refactor command package to remove globals and add dependency injection

cmd/syft/cli/eventloop/event_loop.go

@@ -0,0 +1,98 @@
+package eventloop
+
+import (
+	"errors"
+	"fmt"
+	"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.
+// nolint:funlen
+func EventLoop(workerErrs <-chan error, signals <-chan os.Signal, subscription *partybus.Subscription, cleanupFn func(), uxs ...ui.UI) error {
+	defer cleanupFn()
+	events := subscription.Events()
+	var err error
+	var ux ui.UI
+
+	if ux, err = setupUI(subscription.Unsubscribe, uxs...); err != nil {
+		return err
+	}
+
+	var retErr error
+	var forceTeardown bool
+
+	for {
+		if workerErrs == nil && events == nil {
+			break
+		}
+		select {
+		case err, isOpen := <-workerErrs:
+			if !isOpen {
+				workerErrs = nil
+				continue
+			}
+			if err != nil {
+				// capture the error from the worker and unsubscribe to complete a graceful shutdown
+				retErr = multierror.Append(retErr, err)
+				_ = subscription.Unsubscribe()
+				// the worker has exited, we may have been mid-handling events for the UI which should now be
+				// ignored, in which case forcing a teardown of the UI irregardless of the state is required.
+				forceTeardown = true
+			}
+		case e, isOpen := <-events:
+			if !isOpen {
+				events = nil
+				continue
+			}
+
+			if err := ux.Handle(e); err != nil {
+				if errors.Is(err, partybus.ErrUnsubscribe) {
+					events = nil
+				} else {
+					retErr = multierror.Append(retErr, err)
+					// TODO: should we unsubscribe? should we try to halt execution? or continue?
+				}
+			}
+		case <-signals:
+			// ignore further results from any event source and exit ASAP, but ensure that all cache is cleaned up.
+			// we ignore further errors since cleaning up the tmp directories will affect running catalogers that are
+			// reading/writing from/to their nested temp dirs. This is acceptable since we are bailing without result.
+
+			// TODO: potential future improvement would be to pass context into workers with a cancel function that is
+			// to the event loop. In this way we can have a more controlled shutdown even at the most nested levels
+			// of processing.
+			events = nil
+			workerErrs = nil
+			forceTeardown = true
+		}
+	}
+
+	if err := ux.Teardown(forceTeardown); err != nil {
+		retErr = multierror.Append(retErr, err)
+	}
+
+	return retErr
+}
+
+// setupUI takes one or more UIs that responds to events and takes a event bus unsubscribe function for use
+// during teardown. With the given UIs, the first UI which the ui.Setup() function does not return an error
+// will be utilized in execution. Providing a set of UIs allows for the caller to provide graceful fallbacks
+// when there are environmental problem (e.g. unable to setup a TUI with the current TTY).
+func setupUI(unsubscribe func() error, uis ...ui.UI) (ui.UI, error) {
+	for _, ux := range uis {
+		if err := ux.Setup(unsubscribe); err != nil {
+			log.Warnf("unable to setup given UI, falling back to alternative UI: %+v", err)
+			continue
+		}
+
+		return ux, nil
+	}
+	return nil, fmt.Errorf("unable to setup any UI")
+}