[feature] Implement following hashtags (#3141)

* Implement followed tags API

* Insert statuses with followed tags into home timelines

* Test following and unfollowing tags

* Correct Swagger path params

* Trim conversation caches

* Migration for followed_tags table

* Followed tag caches and DB implementation

* Lint and tests

* Add missing tag info endpoint, reorganize tag API

* Unwrap boosts when timelining based on tags

* Apply visibility filters to tag followers

* Address review comments

2 files changed, 34 insertions, 0 deletions

diff --git a/internal/cache/cache.go b/internal/cache/cache.go
index 8187ba419..2949d528a 100644
--- a/internal/cache/cache.go
+++ b/internal/cache/cache.go
@@ -57,6 +57,7 @@ func (c *Caches) Init() {
 	log.Infof(nil, "init: %p", c)
 
 	c.initAccount()
+	c.initAccountIDsFollowingTag()
 	c.initAccountNote()
 	c.initAccountSettings()
 	c.initAccountStats()
@@ -98,6 +99,7 @@ func (c *Caches) Init() {
 	c.initStatusFave()
 	c.initStatusFaveIDs()
 	c.initTag()
+	c.initTagIDsFollowedByAccount()
 	c.initThreadMute()
 	c.initToken()
 	c.initTombstone()
@@ -134,6 +136,7 @@ func (c *Caches) Stop() {
 // significant overhead to all cache writes.
 func (c *Caches) Sweep(threshold float64) {
 	c.DB.Account.Trim(threshold)
+	c.DB.AccountIDsFollowingTag.Trim(threshold)
 	c.DB.AccountNote.Trim(threshold)
 	c.DB.AccountSettings.Trim(threshold)
 	c.DB.AccountStats.Trim(threshold)
@@ -142,6 +145,8 @@ func (c *Caches) Sweep(threshold float64) {
 	c.DB.BlockIDs.Trim(threshold)
 	c.DB.BoostOfIDs.Trim(threshold)
 	c.DB.Client.Trim(threshold)
+	c.DB.Conversation.Trim(threshold)
+	c.DB.ConversationLastStatusIDs.Trim(threshold)
 	c.DB.Emoji.Trim(threshold)
 	c.DB.EmojiCategory.Trim(threshold)
 	c.DB.Filter.Trim(threshold)
@@ -171,6 +176,7 @@ func (c *Caches) Sweep(threshold float64) {
 	c.DB.StatusFave.Trim(threshold)
 	c.DB.StatusFaveIDs.Trim(threshold)
 	c.DB.Tag.Trim(threshold)
+	c.DB.TagIDsFollowedByAccount.Trim(threshold)
 	c.DB.ThreadMute.Trim(threshold)
 	c.DB.Token.Trim(threshold)
 	c.DB.Tombstone.Trim(threshold)
diff --git a/internal/cache/db.go b/internal/cache/db.go
index 16e1d286a..c1b87ef96 100644
--- a/internal/cache/db.go
+++ b/internal/cache/db.go
@@ -29,6 +29,9 @@ type DBCaches struct {
 	// Account provides access to the gtsmodel Account database cache.
 	Account StructCache[*gtsmodel.Account]
 
+	// AccountIDsFollowingTag caches account IDs following a given tag ID.
+	AccountIDsFollowingTag SliceCache[string]
+
 	// AccountNote provides access to the gtsmodel Note database cache.
 	AccountNote StructCache[*gtsmodel.AccountNote]
 
@@ -160,6 +163,9 @@ type DBCaches struct {
 	// Tag provides access to the gtsmodel Tag database cache.
 	Tag StructCache[*gtsmodel.Tag]
 
+	// TagIDsFollowedByAccount caches tag IDs followed by a given account ID.
+	TagIDsFollowedByAccount SliceCache[string]
+
 	// ThreadMute provides access to the gtsmodel ThreadMute database cache.
 	ThreadMute StructCache[*gtsmodel.ThreadMute]
 
@@ -234,6 +240,17 @@ func (c *Caches) initAccount() {
 	})
 }
 
+func (c *Caches) initAccountIDsFollowingTag() {
+	// Calculate maximum cache size.
+	cap := calculateSliceCacheMax(
+		config.GetCacheAccountIDsFollowingTagMemRatio(),
+	)
+
+	log.Infof(nil, "cache size = %d", cap)
+
+	c.DB.AccountIDsFollowingTag.Init(0, cap)
+}
+
 func (c *Caches) initAccountNote() {
 	// Calculate maximum cache size.
 	cap := calculateResultCacheMax(
@@ -1317,6 +1334,17 @@ func (c *Caches) initTag() {
 	})
 }
 
+func (c *Caches) initTagIDsFollowedByAccount() {
+	// Calculate maximum cache size.
+	cap := calculateSliceCacheMax(
+		config.GetCacheTagIDsFollowedByAccountMemRatio(),
+	)
+
+	log.Infof(nil, "cache size = %d", cap)
+
+	c.DB.TagIDsFollowedByAccount.Init(0, cap)
+}
+
 func (c *Caches) initThreadMute() {
 	cap := calculateResultCacheMax(
 		sizeofThreadMute(), // model in-mem size.