Fix system service panic from early hangup in events

We weren't actually halting the goroutine that sent events, so it
would continue sending even when the channel closed (the most
notable cause being early hangup - e.g. Control-c on a curl
session). Use a context to cancel the events goroutine and stop
sending events.

Fixes #6805

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

pkg/varlinkapi/attach.go

@@ -4,6 +4,7 @@ package varlinkapi
 
 import (
 	"bufio"
+	"context"
 	"io"
 
 	"github.com/containers/libpod/libpod"
@@ -89,7 +90,7 @@ func (i *VarlinkAPI) Attach(call iopodman.VarlinkCall, name string, detachKeys s
 		if ecode, err := ctr.Wait(); err != nil {
 			if errors.Cause(err) == define.ErrNoSuchCtr {
 				// Check events
-				event, err := i.Runtime.GetLastContainerEvent(ctr.ID(), events.Exited)
+				event, err := i.Runtime.GetLastContainerEvent(context.Background(), ctr.ID(), events.Exited)
 				if err != nil {
 					logrus.Errorf("Cannot get exit code: %v", err)
 					exitCode = define.ExecErrorCodeNotFound