Add ExecDied event and use it to retrieve exit codes

When making Exec Cleanup processes mandatory, I introduced a race
wherein attached exec sessions could be cleaned up and removed by
the cleanup process before the frontend had a chance to get their
exit code. Fortunately, we've dealt with this issue before in
containers, and the same solution can be applied here. I added an
event for an exec session's process exiting, `exec_died` (Docker
has an identical event, so this actually improves our
compatibility there) that includes the exit code of the exec
session. If the race happens and the exec session no longer
exists when we go to remove it, pick up exit code from the event
and exit cleanly.

Signed-off-by: Matthew Heon <mheon@redhat.com>

2 files changed, 4 insertions, 0 deletions

diff --git a/libpod/events/config.go b/libpod/events/config.go
index 085fa9d52..d88d7b6e3 100644
--- a/libpod/events/config.go
+++ b/libpod/events/config.go
@@ -127,6 +127,8 @@ const (
 	Create Status = "create"
 	// Exec ...
 	Exec Status = "exec"
+	// ExecDied indicates that an exec session in a container died.
+	ExecDied Status = "exec_died"
 	// Exited indicates that a container's process died
 	Exited Status = "died"
 	// Export ...
diff --git a/libpod/events/events.go b/libpod/events/events.go
index 01ea6a386..e03215eff 100644
--- a/libpod/events/events.go
+++ b/libpod/events/events.go
@@ -149,6 +149,8 @@ func StringToStatus(name string) (Status, error) {
 		return Create, nil
 	case Exec.String():
 		return Exec, nil
+	case ExecDied.String():
+		return ExecDied, nil
 	case Exited.String():
 		return Exited, nil
 	case Export.String():