 2>&1

echo "Options: $@"

exec /usr/local/bin/firecracker "$@"

[kzys]

The Go SDK is able to bind stdout/stderr

https://github.com/firecracker-microvm/firecracker-go-sdk/blob/b704cc3feb1e827244ddbc8a25c55c482f7f6e36/command_builder.go#L103-L119

but we are not utilizing it so far.

firecracker-containerd/runtime/service.go

Lines 478 to 481 in b25fb56

	cmd := firecracker.VMCommandBuilder{}.
	WithBin(s.config.FirecrackerBinaryPath).
	WithSocketPath(s.shimDir.FirecrackerSockPath()).
	Build(s.shimCtx) // shimCtx so the VM process is only killed when the shim shuts down

[kzys]

Note that jailer support (#122) may change the way we invoke Firecracker.