event backend none: return an error when reading events

podman --events-backend none events should return with an error since it
will never be able to actually list events.

Fixes part three of containers#15688

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

cmd/podman/system/events.go

@@ -99,25 +99,28 @@ func eventsCmd(cmd *cobra.Command, _ []string) error {
 		errChannel <- err
 	}()
 
-	for event := range eventChannel {
-		switch {
-		case event == nil:
-			// no-op
-		case doJSON:
-			jsonStr, err := event.ToJSONString()
-			if err != nil {
-				return err
+	for {
+		select {
+		case err := <-errChannel:
+			return err
+		case event := <-eventChannel:
+			switch {
+			case event == nil:
+				// no-op
+			case doJSON:
+				jsonStr, err := event.ToJSONString()
+				if err != nil {
+					return err
+				}
+				fmt.Println(jsonStr)
+			case cmd.Flags().Changed("format"):
+				if err := rpt.Execute(event); err != nil {
+					return err
+				}
+				os.Stdout.WriteString("\n")
+			default:
+				fmt.Println(event.ToHumanReadable(!noTrunc))
 			}
-			fmt.Println(jsonStr)
-		case cmd.Flags().Changed("format"):
-			if err := rpt.Execute(event); err != nil {
-				return err
-			}
-			os.Stdout.WriteString("\n")
-		default:
-			fmt.Println(event.ToHumanReadable(!noTrunc))
 		}
 	}
-
-	return <-errChannel
 }

libpod/events/events_linux.go

@@ -20,7 +20,7 @@ func NewEventer(options EventerOptions) (Eventer, error) {
 	case strings.ToUpper(LogFile.String()):
 		return newLogFileEventer(options)
 	case strings.ToUpper(Null.String()):
-		return NewNullEventer(), nil
+		return newNullEventer(), nil
 	case strings.ToUpper(Memory.String()):
 		return NewMemoryEventer(), nil
 	default:

libpod/events/nullout.go

@@ -2,25 +2,26 @@ package events
 
 import (
 	"context"
+	"errors"
 )
 
-// EventToNull is an eventer type that only performs write operations
-// and only writes to /dev/null. It is meant for unittests only
+// EventToNull is an eventer type that does nothing.
+// It is meant for unittests only
 type EventToNull struct{}
 
 // Write eats the event and always returns nil
 func (e EventToNull) Write(ee Event) error {
 	return nil
 }
 
-// Read does nothing. Do not use it.
+// Read does nothing and returns an error.
 func (e EventToNull) Read(ctx context.Context, options ReadOptions) error {
-	return nil
+	return errors.New("cannot read events with the \"none\" backend")
 }
 
-// NewNullEventer returns a new null eventer.  You should only do this for
+// newNullEventer returns a new null eventer.  You should only do this for
 // the purposes of internal libpod testing.
-func NewNullEventer() Eventer {
+func newNullEventer() Eventer {
 	return EventToNull{}
 }
 

test/system/090-events.bats

@@ -217,3 +217,12 @@ EOF
                --format="{{.Attributes.$lname}}"
     assert "$output" = "$lvalue" "podman-events output includes container label"
 }
+
+@test "events - backend none should error" {
+    skip_if_remote "remote does not support --events-backend"
+
+    run_podman 125 --events-backend none events
+    is "$output" "cannot read events with the \"none\" backend" "correct error message"
+    run_podman 125 --events-backend none events --stream=false
+    is "$output" "cannot read events with the \"none\" backend" "correct error message"
+}