split UI from event handling

Signed-off-by: Alex Goodman <wagoodman@gmail.com>

cmd/event_loop.go

@@ -0,0 +1,77 @@
+package cmd
+
+import (
+	"errors"
+	"os"
+
+	"github.com/anchore/syft/internal/log"
+	"github.com/anchore/syft/internal/ui"
+	"github.com/hashicorp/go-multierror"
+	"github.com/wagoodman/go-partybus"
+)
+
+// eventLoop listens to worker errors (from execution path), worker events (from a partybus subscription), and
+// signal interrupts. Is responsible for handling each event relative to a given UI an to coordinate eventing until
+// an eventual graceful exit.
+func eventLoop(workerErrs <-chan error, signals <-chan os.Signal, subscription *partybus.Subscription, ux ui.UI) error {
+	events := subscription.Events()
+	if err := setupUI(subscription.Unsubscribe, ux); err != nil {
+		return err
+	}
+
+	var retErr error
+
+	for {
+		if workerErrs == nil && events == nil {
+			break
+		}
+		select {
+		case err, isOpen := <-workerErrs:
+			if !isOpen {
+				workerErrs = nil
+				continue
+			}
+			if err != nil {
+				retErr = err
+			}
+		case e, isOpen := <-events:
+			if !isOpen {
+				events = nil
+				continue
+			}
+
+			if err := ux.Handle(e); err != nil {
+				if errors.Is(err, partybus.ErrUnsubscribe) {
+					log.Warnf("unable to unsubscribe from the event bus")
+					events = nil
+				} else {
+					retErr = multierror.Append(retErr, err)
+					// TODO: should we unsubscribe? should we try to halt execution? or continue?
+				}
+			}
+		case <-signals:
+			if err := subscription.Unsubscribe(); err != nil {
+				log.Warnf("unable to unsubscribe from the event bus: %+v", err)
+				events = nil
+			}
+		}
+	}
+
+	if err := ux.Teardown(); err != nil {
+		retErr = multierror.Append(retErr, err)
+	}
+
+	return retErr
+}
+
+func setupUI(unsubscribe func() error, ux ui.UI) error {
+	if err := ux.Setup(unsubscribe); err != nil {
+		ux = ui.NewLoggerUI()
+		if err := ux.Setup(unsubscribe); err != nil {
+			// something is very wrong, bail.
+			return err
+		}
+		log.Errorf("unable to setup given UI, falling back to logger: %+v", err)
+	}
+	return nil
+}

cmd/packages.go

@@ -186,9 +186,12 @@ func bindPackagesConfigOptions(flags *pflag.FlagSet) error {
 }
 
 func packagesExec(_ *cobra.Command, args []string) error {
-	errs := packagesExecWorker(args[0])
-	ux := ui.Select(appConfig.CliOptions.Verbosity > 0, appConfig.Quiet)
-	return ux(errs, eventSubscription)
+	return eventLoop(
+		packagesExecWorker(args[0]),
+		setupSignals(),
+		eventSubscription,
+		ui.Select(appConfig.CliOptions.Verbosity > 0, appConfig.Quiet),
+	)
 }
 
 func packagesExecWorker(userInput string) <-chan error {

cmd/power_user.go

@@ -63,9 +63,12 @@ func init() {
 }
 
 func powerUserExec(_ *cobra.Command, args []string) error {
-	errs := powerUserExecWorker(args[0])
-	ux := ui.Select(appConfig.CliOptions.Verbosity > 0, appConfig.Quiet)
-	return ux(errs, eventSubscription)
+	return eventLoop(
+		powerUserExecWorker(args[0]),
+		setupSignals(),
+		eventSubscription,
+		ui.Select(appConfig.CliOptions.Verbosity > 0, appConfig.Quiet),
+	)
 }
 
 func powerUserExecWorker(userInput string) <-chan error {

cmd/signals.go

@@ -0,0 +1,20 @@
+package cmd
+
+import (
+	"os"
+	"os/signal"
+	"syscall"
+)
+
+func setupSignals() <-chan os.Signal {
+	c := make(chan os.Signal, 1) // Note: A buffered channel is recommended for this; see https://golang.org/pkg/os/signal/#Notify
+
+	interruptions := []os.Signal{
+		syscall.SIGINT,
+		syscall.SIGTERM,
+	}
+
+	signal.Notify(c, interruptions...)
+
+	return c
+}