cli-plugins: use `docker system dial-stdio` to call the daemon

This means that plugins can use whatever methods the monolithic CLI supports,
which is good for consistency.

This relies on `os.Args[0]` being something which can be executed again to
reach the same binary, since it is propagated (via an envvar) to the plugin for
this purpose. This essentially requires that the current working directory and
path are not modified by the monolithic CLI before it launches the plugin nor
by the plugin before it initializes the client. This should be the case.

Previously the fake apiclient used by `TestExperimentalCLI` was not being used,
since `cli.Initialize` was unconditionally overwriting it with a real one
(talking to a real daemon during unit testing, it seems). This wasn't expected
nor desirable and no longer happens with the new arrangements, exposing the
fact that no `pingFunc` is provided, leading to a panic. Add a `pingFunc` to
the fake client to avoid this.

Signed-off-by: Ian Campbell <ijc@docker.com>

cli-plugins/manager/manager.go

@@ -12,6 +12,12 @@ import (
 	"github.com/spf13/cobra"
 )
 
+// ReexecEnvvar is the name of an ennvar which is set to the command
+// used to originally invoke the docker CLI when executing a
+// plugin. Assuming $PATH and $CWD remain unchanged this should allow
+// the plugin to re-execute the original CLI.
+const ReexecEnvvar = "DOCKER_CLI_PLUGIN_ORIGINAL_CLI_COMMAND"
+
 // errPluginNotFound is the error returned when a plugin could not be found.
 type errPluginNotFound string
 
@@ -155,6 +161,9 @@ func PluginRunCommand(dockerCli command.Cli, name string, rootcmd *cobra.Command
 		cmd.Stdout = os.Stdout
 		cmd.Stderr = os.Stderr
 
+		cmd.Env = os.Environ()
+		cmd.Env = append(cmd.Env, ReexecEnvvar+"="+os.Args[0])
+
 		return cmd, nil
 	}
 	return nil, errPluginNotFound(name)

cli-plugins/plugin/plugin.go

@@ -9,7 +9,9 @@ import (
 	"github.com/docker/cli/cli"
 	"github.com/docker/cli/cli-plugins/manager"
 	"github.com/docker/cli/cli/command"
+	"github.com/docker/cli/cli/connhelper"
 	cliflags "github.com/docker/cli/cli/flags"
+	"github.com/docker/docker/client"
 	"github.com/spf13/cobra"
 	"github.com/spf13/pflag"
 )
@@ -49,6 +51,7 @@ func Run(makeCmd func(command.Cli) *cobra.Command, meta manager.Metadata) {
 // own use of that hook will shadow anything we add to the top-level
 // command meaning the CLI is never Initialized.
 var options struct {
+	name         string
 	init, prerun sync.Once
 	opts         *cliflags.ClientOptions
 	flags        *pflag.FlagSet
@@ -71,11 +74,43 @@ func PersistentPreRunE(cmd *cobra.Command, args []string) error {
 		}
 		// flags must be the original top-level command flags, not cmd.Flags()
 		options.opts.Common.SetDefaultOptions(options.flags)
-		err = options.dockerCli.Initialize(options.opts)
+		err = options.dockerCli.Initialize(options.opts, withPluginClientConn(options.name))
 	})
 	return err
 }
 
+func withPluginClientConn(name string) command.InitializeOpt {
+	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))
+	})
+}
+
 func newPluginCommand(dockerCli *command.DockerCli, plugin *cobra.Command, meta manager.Metadata) *cobra.Command {
 	name := plugin.Name()
 	fullname := manager.NamePrefix + name
@@ -101,6 +136,7 @@ func newPluginCommand(dockerCli *command.DockerCli, plugin *cobra.Command, meta
 	cli.DisableFlagsInUseLine(cmd)
 
 	options.init.Do(func() {
+		options.name = name
 		options.opts = opts
 		options.flags = flags
 		options.dockerCli = dockerCli
@@ -115,6 +151,8 @@ func newMetadataSubcommand(plugin *cobra.Command, meta manager.Metadata) *cobra.
 	cmd := &cobra.Command{
 		Use:    manager.MetadataSubcommandName,
 		Hidden: true,
+		// Suppress the global/parent PersistentPreRunE, which needlessly initializes the client and tries to connect to the daemon.
+		PersistentPreRun: func(cmd *cobra.Command, args []string) {},
 		RunE: func(cmd *cobra.Command, args []string) error {
 			enc := json.NewEncoder(os.Stdout)
 			enc.SetEscapeHTML(false)

cli/command/cli.go

@@ -175,9 +175,28 @@ func (cli *DockerCli) RegistryClient(allowInsecure bool) registryclient.Registry
 	return registryclient.NewRegistryClient(resolver, UserAgent(), allowInsecure)
 }
 
+// InitializeOpt is the type of the functional options passed to DockerCli.Initialize
+type InitializeOpt func(dockerCli *DockerCli) error
+
+// WithInitializeClient is passed to DockerCli.Initialize by callers who wish to set a particular API Client for use by the CLI.
+func WithInitializeClient(makeClient func(dockerCli *DockerCli) (client.APIClient, error)) InitializeOpt {
+	return func(dockerCli *DockerCli) error {
+		var err error
+		dockerCli.client, err = makeClient(dockerCli)
+		return err
+	}
+}
+
 // Initialize the dockerCli runs initialization that must happen after command
 // line flags are parsed.
-func (cli *DockerCli) Initialize(opts *cliflags.ClientOptions) error {
+func (cli *DockerCli) Initialize(opts *cliflags.ClientOptions, ops ...InitializeOpt) error {
+	var err error
+
+	for _, o := range ops {
+		if err := o(cli); err != nil {
+			return err
+		}
+	}
 	cliflags.SetLogLevel(opts.Common.LogLevel)
 
 	if opts.ConfigDir != "" {
@@ -189,7 +208,8 @@ func (cli *DockerCli) Initialize(opts *cliflags.ClientOptions) error {
 	}
 
 	cli.configFile = cliconfig.LoadDefaultConfigFile(cli.err)
-	var err error
+
+	if cli.client == nil {
 		cli.contextStore = store.New(cliconfig.ContextStoreDir(), cli.contextStoreConfig)
 		cli.currentContext, err = resolveContextName(opts.Common, cli.configFile, cli.contextStore)
 		if err != nil {
@@ -213,6 +233,7 @@ func (cli *DockerCli) Initialize(opts *cliflags.ClientOptions) error {
 		if err != nil {
 			return err
 		}
+	}
 	var experimentalValue string
 	// Environment variable always overrides configuration
 	if experimentalValue = os.Getenv("DOCKER_CLI_EXPERIMENTAL"); experimentalValue == "" {

cli/command/cli_test.go

@@ -175,6 +175,9 @@ func TestExperimentalCLI(t *testing.T) {
 			defer dir.Remove()
 			apiclient := &fakeClient{
 				version: defaultVersion,
+				pingFunc: func() (types.Ping, error) {
+					return types.Ping{Experimental: true, OSType: "linux", APIVersion: defaultVersion}, nil
+				},
 			}
 
 			cli := &DockerCli{client: apiclient, err: os.Stderr}

cli/connhelper/connhelper.go

@@ -53,6 +53,16 @@ func GetConnectionHelper(daemonURL string) (*ConnectionHelper, error) {
 	return nil, err
 }
 
+// GetCommandConnectionHelper returns a ConnectionHelp constructed from an arbitrary command.
+func GetCommandConnectionHelper(cmd string, flags ...string) (*ConnectionHelper, error) {
+	return &ConnectionHelper{
+		Dialer: func(ctx context.Context, network, addr string) (net.Conn, error) {
+			return newCommandConn(ctx, cmd, flags...)
+		},
+		Host: "http://docker",
+	}, nil
+}
+
 func newCommandConn(ctx context.Context, cmd string, args ...string) (net.Conn, error) {
 	var (
 		c   commandConn

docs/extend/cli_plugins.md

@@ -75,6 +75,19 @@ A plugin is required to support all of the global options of the
 top-level CLI, i.e. those listed by `man docker 1` with the exception
 of `-v`.
 
+## Connecting to the docker engine
+
+For consistency plugins should prefer to dial the engine by using the
+`system dial-stdio` subcommand of the main Docker CLI binary.
+
+To facilitate this plugins will be executed with the
+`$DOCKER_CLI_PLUGIN_ORIGINAL_CLI_COMMAND` environment variable
+pointing back to the main Docker CLI binary.
+
+All global options (everything from after the binary name up to, but
+not including, the primary entry point subcommand name) should be
+passed back to the CLI.
+
 ## Installation
 
 Plugins distributed in packages for system wide installation on

e2e/cli-plugins/dial_test.go

@@ -0,0 +1,26 @@
+package cliplugins
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/docker/cli/cli-plugins/manager"
+	"gotest.tools/assert"
+	is "gotest.tools/assert/cmp"
+	"gotest.tools/icmd"
+)
+
+func TestDialStdio(t *testing.T) {
+	// Run the helloworld plugin forcing /bin/true as the `system
+	// dial-stdio` target. It should be passed all arguments from
+	// before the `helloworld` arg, but not the --who=foo which
+	// follows. We observe this from the debug level logging from
+	// the connhelper stuff.
+	helloworld := filepath.Join(os.Getenv("DOCKER_CLI_E2E_PLUGINS_EXTRA_DIRS"), "docker-helloworld")
+	cmd := icmd.Command(helloworld, "--config=blah", "--tls", "--log-level", "debug", "helloworld", "--who=foo")
+	res := icmd.RunCmd(cmd, icmd.WithEnv(manager.ReexecEnvvar+"=/bin/true"))
+	res.Assert(t, icmd.Success)
+	assert.Assert(t, is.Contains(res.Stderr(), `msg="connhelper: starting /bin/true with [--config=blah --tls --log-level debug system dial-stdio]"`))
+	assert.Assert(t, is.Equal(res.Stdout(), "Hello foo!\n"))
+}