fix: restore os.Args after plugin completion and fix error return

cli-plugins/manager/cobra.go

@@ -47,7 +47,7 @@ func AddPluginCommandStubs(dockerCLI config.Provider, rootCmd *cobra.Command) (e
 					flags.SetOutput(nil)
 					perr := flags.Parse(args)
 					if perr != nil {
-						return err
+						return perr
 					}
 					if flags.Changed("help") {
 						cmd.HelpFunc()(rootCmd, args)
@@ -60,7 +60,11 @@ func AddPluginCommandStubs(dockerCLI config.Provider, rootCmd *cobra.Command) (e
 					cargs := []string{p.Path, cobra.ShellCompRequestCmd, p.Name} //nolint:prealloc // no need to over-complicate things.
 					cargs = append(cargs, args...)
 					cargs = append(cargs, toComplete)
+					origArgs := os.Args
 					os.Args = cargs
+					defer func() {
+						os.Args = origArgs
+					}()
 					runCommand, runErr := PluginRunCommand(dockerCLI, p.Name, cmd)
 					if runErr != nil {
 						return nil, cobra.ShellCompDirectiveError

cli-plugins/manager/cobra_test.go

@@ -1,10 +1,15 @@
 package manager
 
 import (
+	"os"
+	"sync"
 	"testing"
 
+	"github.com/docker/cli/cli/config/configfile"
+	"github.com/docker/cli/internal/test"
 	"github.com/spf13/cobra"
 	"gotest.tools/v3/assert"
+	"gotest.tools/v3/fs"
 )
 
 func TestPluginResourceAttributesEnvvar(t *testing.T) {
@@ -24,3 +29,47 @@ func TestPluginResourceAttributesEnvvar(t *testing.T) {
 	env = appendPluginResourceAttributesEnvvar(nil, cmd, Plugin{Name: "compose"})
 	assert.DeepEqual(t, []string{"OTEL_RESOURCE_ATTRIBUTES=a.b.c=foo,docker.cli.cobra.command_path=docker%20compose"}, env)
 }
+
+func TestPluginStubRunEReturnsParseError(t *testing.T) {
+	cmd := preparePluginStubCommand(t, `{"SchemaVersion":"0.1.0","Vendor":"e2e-testing"}`)
+
+	err := cmd.RunE(cmd, []string{"--definitely-not-a-real-flag"})
+	assert.ErrorContains(t, err, "unknown flag: --definitely-not-a-real-flag")
+}
+
+func TestPluginStubCompletionRestoresOSArgs(t *testing.T) {
+	cmd := preparePluginStubCommand(t, `{"SchemaVersion":"0.1.0"}`)
+
+	originalArgs := []string{"docker", "image", "ls"}
+	os.Args = append([]string(nil), originalArgs...)
+
+	_, directive := cmd.ValidArgsFunction(cmd, []string{"--all"}, "alp")
+	assert.Equal(t, directive, cobra.ShellCompDirectiveError)
+	assert.DeepEqual(t, os.Args, originalArgs)
+}
+
+func preparePluginStubCommand(t *testing.T, metadata string) *cobra.Command {
+	t.Helper()
+	pluginCommandStubsOnce = sync.Once{}
+
+	dir := fs.NewDir(t, t.Name(),
+		fs.WithFile("docker-testplugin", "#!/bin/sh\nprintf '%s' '"+metadata+"'\n", fs.WithMode(0o777)),
+	)
+	t.Cleanup(func() { dir.Remove() })
+
+	cli := test.NewFakeCli(nil)
+	cli.SetConfigFile(&configfile.ConfigFile{
+		CLIPluginsExtraDirs: []string{dir.Path()},
+	})
+
+	root := &cobra.Command{Use: "docker"}
+	root.PersistentFlags().Bool("debug", false, "")
+
+	err := AddPluginCommandStubs(cli, root)
+	assert.NilError(t, err)
+
+	cmd, _, err := root.Find([]string{"testplugin"})
+	assert.NilError(t, err)
+	assert.Assert(t, cmd != nil)
+	return cmd
+}