[MM-55143] Disallow reacting with an emoji that does not exist, limit the total number of unique reactions per post (#25331)

* [MM-55143] Disallow reacting with an emoji that does not exist

* WIP for server limit on emoji reactions

* WIP

* Implement default limit of 25 unique emoji reactions

* Add modal for reaction limit

* Fix test

* PR feedback

* Fix i18n

* Update admin string

* Merge'd

* Fixing some issues, check limits correctly based on other users reactions

* Fix typos

* Fix lint/test

* Add tests, fix other tests

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/reaction_test.go

@@ -13,6 +13,89 @@ import (
 	"github.com/mattermost/mattermost/server/v8/channels/testlib"
 )
 
+func TestSaveReactionForPost(t *testing.T) {
+	th := Setup(t).InitBasic()
+
+	post := th.CreatePost(th.BasicChannel)
+	reaction1, err := th.App.SaveReactionForPost(th.Context, &model.Reaction{
+		UserId:    th.BasicUser.Id,
+		PostId:    post.Id,
+		EmojiName: "cry",
+	})
+	require.NotNil(t, reaction1)
+	require.Nil(t, err)
+	reaction2, err := th.App.SaveReactionForPost(th.Context, &model.Reaction{
+		UserId:    th.BasicUser.Id,
+		PostId:    post.Id,
+		EmojiName: "smile",
+	})
+	require.NotNil(t, reaction2)
+	require.Nil(t, err)
+	reaction3, err := th.App.SaveReactionForPost(th.Context, &model.Reaction{
+		UserId:    th.BasicUser.Id,
+		PostId:    post.Id,
+		EmojiName: "rofl",
+	})
+	require.NotNil(t, reaction3)
+	require.Nil(t, err)
+
+	t.Run("should not add reaction if it does not exist on the system", func(t *testing.T) {
+		reaction := &model.Reaction{
+			UserId:    th.BasicUser.Id,
+			PostId:    th.BasicPost.Id,
+			EmojiName: "definitely-not-a-real-emoji",
+		}
+
+		result, err := th.App.SaveReactionForPost(th.Context, reaction)
+		require.NotNil(t, err)
+		require.Nil(t, result)
+	})
+
+	t.Run("should not add reaction if we are over the limit", func(t *testing.T) {
+		var originalLimit *int
+		th.UpdateConfig(func(cfg *model.Config) {
+			originalLimit = cfg.ServiceSettings.UniqueEmojiReactionLimitPerPost
+			*cfg.ServiceSettings.UniqueEmojiReactionLimitPerPost = 3
+		})
+		defer th.UpdateConfig(func(cfg *model.Config) {
+			cfg.ServiceSettings.UniqueEmojiReactionLimitPerPost = originalLimit
+		})
+
+		reaction := &model.Reaction{
+			UserId:    th.BasicUser.Id,
+			PostId:    post.Id,
+			EmojiName: "joy",
+		}
+
+		result, err := th.App.SaveReactionForPost(th.Context, reaction)
+		require.NotNil(t, err)
+		require.Nil(t, result)
+	})
+
+	t.Run("should always add reaction if we are over the limit but the reaction is not unique", func(t *testing.T) {
+		user := th.CreateUser()
+
+		var originalLimit *int
+		th.UpdateConfig(func(cfg *model.Config) {
+			originalLimit = cfg.ServiceSettings.UniqueEmojiReactionLimitPerPost
+			*cfg.ServiceSettings.UniqueEmojiReactionLimitPerPost = 3
+		})
+		defer th.UpdateConfig(func(cfg *model.Config) {
+			cfg.ServiceSettings.UniqueEmojiReactionLimitPerPost = originalLimit
+		})
+
+		reaction := &model.Reaction{
+			UserId:    user.Id,
+			PostId:    post.Id,
+			EmojiName: "cry",
+		}
+
+		result, err := th.App.SaveReactionForPost(th.Context, reaction)
+		require.Nil(t, err)
+		require.NotNil(t, result)
+	})
+}
+
 func TestSharedChannelSyncForReactionActions(t *testing.T) {
 	t.Run("adding a reaction in a shared channel performs a content sync when sync service is running on that node", func(t *testing.T) {
 		th := Setup(t).InitBasic()
@@ -84,3 +167,15 @@ func TestSharedChannelSyncForReactionActions(t *testing.T) {
 		assert.Equal(t, channel.Id, sharedChannelService.channelNotifications[1])
 	})
 }
+
+func (th *TestHelper) UpdateConfig(f func(*model.Config)) {
+	if th.ConfigStore.IsReadOnly() {
+		return
+	}
+	old := th.ConfigStore.Get()
+	updated := old.Clone()
+	f(updated)
+	if _, _, err := th.ConfigStore.Set(updated); err != nil {
+		panic(err)
+	}
+}