nebula/localapi/wire.go

package localapi

import (
	"context"
	"errors"
	"log/slog"

	"github.com/slackhq/nebula/commands"
	"github.com/slackhq/nebula/config"
)

// Wire brings up the local control socket for a running nebula and keeps it pointed at
// the configured path across reloads.
//
// Every entry point that runs nebula as a daemon calls this, so the endpoint behaves the
// same whether nebula was started from a shell or by a service manager. It deliberately
// does not live in nebula.Main: the endpoint is on by default with no key to disable it,
// so an embedder inside an app sandbox would have no way to decline a socket it cannot
// use. Wiring it at the daemon entry points means embedders get the command registry and
// no listener.
//
// Every failure below is a warning, bar one. Nebula is expected to keep forwarding
// packets whether or not anyone can reach it; but a name that another process holds is
// not a misconfiguration, it is a process that will be answering nebula ctl in the
// daemon's place, and that is logged as an error.
//
// Call it after Control.Start, so the listener only exists once the interface is up.
func Wire(ctx context.Context, l *slog.Logger, c *config.C, reg *commands.Registry) *Server {
	l = l.With("subsystem", "localapi")
	srv := NewServer(l, reg)

	// Registered before the first attempt, so that a path or bind that is bad at startup
	// can be fixed with a config edit and a reload, the same as one that goes bad later,
	// and so that a HUP landing in between is harmless: Rebind on a server already at
	// the path is a no-op.
	reconcile := followControlConfig(ctx, l, c, srv)
	reconcile(c)
	return srv
}

// followControlConfig registers the reload callback that keeps the socket where
// local_api.control.socket says it is, moving it when the key changes and bringing one
// up while there is none, and returns it for the first attempt to call.
//
// The key is reloadable because the client finds the socket by reading the config: if the
// two could disagree, the file on disk would name a socket that is not there.
//
// Every reload is reconciled rather than gated on the key having changed. Rebind on a
// server already at the path is a no-op, so that costs nothing; and a bind that failed
// for a reason outside the config, a runtime directory that was not there yet, is fixed
// outside the config too, so the reload that follows the fix is a plain one. That holds
// for a move as much as for the first bind: a daemon left on its old path by a move that
// could not bind is one the client, reading the new path from the same file, cannot
// reach, and the operator's fix is the directory and a reload, not an edit to an
// unrelated line to make the key read as changed.
//
// The callback recovers from its own panics rather than relying on the config package,
// whose callback loop has no isolation of its own. A panic there would take the daemon
// down and skip every callback queued behind this one.
func followControlConfig(ctx context.Context, l *slog.Logger, c *config.C, srv *Server) func(*config.C) {
	reconcile := func(c *config.C) {
		want, err := SocketPathFromConfig(c)
		if err != nil {
			if srv.Path() == "" {
				l.Warn("Could not determine the local control socket path, control commands will not be available", "error", err)
			} else {
				l.Warn("Ignoring unusable local control socket path from reloaded config", "error", err)
			}
			return
		}

		acc, err := controlAccessFrom(c)
		if err != nil {
			if srv.Path() == "" {
				l.Warn("Local control socket is not available, its group is unusable", "error", err)
			} else {
				l.Warn("Ignoring unusable local control socket group from reloaded config, keeping the current socket", "error", err)
			}
			return
		}

		old := srv.Path()
		err = srv.Rebind(ctx, want, acc)
		reportRebind(l, old, want, err, srv.Path())
	}

	c.RegisterReloadCallback(func(c *config.C, _ *config.Reload) {
		defer func() {
			if r := recover(); r != nil {
				l.Error("Recovered from panic reloading the local control socket", "panic", r)
			}
		}()
		reconcile(c)
	})
	return reconcile
}

// reportRebind logs what a Rebind did beyond the listening line the bind itself writes:
// nothing for a server left where it was, a move with both paths, and a failure with
// what the operator is left with. old is the path served before the call, "" for none,
// want the one asked for, and now the path served after it: on unix a failed move keeps
// the old socket, and now is old; on windows a change of access closes the pipe before
// creating it again, and a failure there leaves nothing serving, so now is "".
func reportRebind(l *slog.Logger, old, want string, err error, now string) {
	switch {
	case err == nil:
		if old != "" && old != want {
			l.Info("Moved the local control socket", "from", old, "to", want)
		}
	case old == "":
		bindFailure(l, err)("Could not start the local control socket, control commands will not be available until a reload binds it", "error", err, "path", want)
	default:
		bindFailure(l, err)("Could not bind the local control socket at the requested path",
			"error", err, "requested", want, "serving", now,
		)
	}
}

// errEndpointHeld is returned by listen when the endpoint's name is already held by
// some other process, which nebula will not serve alongside: whoever holds it would be
// answering nebula ctl. It is only raised on windows, where the default name can be
// held by another administrator's process and an override outside the protected
// namespace by any account's; on unix a path that is still being served is an earlier
// nebula, which clearStale reports as in use.
var errEndpointHeld = errors.New("the local control socket's name is held by another process")

// bindFailure picks the level a failed bind is logged at. Nearly every failure is the
// operator's environment, a missing runtime directory or a bad path, and a warning; a
// name held by another process is logged as an error, since nothing else will say that
// something is sitting where the daemon should be.
func bindFailure(l *slog.Logger, err error) func(msg string, args ...any) {
	if errors.Is(err, errEndpointHeld) {
		return l.Error
	}
	return l.Warn
}