kill background agents if versions don't match

pkg/agent/client.go

@@ -6,11 +6,14 @@ import (
 	"net"
 	"os"
 	"strings"
+	"time"
 
 	"github.com/blang/semver"
 	"github.com/pkg/errors"
 	"github.com/superfly/flyctl/api"
+	"github.com/superfly/flyctl/internal/buildinfo"
 	"github.com/superfly/flyctl/internal/wireguard"
+	"github.com/superfly/flyctl/terminal"
 )
 
 /// Establish starts the daemon if necessary and returns a client
@@ -21,9 +24,25 @@ func Establish(ctx context.Context, apiClient *api.Client) (*Client, error) {
 
 	c, err := DefaultClient(apiClient)
 	if err == nil {
-		_, err := c.Ping(ctx)
+		resp, err := c.Ping(ctx)
 		if err == nil {
-			return c, nil
+			if buildinfo.Version().EQ(resp.Version) {
+				return c, nil
+			}
+
+			msg := fmt.Sprintf("flyctl version %s does not match agent version %s", buildinfo.Version(), resp.Version)
+
+			if !resp.Background {
+				terminal.Warn(msg)
+				return c, nil
+			}
+
+			if err := c.Kill(ctx); err != nil {
+				terminal.Warn(msg)
+				return nil, errors.Wrap(err, "kill failed")
+			}
+			// this is gross, but we need to wait for the agent to exit
+			time.Sleep(1 * time.Second)
 		}
 	}