MM-21343: Recognize multi-line slash commands without requiring trailing space after trigger word (#13646)

* change how we are parsing the args.command

* change comment

* atttempt to write unit test

* remove quotes

* use unicode.IsSpace

* MM-21343 fix panic

* use table

* test cases

* bad request, add test for missing '/'

* mispelled err

* Update app/command.go

Co-Authored-By: Jesse Hallam <jesse.hallam@gmail.com>

* Update app/command.go

Co-Authored-By: Jesse Hallam <jesse.hallam@gmail.com>

* use a map

* use subtest and add test case of empty command

* go fmt

* do not use t.fatal

* Update app/command.go

Co-Authored-By: Bot from GolangCI <42910462+golangcibot@users.noreply.github.com>

* fix missing unicode import

Co-authored-by: wiggin77 <wiggin77@warpmail.net>
Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>
Co-authored-by: mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Bot from GolangCI <42910462+golangcibot@users.noreply.github.com>

app/command.go

@@ -10,6 +10,7 @@ import (
 	"net/url"
 	"strings"
 	"sync"
+	"unicode"
 
 	goi18n "github.com/mattermost/go-i18n/i18n"
 	"github.com/mattermost/mattermost-server/v5/mlog"
@@ -159,10 +160,20 @@ func (a *App) ListAllCommands(teamId string, T goi18n.TranslateFunc) ([]*model.C
 
 // @openTracingParams args
 func (a *App) ExecuteCommand(args *model.CommandArgs) (*model.CommandResponse, *model.AppError) {
-	parts := strings.Split(args.Command, " ")
-	trigger := parts[0][1:]
+	trigger := ""
+	message := ""
+	index := strings.IndexFunc(args.Command, unicode.IsSpace)
+	if index != -1 {
+		trigger = args.Command[:index]
+		message = args.Command[index+1:]
+	} else {
+		trigger = args.Command
+	}
 	trigger = strings.ToLower(trigger)
-	message := strings.Join(parts[1:], " ")
+	if !strings.HasPrefix(trigger, "/") {
+		return nil, model.NewAppError("command", "api.command.execute_command.format.app_error", map[string]interface{}{"Trigger": trigger}, "", http.StatusBadRequest)
+	}
+	trigger = strings.TrimPrefix(trigger, "/")
 
 	clientTriggerId, triggerId, appErr := model.GenerateTriggerId(args.UserId, a.AsymmetricSigningKey())
 	if appErr != nil {