Allow open to be asynchronous

This stops the ui being blocked while the resource is opened. The wait
ensures that resources are reclaimed when the process finishes while
aerc is still running.

commands/msg/unsubscribe.go

@@ -115,6 +115,6 @@ func unsubscribeMailto(aerc *widgets.Aerc, u *url.URL) error {
 }
 
 func unsubscribeHTTP(u *url.URL) error {
-	go lib.OpenFile(u.String())
+	lib.OpenFile(u.String(), nil)
 	return nil
 }

commands/msgview/open.go

@@ -60,10 +60,9 @@ func (Open) Execute(aerc *widgets.Aerc, args []string) error {
 			return
 		}
 
-		err = lib.OpenFile(tmpFile.Name())
+		lib.OpenFile(tmpFile.Name(), func(err error) {
-		if err != nil {
 			aerc.PushError(" " + err.Error())
-		}
+		})
 
 		aerc.PushStatus("Opened", 10*time.Second)
 	})

lib/open.go

@@ -6,7 +6,18 @@ import (
 	"os/exec"
 )
 
-func OpenFile(filename string) error {
+func OpenFile(filename string, onErr func(error)) {
 	cmd := exec.Command("xdg-open", filename)
-	return cmd.Run()
+	err := cmd.Start()
+	if err != nil && onErr != nil {
+		onErr(err)
+		return
+	}
+
+	go func() {
+		err := cmd.Wait()
+		if err != nil && onErr != nil {
+			onErr(err)
+		}
+	}()
 }

lib/open_darwin.go

@@ -4,7 +4,18 @@ import (
 	"os/exec"
 )
 
-func OpenFile(filename string) error {
+func OpenFile(filename string, onErr func(error)) {
 	cmd := exec.Command("open", filename)
-	return cmd.Run()
+	err := cmd.Start()
+	if err != nil && onErr != nil {
+		onErr(err)
+		return
+	}
+
+	go func() {
+		err := cmd.Wait()
+		if err != nil && onErr != nil {
+			onErr(err)
+		}
+	}()
 }