plugins: fix plugin socket being closed before use

Signed-off-by: Laura Brehm <laurabrehm@hey.com>

cli-plugins/socket/socket.go

@@ -14,21 +14,20 @@ import (
 const EnvKey = "DOCKER_CLI_PLUGIN_SOCKET"
 
 // SetupConn sets up a Unix socket listener, establishes a goroutine to handle connections
-// and update the conn pointer, and returns the environment variable to pass to the plugin.
+// and update the conn pointer, and returns the listener for the socket (which the caller
-func SetupConn(conn **net.UnixConn) (string, error) {
+// is responsible for closing when it's no longer needed).
+func SetupConn(conn **net.UnixConn) (*net.UnixListener, error) {
 	listener, err := listen("docker_cli_" + uuid.Generate().String())
 	if err != nil {
-		return "", err
+		return nil, err
 	}
 
 	accept(listener, conn)
 
-	return EnvKey + "=" + listener.Addr().String(), nil
+	return listener, nil
 }
 
 func accept(listener *net.UnixListener, conn **net.UnixConn) {
-	defer listener.Close()
-
 	go func() {
 		for {
 			// ignore error here, if we failed to accept a connection,

cmd/docker/docker.go

@@ -223,9 +223,10 @@ func tryPluginRun(dockerCli command.Cli, cmd *cobra.Command, subcommand string,
 
 	// Establish the plugin socket, adding it to the environment under a well-known key if successful.
 	var conn *net.UnixConn
-	socketenv, err := socket.SetupConn(&conn)
+	listener, err := socket.SetupConn(&conn)
 	if err == nil {
-		envs = append(envs, socketenv)
+		envs = append(envs, socket.EnvKey+"="+listener.Addr().String())
+		defer listener.Close()
 	}
 
 	plugincmd.Env = append(envs, plugincmd.Env...)