[MM-63774] Allow users to leave private channels when there is only 1 member (#30746)

* allow last user to leave private channel
---------
Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/channel.go

@@ -2378,13 +2378,6 @@ func (a *App) LeaveChannel(c request.CTX, channelID string, userID string) *mode
 		close(uc)
 	}()
 
-	mcc := make(chan store.StoreResult[int64], 1)
-	go func() {
-		count, err := a.Srv().Store().Channel().GetMemberCount(channelID, false)
-		mcc <- store.StoreResult[int64]{Data: count, NErr: err}
-		close(mcc)
-	}()
-
 	cresult := <-sc
 	if cresult.NErr != nil {
 		errCtx := map[string]any{"channel_id": channelID}
@@ -2406,25 +2399,15 @@ func (a *App) LeaveChannel(c request.CTX, channelID string, userID string) *mode
 			return model.NewAppError("LeaveChannel", "app.user.get.app_error", nil, "", http.StatusInternalServerError).Wrap(uresult.NErr)
 		}
 	}
-	ccresult := <-mcc
-	if ccresult.NErr != nil {
-		return model.NewAppError("LeaveChannel", "app.channel.get_member_count.app_error", nil, "", http.StatusInternalServerError).Wrap(ccresult.NErr)
-	}
 
 	channel := cresult.Data
 	user := uresult.Data
-	membersCount := ccresult.Data
 
 	if channel.IsGroupOrDirect() {
 		err := model.NewAppError("LeaveChannel", "api.channel.leave.direct.app_error", nil, "", http.StatusBadRequest)
 		return err
 	}
 
-	if channel.Type == model.ChannelTypePrivate && membersCount == 1 {
-		err := model.NewAppError("LeaveChannel", "api.channel.leave.last_member.app_error", nil, "userId="+user.Id, http.StatusBadRequest)
-		return err
-	}
-
 	if err := a.removeUserFromChannel(c, userID, userID, channel); err != nil {
 		return err
 	}

server/channels/app/channel_test.go

@@ -897,6 +897,17 @@ func TestLeaveChannel(t *testing.T) {
 		require.Nil(t, appErr)
 		require.Len(t, threads.Threads, 1)
 	})
+
+	t.Run("can leave private channel as last member", func(t *testing.T) {
+		channel := th.createChannel(th.Context, th.BasicTeam, model.ChannelTypePrivate)
+
+		count, appErr := th.App.GetChannelMemberCount(th.Context, th.BasicChannel.Id)
+		require.Nil(t, appErr, "It should remove channel membership")
+		require.Equal(t, int64(1), count)
+
+		appErr = th.App.LeaveChannel(th.Context, channel.Id, th.BasicUser.Id)
+		require.Nil(t, appErr)
+	})
 }
 
 func TestLeaveLastChannel(t *testing.T) {

server/i18n/en.json

@@ -419,10 +419,6 @@
     "id": "api.channel.leave.direct.app_error",
     "translation": "Unable to leave a direct message channel."
   },
-  {
-    "id": "api.channel.leave.last_member.app_error",
-    "translation": "You're the only member left, try removing the Private Channel instead of leaving."
-  },
   {
     "id": "api.channel.leave.left",
     "translation": "%v left the channel."