[MM-24987] Dont call GetSchemeRolesForChannel and instead load the scheme from channel (#14514)

* MM-24987 Dont call a.GetSchemeRolesForChannel and instead load the scheme using the channel directly

* Trigger CI

* MM-24987 Add a test for concurrent patch to channel moderation

* MM-24987 Cleanup

app/channel.go

@@ -494,8 +494,10 @@ func (a *App) CreateChannelScheme(channel *model.Channel) (*model.Scheme, *model
 
 // DeleteChannelScheme deletes a channels scheme and sets its SchemeId to nil.
 func (a *App) DeleteChannelScheme(channel *model.Channel) (*model.Channel, *model.AppError) {
-	if _, err := a.DeleteScheme(*channel.SchemeId); err != nil {
+	if channel.SchemeId != nil && len(*channel.SchemeId) != 0 {
-		return nil, err
+		if _, err := a.DeleteScheme(*channel.SchemeId); err != nil {
+			return nil, err
+		}
 	}
 	channel.SchemeId = nil
 	return a.UpdateChannelScheme(channel)
@@ -760,22 +762,26 @@ func (a *App) PatchChannelModerationsForChannel(channel *model.Channel, channelM
 		}
 	}
 
+	var scheme *model.Scheme
 	// Channel has no scheme so create one
 	if channel.SchemeId == nil || len(*channel.SchemeId) == 0 {
-		if _, err = a.CreateChannelScheme(channel); err != nil {
+		scheme, err = a.CreateChannelScheme(channel)
+		if err != nil {
 			return nil, err
 		}
 
 		message := model.NewWebSocketEvent(model.WEBSOCKET_EVENT_CHANNEL_SCHEME_UPDATED, "", channel.Id, "", nil)
 		a.Publish(message)
 		mlog.Info("Permission scheme created.", mlog.String("channel_id", channel.Id), mlog.String("channel_name", channel.Name))
+	} else {
+		scheme, err = a.GetScheme(*channel.SchemeId)
+		if err != nil {
+			return nil, err
+		}
 	}
 
-	guestRoleName, memberRoleName, _, err := a.GetSchemeRolesForChannel(channel.Id)
+	guestRoleName := scheme.DefaultChannelGuestRole
-	if err != nil {
+	memberRoleName := scheme.DefaultChannelUserRole
-		return nil, err
-	}
-
 	memberRole, err := a.GetRoleByName(memberRoleName)
 	if err != nil {
 		return nil, err

app/channel_test.go

@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"sort"
 	"strings"
+	"sync"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -1657,6 +1658,52 @@ func TestPatchChannelModerationsForChannel(t *testing.T) {
 			}
 		})
 	}
+
+	t.Run("Handles concurrent patch requests gracefully", func(t *testing.T) {
+		addCreatePosts := []*model.ChannelModerationPatch{
+			{
+				Name: &createPosts,
+				Roles: &model.ChannelModeratedRolesPatch{
+					Members: model.NewBool(false),
+					Guests:  model.NewBool(false),
+				},
+			},
+		}
+		removeCreatePosts := []*model.ChannelModerationPatch{
+			{
+				Name: &createPosts,
+				Roles: &model.ChannelModeratedRolesPatch{
+					Members: model.NewBool(false),
+					Guests:  model.NewBool(false),
+				},
+			},
+		}
+
+		wg := sync.WaitGroup{}
+		wg.Add(20)
+		for i := 0; i < 10; i++ {
+			go func() {
+				th.App.PatchChannelModerationsForChannel(channel, addCreatePosts)
+				th.App.PatchChannelModerationsForChannel(channel, removeCreatePosts)
+				wg.Done()
+			}()
+		}
+		for i := 0; i < 10; i++ {
+			go func() {
+				th.App.PatchChannelModerationsForChannel(channel, addCreatePosts)
+				th.App.PatchChannelModerationsForChannel(channel, removeCreatePosts)
+				wg.Done()
+			}()
+		}
+		wg.Wait()
+
+		higherScopedGuestRoleName, higherScopedMemberRoleName, _, _ := th.App.GetTeamSchemeChannelRoles(channel.TeamId)
+		higherScopedMemberRole, _ := th.App.GetRoleByName(higherScopedMemberRoleName)
+		higherScopedGuestRole, _ := th.App.GetRoleByName(higherScopedGuestRoleName)
+		assert.Contains(t, higherScopedMemberRole.Permissions, createPosts)
+		assert.Contains(t, higherScopedGuestRole.Permissions, createPosts)
+	})
+
 }
 
 // TestMarkChannelsAsViewedPanic verifies that returning an error from a.GetUser