rcserver: fix 500 error when marshalling errors from core/command

Before this change attempting to return an error from core/command
failed with a 500 error and a message about unmarshable types.

This is because it was attempting to marshal the input parameters
which get _response added to them which contains an unmarshalable
field.

This was fixed by using the original parameters in the error response
rather than the one modified during the error handling.

This also adds end to end tests for the streaming facilities as used
in core/command.

fs/rc/rcserver/rcserver_test.go

@@ -8,6 +8,7 @@ import (
 	"io/ioutil"
 	"net/http"
 	"net/http/httptest"
+	"os"
 	"regexp"
 	"strings"
 	"testing"
@@ -17,6 +18,7 @@ import (
 	"github.com/stretchr/testify/require"
 
 	_ "github.com/rclone/rclone/backend/local"
+	"github.com/rclone/rclone/fs"
 	"github.com/rclone/rclone/fs/accounting"
 	"github.com/rclone/rclone/fs/rc"
 )
@@ -28,6 +30,21 @@ const (
 	remoteURL       = "[" + testFs + "]/" // initial URL path to fetch from that remote
 )
 
+func TestMain(m *testing.M) {
+	// Pretend to be rclone version if we have a version string parameter
+	if os.Args[len(os.Args)-1] == "version" {
+		fmt.Printf("rclone %s\n", fs.Version)
+		os.Exit(0)
+	}
+	// Pretend to error if we have an unknown command
+	if os.Args[len(os.Args)-1] == "unknown_command" {
+		fmt.Printf("rclone %s\n", fs.Version)
+		fmt.Fprintf(os.Stderr, "Unknown command\n")
+		os.Exit(1)
+	}
+	os.Exit(m.Run())
+}
+
 // Test the RC server runs and we can do HTTP fetches from it.
 // We'll do the majority of the testing with the httptest framework
 func TestRcServer(t *testing.T) {
@@ -456,6 +473,73 @@ func TestRC(t *testing.T) {
 	testServer(t, tests, &opt)
 }
 
+func TestRCWithAuth(t *testing.T) {
+	tests := []testRun{{
+		Name:        "core-command",
+		URL:         "core/command",
+		Method:      "POST",
+		Body:        `command=version`,
+		ContentType: "application/x-www-form-urlencoded",
+		Status:      http.StatusOK,
+		Expected: fmt.Sprintf(`{
+	"error": false,
+	"result": "rclone %s\n"
+}
+`, fs.Version),
+	}, {
+		Name:        "core-command-bad-returnType",
+		URL:         "core/command",
+		Method:      "POST",
+		Body:        `command=version&returnType=POTATO`,
+		ContentType: "application/x-www-form-urlencoded",
+		Status:      http.StatusInternalServerError,
+		Expected: `{
+	"error": "Unknown returnType \"POTATO\"",
+	"input": {
+		"command": "version",
+		"returnType": "POTATO"
+	},
+	"path": "core/command",
+	"status": 500
+}
+`,
+	}, {
+		Name:        "core-command-stream",
+		URL:         "core/command",
+		Method:      "POST",
+		Body:        `command=version&returnType=STREAM`,
+		ContentType: "application/x-www-form-urlencoded",
+		Status:      http.StatusOK,
+		Expected: fmt.Sprintf(`rclone %s
+{}
+`, fs.Version),
+	}, {
+		Name:        "core-command-stream-error",
+		URL:         "core/command",
+		Method:      "POST",
+		Body:        `command=unknown_command&returnType=STREAM`,
+		ContentType: "application/x-www-form-urlencoded",
+		Status:      http.StatusOK,
+		Expected: fmt.Sprintf(`rclone %s
+Unknown command
+{
+	"error": "exit status 1",
+	"input": {
+		"command": "unknown_command",
+		"returnType": "STREAM"
+	},
+	"path": "core/command",
+	"status": 500
+}
+`, fs.Version),
+	}}
+	opt := newTestOpt()
+	opt.Serve = true
+	opt.Files = testFs
+	opt.NoAuth = true
+	testServer(t, tests, &opt)
+}
+
 func TestMethods(t *testing.T) {
 	tests := []testRun{{
 		Name:     "options",