Do not error on installing duplicate shutdown handler

Installing a duplicate shutdown handler fails, but if a handler
with the same name is already present, we should be set to go.
There's no reason to print a user-facing error about it.

This comes up almost nowhere because Podman never makes more than
one Libpod runtime, but there is one exception (`system reset`)
and the error messages, while harmless, were making people very
confused (we got several bug reports that `system reset` was
nonfunctional).

Signed-off-by: Matthew Heon <[email protected]>

libpod/runtime.go

@@ -190,7 +190,7 @@ func newRuntimeFromConfig(ctx context.Context, conf *config.Config, options ...R
 	if err := shutdown.Register("libpod", func(sig os.Signal) error {
 		os.Exit(1)
 		return nil
-	}); err != nil {
+	}); err != nil && errors.Cause(err) != shutdown.ErrHandlerExists {
 		logrus.Errorf("Error registering shutdown handler for libpod: %v", err)
 	}
 

libpod/shutdown/handler.go

@@ -10,6 +10,10 @@ import (
 	"github.com/sirupsen/logrus"
 )
 
+var (
+	ErrHandlerExists error = errors.New("handler with given name already exists")
+)
+
 var (
 	stopped    bool
 	sigChan    chan os.Signal
@@ -98,7 +102,7 @@ func Register(name string, handler func(os.Signal) error) error {
 	}
 
 	if _, ok := handlers[name]; ok {
-		return errors.Errorf("handler with name %s already exists", name)
+		return ErrHandlerExists
 	}
 
 	handlers[name] = handler