plugins: fix "dial-stdio" feature or remove the feature

[thaJeztah]

Description

(I noticed this while reviewing #4599)

• relates to cli-plugins: disable use of dial-stdio #1745
• relates to cli-plugins: use system dial-stdio to contact the engine. #1654
• relates to dial-stdio: handle connections which lack CloseRead method. #1718

Relates to (#1661):

• plugins should call back to main cli using dial-stdio to connect to engine. From CLI Plugins Design CLI Plugins Design #1534 (comment) → See PR cli-plugins: use system dial-stdio to contact the engine. #1654.

Commit ff2ed6e (#1745) disabled dial-stdio for plugins, adding an (undocumnented) DOCKER_CLI_PLUGIN_USE_DIAL_STDIO environment-variable to re-enable the feature. This feature was added in #1654, but caused CI failures and didn't work as anticipated.

We should either debug (and fix) the feature, or consider removing it.

[AkihiroSuda]

Does this proposal affect DOCKER_HOST=ssh:// ?

[thaJeztah]

No it shouldn't, AFAICS, or at least it currently is disabled by default (which is why my eye dropped on it);

cli/cli-plugins/plugin/plugin.go

Lines 86 to 90 in 1f97a34

	var opts []command.CLIOption
	if os.Getenv("DOCKER_CLI_PLUGIN_USE_DIAL_STDIO") != "" {
	opts = append(opts, withPluginClientConn(plugin.Name()))
	}
	err = tcmd.Initialize(opts...)

Setting that env-var enables this option, which is disabled by default because (from links above) it was broken;

cli/cli-plugins/plugin/plugin.go

Lines 132 to 162 in 1f97a34

	func withPluginClientConn(name string) command.CLIOption {
	return command.WithInitializeClient(func(dockerCli *command.DockerCli) (client.APIClient, error) {
	cmd := "docker"
	if x := os.Getenv(manager.ReexecEnvvar); x != "" {
	cmd = x
	}
	var flags []string
	// Accumulate all the global arguments, that is those
	// up to (but not including) the plugin's name. This
	// ensures that `docker system dial-stdio` is
	// evaluating the same set of `--config`, `--tls*` etc
	// global options as the plugin was called with, which
	// in turn is the same as what the original docker
	// invocation was passed.
	for _, a := range os.Args[1:] {
	if a == name {
	break
	}
	flags = append(flags, a)
	}
	flags = append(flags, "system", "dial-stdio")
	helper, err := connhelper.GetCommandConnectionHelper(cmd, flags...)
	if err != nil {
	return nil, err
	}
	return client.NewClientWithOpts(client.WithDialContext(helper.Dialer))
	})
	}