[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, 812 insertions, 40 deletions

diff --git a/internal/processing/workers/fromclientapi_test.go b/internal/processing/workers/fromclientapi_test.go
index 35c2c31b7..b4eae0be0 100644
--- a/internal/processing/workers/fromclientapi_test.go
+++ b/internal/processing/workers/fromclientapi_test.go
@@ -52,6 +52,7 @@ func (suite *FromClientAPITestSuite) newStatus(
 	boostOfStatus *gtsmodel.Status,
 	mentionedAccounts []*gtsmodel.Account,
 	createThread bool,
+	tagIDs []string,
 ) *gtsmodel.Status {
 	var (
 		protocol = config.GetProtocol()
@@ -65,6 +66,7 @@ func (suite *FromClientAPITestSuite) newStatus(
 		URI:                 protocol + "://" + host + "/users/" + account.Username + "/statuses/" + statusID,
 		URL:                 protocol + "://" + host + "/@" + account.Username + "/statuses/" + statusID,
 		Content:             "pee pee poo poo",
+		TagIDs:              tagIDs,
 		Local:               util.Ptr(true),
 		AccountURI:          account.URI,
 		AccountID:           account.ID,
@@ -256,6 +258,7 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusWithNotification() {
 			nil,
 			nil,
 			false,
+			nil,
 		)
 	)
 
@@ -367,6 +370,7 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusReply() {
 			nil,
 			nil,
 			false,
+			nil,
 		)
 	)
 
@@ -428,6 +432,7 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusReplyMuted() {
 			nil,
 			nil,
 			false,
+			nil,
 		)
 		threadMute = &gtsmodel.ThreadMute{
 			ID:        "01HD3KRMBB1M85QRWHD912QWRE",
@@ -488,6 +493,7 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusBoostMuted() {
 			suite.testStatuses["local_account_1_status_1"],
 			nil,
 			false,
+			nil,
 		)
 		threadMute = &gtsmodel.ThreadMute{
 			ID:        "01HD3KRMBB1M85QRWHD912QWRE",
@@ -553,6 +559,7 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusListRepliesPolicyLis
 			nil,
 			nil,
 			false,
+			nil,
 		)
 	)
 
@@ -628,6 +635,7 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusListRepliesPolicyLis
 			nil,
 			nil,
 			false,
+			nil,
 		)
 	)
 
@@ -708,6 +716,7 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusReplyListRepliesPoli
 			nil,
 			nil,
 			false,
+			nil,
 		)
 	)
 
@@ -780,6 +789,7 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusBoost() {
 			suite.testStatuses["local_account_2_status_1"],
 			nil,
 			false,
+			nil,
 		)
 	)
 
@@ -843,6 +853,7 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusBoostNoReblogs() {
 			suite.testStatuses["local_account_2_status_1"],
 			nil,
 			false,
+			nil,
 		)
 	)
 
@@ -912,6 +923,7 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusWhichBeginsConversat
 			nil,
 			[]*gtsmodel.Account{receivingAccount},
 			true,
+			nil,
 		)
 	)
 
@@ -997,6 +1009,7 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusWhichShouldNotCreate
 			nil,
 			[]*gtsmodel.Account{receivingAccount},
 			true,
+			nil,
 		)
 	)
 
@@ -1038,6 +1051,555 @@ func (suite *FromClientAPITestSuite) TestProcessCreateStatusWhichShouldNotCreate
 	)
 }
 
+// A public status with a hashtag followed by a local user who does not otherwise follow the author
+// should end up in the tag-following user's home timeline.
+func (suite *FromClientAPITestSuite) TestProcessCreateStatusWithFollowedHashtag() {
+	testStructs := suite.SetupTestStructs()
+	defer suite.TearDownTestStructs(testStructs)
+
+	var (
+		ctx              = context.Background()
+		postingAccount   = suite.testAccounts["admin_account"]
+		receivingAccount = suite.testAccounts["local_account_2"]
+		streams          = suite.openStreams(ctx,
+			testStructs.Processor,
+			receivingAccount,
+			nil,
+		)
+		homeStream = streams[stream.TimelineHome]
+		testTag    = suite.testTags["welcome"]
+
+		// postingAccount posts a new public status not mentioning anyone but using testTag.
+		status = suite.newStatus(
+			ctx,
+			testStructs.State,
+			postingAccount,
+			gtsmodel.VisibilityPublic,
+			nil,
+			nil,
+			nil,
+			false,
+			[]string{testTag.ID},
+		)
+	)
+
+	// Check precondition: receivingAccount does not follow postingAccount.
+	following, err := testStructs.State.DB.IsFollowing(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(following)
+
+	// Check precondition: receivingAccount does not block postingAccount or vice versa.
+	blocking, err := testStructs.State.DB.IsEitherBlocked(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(blocking)
+
+	// Setup: receivingAccount follows testTag.
+	if err := testStructs.State.DB.PutFollowedTag(ctx, receivingAccount.ID, testTag.ID); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Process the new status.
+	if err := testStructs.Processor.Workers().ProcessFromClientAPI(
+		ctx,
+		&messages.FromClientAPI{
+			APObjectType:   ap.ObjectNote,
+			APActivityType: ap.ActivityCreate,
+			GTSModel:       status,
+			Origin:         postingAccount,
+		},
+	); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Check status in home stream.
+	suite.checkStreamed(
+		homeStream,
+		true,
+		"",
+		stream.EventTypeUpdate,
+	)
+}
+
+// A public status with a hashtag followed by a local user who does not otherwise follow the author
+// should not end up in the tag-following user's home timeline
+// if the user has the author blocked.
+func (suite *FromClientAPITestSuite) TestProcessCreateStatusWithFollowedHashtagAndBlock() {
+	testStructs := suite.SetupTestStructs()
+	defer suite.TearDownTestStructs(testStructs)
+
+	var (
+		ctx              = context.Background()
+		postingAccount   = suite.testAccounts["remote_account_1"]
+		receivingAccount = suite.testAccounts["local_account_2"]
+		streams          = suite.openStreams(ctx,
+			testStructs.Processor,
+			receivingAccount,
+			nil,
+		)
+		homeStream = streams[stream.TimelineHome]
+		testTag    = suite.testTags["welcome"]
+
+		// postingAccount posts a new public status not mentioning anyone but using testTag.
+		status = suite.newStatus(
+			ctx,
+			testStructs.State,
+			postingAccount,
+			gtsmodel.VisibilityPublic,
+			nil,
+			nil,
+			nil,
+			false,
+			[]string{testTag.ID},
+		)
+	)
+
+	// Check precondition: receivingAccount does not follow postingAccount.
+	following, err := testStructs.State.DB.IsFollowing(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(following)
+
+	// Check precondition: postingAccount does not block receivingAccount.
+	blocking, err := testStructs.State.DB.IsBlocked(ctx, postingAccount.ID, receivingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(blocking)
+
+	// Check precondition: receivingAccount blocks postingAccount.
+	blocking, err = testStructs.State.DB.IsBlocked(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.True(blocking)
+
+	// Setup: receivingAccount follows testTag.
+	if err := testStructs.State.DB.PutFollowedTag(ctx, receivingAccount.ID, testTag.ID); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Process the new status.
+	if err := testStructs.Processor.Workers().ProcessFromClientAPI(
+		ctx,
+		&messages.FromClientAPI{
+			APObjectType:   ap.ObjectNote,
+			APActivityType: ap.ActivityCreate,
+			GTSModel:       status,
+			Origin:         postingAccount,
+		},
+	); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Check status in home stream.
+	suite.checkStreamed(
+		homeStream,
+		false,
+		"",
+		"",
+	)
+}
+
+// A boost of a public status with a hashtag followed by a local user
+// who does not otherwise follow the author or booster
+// should end up in the tag-following user's home timeline as the original status.
+func (suite *FromClientAPITestSuite) TestProcessCreateBoostWithFollowedHashtag() {
+	testStructs := suite.SetupTestStructs()
+	defer suite.TearDownTestStructs(testStructs)
+
+	var (
+		ctx              = context.Background()
+		postingAccount   = suite.testAccounts["remote_account_2"]
+		boostingAccount  = suite.testAccounts["admin_account"]
+		receivingAccount = suite.testAccounts["local_account_2"]
+		streams          = suite.openStreams(ctx,
+			testStructs.Processor,
+			receivingAccount,
+			nil,
+		)
+		homeStream = streams[stream.TimelineHome]
+		testTag    = suite.testTags["welcome"]
+
+		// postingAccount posts a new public status not mentioning anyone but using testTag.
+		status = suite.newStatus(
+			ctx,
+			testStructs.State,
+			postingAccount,
+			gtsmodel.VisibilityPublic,
+			nil,
+			nil,
+			nil,
+			false,
+			[]string{testTag.ID},
+		)
+
+		// boostingAccount boosts that status.
+		boost = suite.newStatus(
+			ctx,
+			testStructs.State,
+			boostingAccount,
+			gtsmodel.VisibilityPublic,
+			nil,
+			status,
+			nil,
+			false,
+			nil,
+		)
+	)
+
+	// Check precondition: receivingAccount does not follow postingAccount.
+	following, err := testStructs.State.DB.IsFollowing(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(following)
+
+	// Check precondition: receivingAccount does not block postingAccount or vice versa.
+	blocking, err := testStructs.State.DB.IsEitherBlocked(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(blocking)
+
+	// Check precondition: receivingAccount does not follow boostingAccount.
+	following, err = testStructs.State.DB.IsFollowing(ctx, receivingAccount.ID, boostingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(following)
+
+	// Check precondition: receivingAccount does not block boostingAccount or vice versa.
+	blocking, err = testStructs.State.DB.IsEitherBlocked(ctx, receivingAccount.ID, boostingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(blocking)
+
+	// Setup: receivingAccount follows testTag.
+	if err := testStructs.State.DB.PutFollowedTag(ctx, receivingAccount.ID, testTag.ID); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Process the boost.
+	if err := testStructs.Processor.Workers().ProcessFromClientAPI(
+		ctx,
+		&messages.FromClientAPI{
+			APObjectType:   ap.ActivityAnnounce,
+			APActivityType: ap.ActivityCreate,
+			GTSModel:       boost,
+			Origin:         postingAccount,
+		},
+	); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Check status in home stream.
+	suite.checkStreamed(
+		homeStream,
+		true,
+		"",
+		stream.EventTypeUpdate,
+	)
+}
+
+// A boost of a public status with a hashtag followed by a local user
+// who does not otherwise follow the author or booster
+// should not end up in the tag-following user's home timeline
+// if the user has the author blocked.
+func (suite *FromClientAPITestSuite) TestProcessCreateBoostWithFollowedHashtagAndBlock() {
+	testStructs := suite.SetupTestStructs()
+	defer suite.TearDownTestStructs(testStructs)
+
+	var (
+		ctx              = context.Background()
+		postingAccount   = suite.testAccounts["remote_account_1"]
+		boostingAccount  = suite.testAccounts["admin_account"]
+		receivingAccount = suite.testAccounts["local_account_2"]
+		streams          = suite.openStreams(ctx,
+			testStructs.Processor,
+			receivingAccount,
+			nil,
+		)
+		homeStream = streams[stream.TimelineHome]
+		testTag    = suite.testTags["welcome"]
+
+		// postingAccount posts a new public status not mentioning anyone but using testTag.
+		status = suite.newStatus(
+			ctx,
+			testStructs.State,
+			postingAccount,
+			gtsmodel.VisibilityPublic,
+			nil,
+			nil,
+			nil,
+			false,
+			[]string{testTag.ID},
+		)
+
+		// boostingAccount boosts that status.
+		boost = suite.newStatus(
+			ctx,
+			testStructs.State,
+			boostingAccount,
+			gtsmodel.VisibilityPublic,
+			nil,
+			status,
+			nil,
+			false,
+			nil,
+		)
+	)
+
+	// Check precondition: receivingAccount does not follow postingAccount.
+	following, err := testStructs.State.DB.IsFollowing(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(following)
+
+	// Check precondition: postingAccount does not block receivingAccount.
+	blocking, err := testStructs.State.DB.IsBlocked(ctx, postingAccount.ID, receivingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(blocking)
+
+	// Check precondition: receivingAccount blocks postingAccount.
+	blocking, err = testStructs.State.DB.IsBlocked(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.True(blocking)
+
+	// Check precondition: receivingAccount does not follow boostingAccount.
+	following, err = testStructs.State.DB.IsFollowing(ctx, receivingAccount.ID, boostingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(following)
+
+	// Check precondition: receivingAccount does not block boostingAccount or vice versa.
+	blocking, err = testStructs.State.DB.IsEitherBlocked(ctx, receivingAccount.ID, boostingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(blocking)
+
+	// Setup: receivingAccount follows testTag.
+	if err := testStructs.State.DB.PutFollowedTag(ctx, receivingAccount.ID, testTag.ID); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Process the boost.
+	if err := testStructs.Processor.Workers().ProcessFromClientAPI(
+		ctx,
+		&messages.FromClientAPI{
+			APObjectType:   ap.ActivityAnnounce,
+			APActivityType: ap.ActivityCreate,
+			GTSModel:       boost,
+			Origin:         postingAccount,
+		},
+	); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Check status in home stream.
+	suite.checkStreamed(
+		homeStream,
+		false,
+		"",
+		"",
+	)
+}
+
+// A boost of a public status with a hashtag followed by a local user
+// who does not otherwise follow the author or booster
+// should not end up in the tag-following user's home timeline
+// if the user has the booster blocked.
+func (suite *FromClientAPITestSuite) TestProcessCreateBoostWithFollowedHashtagAndBlockedBoost() {
+	testStructs := suite.SetupTestStructs()
+	defer suite.TearDownTestStructs(testStructs)
+
+	var (
+		ctx              = context.Background()
+		postingAccount   = suite.testAccounts["admin_account"]
+		boostingAccount  = suite.testAccounts["remote_account_1"]
+		receivingAccount = suite.testAccounts["local_account_2"]
+		streams          = suite.openStreams(ctx,
+			testStructs.Processor,
+			receivingAccount,
+			nil,
+		)
+		homeStream = streams[stream.TimelineHome]
+		testTag    = suite.testTags["welcome"]
+
+		// postingAccount posts a new public status not mentioning anyone but using testTag.
+		status = suite.newStatus(
+			ctx,
+			testStructs.State,
+			postingAccount,
+			gtsmodel.VisibilityPublic,
+			nil,
+			nil,
+			nil,
+			false,
+			[]string{testTag.ID},
+		)
+
+		// boostingAccount boosts that status.
+		boost = suite.newStatus(
+			ctx,
+			testStructs.State,
+			boostingAccount,
+			gtsmodel.VisibilityPublic,
+			nil,
+			status,
+			nil,
+			false,
+			nil,
+		)
+	)
+
+	// Check precondition: receivingAccount does not follow postingAccount.
+	following, err := testStructs.State.DB.IsFollowing(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(following)
+
+	// Check precondition: receivingAccount does not block postingAccount or vice versa.
+	blocking, err := testStructs.State.DB.IsEitherBlocked(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(blocking)
+
+	// Check precondition: receivingAccount does not follow boostingAccount.
+	following, err = testStructs.State.DB.IsFollowing(ctx, receivingAccount.ID, boostingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(following)
+
+	// Check precondition: boostingAccount does not block receivingAccount.
+	blocking, err = testStructs.State.DB.IsBlocked(ctx, boostingAccount.ID, receivingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(blocking)
+
+	// Check precondition: receivingAccount blocks boostingAccount.
+	blocking, err = testStructs.State.DB.IsBlocked(ctx, receivingAccount.ID, boostingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.True(blocking)
+
+	// Setup: receivingAccount follows testTag.
+	if err := testStructs.State.DB.PutFollowedTag(ctx, receivingAccount.ID, testTag.ID); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Process the boost.
+	if err := testStructs.Processor.Workers().ProcessFromClientAPI(
+		ctx,
+		&messages.FromClientAPI{
+			APObjectType:   ap.ActivityAnnounce,
+			APActivityType: ap.ActivityCreate,
+			GTSModel:       boost,
+			Origin:         postingAccount,
+		},
+	); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Check status in home stream.
+	suite.checkStreamed(
+		homeStream,
+		false,
+		"",
+		"",
+	)
+}
+
+// Updating a public status with a hashtag followed by a local user who does not otherwise follow the author
+// should stream a status update to the tag-following user's home timeline.
+func (suite *FromClientAPITestSuite) TestProcessUpdateStatusWithFollowedHashtag() {
+	testStructs := suite.SetupTestStructs()
+	defer suite.TearDownTestStructs(testStructs)
+
+	var (
+		ctx              = context.Background()
+		postingAccount   = suite.testAccounts["admin_account"]
+		receivingAccount = suite.testAccounts["local_account_2"]
+		streams          = suite.openStreams(ctx,
+			testStructs.Processor,
+			receivingAccount,
+			nil,
+		)
+		homeStream = streams[stream.TimelineHome]
+		testTag    = suite.testTags["welcome"]
+
+		// postingAccount posts a new public status not mentioning anyone but using testTag.
+		status = suite.newStatus(
+			ctx,
+			testStructs.State,
+			postingAccount,
+			gtsmodel.VisibilityPublic,
+			nil,
+			nil,
+			nil,
+			false,
+			[]string{testTag.ID},
+		)
+	)
+
+	// Check precondition: receivingAccount does not follow postingAccount.
+	following, err := testStructs.State.DB.IsFollowing(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(following)
+
+	// Check precondition: receivingAccount does not block postingAccount or vice versa.
+	blocking, err := testStructs.State.DB.IsEitherBlocked(ctx, receivingAccount.ID, postingAccount.ID)
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+	suite.False(blocking)
+
+	// Setup: receivingAccount follows testTag.
+	if err := testStructs.State.DB.PutFollowedTag(ctx, receivingAccount.ID, testTag.ID); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Update the status.
+	if err := testStructs.Processor.Workers().ProcessFromClientAPI(
+		ctx,
+		&messages.FromClientAPI{
+			APObjectType:   ap.ObjectNote,
+			APActivityType: ap.ActivityUpdate,
+			GTSModel:       status,
+			Origin:         postingAccount,
+		},
+	); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Check status in home stream.
+	suite.checkStreamed(
+		homeStream,
+		true,
+		"",
+		stream.EventTypeStatusUpdate,
+	)
+}
+
 func (suite *FromClientAPITestSuite) TestProcessStatusDelete() {
 	testStructs := suite.SetupTestStructs()
 	defer suite.TearDownTestStructs(testStructs)
diff --git a/internal/processing/workers/surfacetimeline.go b/internal/processing/workers/surfacetimeline.go
index 7bd0a51c6..c0987effd 100644
--- a/internal/processing/workers/surfacetimeline.go
+++ b/internal/processing/workers/surfacetimeline.go
@@ -30,10 +30,12 @@ import (
 	"github.com/superseriousbusiness/gotosocial/internal/log"
 	"github.com/superseriousbusiness/gotosocial/internal/stream"
 	"github.com/superseriousbusiness/gotosocial/internal/timeline"
+	"github.com/superseriousbusiness/gotosocial/internal/util"
 )
 
 // timelineAndNotifyStatus inserts the given status into the HOME
-// and LIST timelines of accounts that follow the status author.
+// and LIST timelines of accounts that follow the status author,
+// as well as the HOME timelines of accounts that follow tags used by the status.
 //
 // It will also handle notifications for any mentions attached to
 // the account, notifications for any local accounts that want
@@ -56,18 +58,24 @@ func (s *Surface) timelineAndNotifyStatus(ctx context.Context, status *gtsmodel.
 		follows = append(follows, &gtsmodel.Follow{
 			AccountID:   status.AccountID,
 			Account:     status.Account,
-			Notify:      func() *bool { b := false; return &b }(), // Account shouldn't notify itself.
-			ShowReblogs: func() *bool { b := true; return &b }(),  // Account should show own reblogs.
+			Notify:      util.Ptr(false), // Account shouldn't notify itself.
+			ShowReblogs: util.Ptr(true),  // Account should show own reblogs.
 		})
 	}
 
 	// Timeline the status for each local follower of this account.
 	// This will also handle notifying any followers with notify
 	// set to true on their follow.
-	if err := s.timelineAndNotifyStatusForFollowers(ctx, status, follows); err != nil {
+	homeTimelinedAccountIDs, err := s.timelineAndNotifyStatusForFollowers(ctx, status, follows)
+	if err != nil {
 		return gtserror.Newf("error timelining status %s for followers: %w", status.ID, err)
 	}
 
+	// Timeline the status for each local account who follows a tag used by this status.
+	if err := s.timelineAndNotifyStatusForTagFollowers(ctx, status, homeTimelinedAccountIDs); err != nil {
+		return gtserror.Newf("error timelining status %s for tag followers: %w", status.ID, err)
+	}
+
 	// Notify each local account that's mentioned by this status.
 	if err := s.notifyMentions(ctx, status); err != nil {
 		return gtserror.Newf("error notifying status mentions for status %s: %w", status.ID, err)
@@ -90,15 +98,18 @@ func (s *Surface) timelineAndNotifyStatus(ctx context.Context, status *gtsmodel.
 // adding the status to list timelines + home timelines of each
 // follower, as appropriate, and notifying each follower of the
 // new status, if the status is eligible for notification.
+//
+// Returns a list of accounts which had this status inserted into their home timelines.
 func (s *Surface) timelineAndNotifyStatusForFollowers(
 	ctx context.Context,
 	status *gtsmodel.Status,
 	follows []*gtsmodel.Follow,
-) error {
+) ([]string, error) {
 	var (
-		errs  gtserror.MultiError
-		boost = status.BoostOfID != ""
-		reply = status.InReplyToURI != ""
+		errs                    gtserror.MultiError
+		boost                   = status.BoostOfID != ""
+		reply                   = status.InReplyToURI != ""
+		homeTimelinedAccountIDs = []string{}
 	)
 
 	for _, follow := range follows {
@@ -122,16 +133,11 @@ func (s *Surface) timelineAndNotifyStatusForFollowers(
 			continue
 		}
 
-		filters, err := s.State.DB.GetFiltersForAccountID(ctx, follow.AccountID)
-		if err != nil {
-			return gtserror.Newf("couldn't retrieve filters for account %s: %w", follow.AccountID, err)
-		}
-
-		mutes, err := s.State.DB.GetAccountMutes(gtscontext.SetBarebones(ctx), follow.AccountID, nil)
+		filters, mutes, err := s.getFiltersAndMutes(ctx, follow.AccountID)
 		if err != nil {
-			return gtserror.Newf("couldn't retrieve mutes for account %s: %w", follow.AccountID, err)
+			errs.Append(err)
+			continue
 		}
-		compiledMutes := usermute.NewCompiledUserMuteList(mutes)
 
 		// Add status to any relevant lists
 		// for this follow, if applicable.
@@ -141,7 +147,7 @@ func (s *Surface) timelineAndNotifyStatusForFollowers(
 			follow,
 			&errs,
 			filters,
-			compiledMutes,
+			mutes,
 		)
 
 		// Add status to home timeline for owner
@@ -154,7 +160,7 @@ func (s *Surface) timelineAndNotifyStatusForFollowers(
 			status,
 			stream.TimelineHome,
 			filters,
-			compiledMutes,
+			mutes,
 		)
 		if err != nil {
 			errs.Appendf("error home timelining status: %w", err)
@@ -166,6 +172,7 @@ func (s *Surface) timelineAndNotifyStatusForFollowers(
 			// timeline, we shouldn't notify it.
 			continue
 		}
+		homeTimelinedAccountIDs = append(homeTimelinedAccountIDs, follow.AccountID)
 
 		if !*follow.Notify {
 			// This follower doesn't have notifs
@@ -196,7 +203,7 @@ func (s *Surface) timelineAndNotifyStatusForFollowers(
 		}
 	}
 
-	return errs.Combine()
+	return homeTimelinedAccountIDs, errs.Combine()
 }
 
 // listTimelineStatusForFollow puts the given status
@@ -259,6 +266,22 @@ func (s *Surface) listTimelineStatusForFollow(
 	}
 }
 
+// getFiltersAndMutes returns an account's filters and mutes.
+func (s *Surface) getFiltersAndMutes(ctx context.Context, accountID string) ([]*gtsmodel.Filter, *usermute.CompiledUserMuteList, error) {
+	filters, err := s.State.DB.GetFiltersForAccountID(ctx, accountID)
+	if err != nil {
+		return nil, nil, gtserror.Newf("couldn't retrieve filters for account %s: %w", accountID, err)
+	}
+
+	mutes, err := s.State.DB.GetAccountMutes(gtscontext.SetBarebones(ctx), accountID, nil)
+	if err != nil {
+		return nil, nil, gtserror.Newf("couldn't retrieve mutes for account %s: %w", accountID, err)
+	}
+	compiledMutes := usermute.NewCompiledUserMuteList(mutes)
+
+	return filters, compiledMutes, err
+}
+
 // listEligible checks if the given status is eligible
 // for inclusion in the list that that the given listEntry
 // belongs to, based on the replies policy of the list.
@@ -391,6 +414,138 @@ func (s *Surface) timelineStatus(
 	return true, nil
 }
 
+// timelineAndNotifyStatusForTagFollowers inserts the status into the
+// home timeline of each local account which follows a useable tag from the status,
+// skipping accounts for which it would have already been inserted.
+func (s *Surface) timelineAndNotifyStatusForTagFollowers(
+	ctx context.Context,
+	status *gtsmodel.Status,
+	alreadyHomeTimelinedAccountIDs []string,
+) error {
+	tagFollowerAccounts, err := s.tagFollowersForStatus(ctx, status, alreadyHomeTimelinedAccountIDs)
+	if err != nil {
+		return err
+	}
+
+	if status.BoostOf != nil {
+		// Unwrap boost and work with the original status.
+		status = status.BoostOf
+	}
+
+	// Insert the status into the home timeline of each tag follower.
+	errs := gtserror.MultiError{}
+	for _, tagFollowerAccount := range tagFollowerAccounts {
+		filters, mutes, err := s.getFiltersAndMutes(ctx, tagFollowerAccount.ID)
+		if err != nil {
+			errs.Append(err)
+			continue
+		}
+
+		if _, err := s.timelineStatus(
+			ctx,
+			s.State.Timelines.Home.IngestOne,
+			tagFollowerAccount.ID, // home timelines are keyed by account ID
+			tagFollowerAccount,
+			status,
+			stream.TimelineHome,
+			filters,
+			mutes,
+		); err != nil {
+			errs.Appendf(
+				"error inserting status %s into home timeline for account %s: %w",
+				status.ID,
+				tagFollowerAccount.ID,
+				err,
+			)
+		}
+	}
+	return errs.Combine()
+}
+
+// tagFollowersForStatus gets local accounts which follow any useable tags from the status,
+// skipping any with IDs in the provided list, and any that shouldn't be able to see it due to blocks.
+func (s *Surface) tagFollowersForStatus(
+	ctx context.Context,
+	status *gtsmodel.Status,
+	skipAccountIDs []string,
+) ([]*gtsmodel.Account, error) {
+	// If the status is a boost, look at the tags from the boosted status.
+	taggedStatus := status
+	if status.BoostOf != nil {
+		taggedStatus = status.BoostOf
+	}
+
+	if taggedStatus.Visibility != gtsmodel.VisibilityPublic || len(taggedStatus.Tags) == 0 {
+		// Only public statuses with tags are eligible for tag processing.
+		return nil, nil
+	}
+
+	// Build list of useable tag IDs.
+	useableTagIDs := make([]string, 0, len(taggedStatus.Tags))
+	for _, tag := range taggedStatus.Tags {
+		if *tag.Useable {
+			useableTagIDs = append(useableTagIDs, tag.ID)
+		}
+	}
+	if len(useableTagIDs) == 0 {
+		return nil, nil
+	}
+
+	// Get IDs for all accounts who follow one or more of the useable tags from this status.
+	allTagFollowerAccountIDs, err := s.State.DB.GetAccountIDsFollowingTagIDs(ctx, useableTagIDs)
+	if err != nil {
+		return nil, gtserror.Newf("DB error getting followers for tags of status %s: %w", taggedStatus.ID, err)
+	}
+	if len(allTagFollowerAccountIDs) == 0 {
+		return nil, nil
+	}
+
+	// Build set for faster lookup of account IDs to skip.
+	skipAccountIDSet := make(map[string]struct{}, len(skipAccountIDs))
+	for _, accountID := range skipAccountIDs {
+		skipAccountIDSet[accountID] = struct{}{}
+	}
+
+	// Build list of tag follower account IDs,
+	// except those which have already had this status inserted into their timeline.
+	tagFollowerAccountIDs := make([]string, 0, len(allTagFollowerAccountIDs))
+	for _, accountID := range allTagFollowerAccountIDs {
+		if _, skip := skipAccountIDSet[accountID]; skip {
+			continue
+		}
+		tagFollowerAccountIDs = append(tagFollowerAccountIDs, accountID)
+	}
+	if len(tagFollowerAccountIDs) == 0 {
+		return nil, nil
+	}
+
+	// Retrieve accounts for remaining tag followers.
+	tagFollowerAccounts, err := s.State.DB.GetAccountsByIDs(ctx, tagFollowerAccountIDs)
+	if err != nil {
+		return nil, gtserror.Newf("DB error getting accounts for followers of tags of status %s: %w", taggedStatus.ID, err)
+	}
+
+	// Check the visibility of the *input* status for each account.
+	// This accounts for the visibility of the boost as well as the original, if the input status is a boost.
+	errs := gtserror.MultiError{}
+	visibleTagFollowerAccounts := make([]*gtsmodel.Account, 0, len(tagFollowerAccounts))
+	for _, account := range tagFollowerAccounts {
+		visible, err := s.VisFilter.StatusVisible(ctx, account, status)
+		if err != nil {
+			errs.Appendf(
+				"error checking visibility of status %s to account %s",
+				status.ID,
+				account.ID,
+			)
+		}
+		if visible {
+			visibleTagFollowerAccounts = append(visibleTagFollowerAccounts, account)
+		}
+	}
+
+	return visibleTagFollowerAccounts, errs.Combine()
+}
+
 // deleteStatusFromTimelines completely removes the given status from all timelines.
 // It will also stream deletion of the status to all open streams.
 func (s *Surface) deleteStatusFromTimelines(ctx context.Context, statusID string) error {
@@ -425,7 +580,7 @@ func (s *Surface) invalidateStatusFromTimelines(ctx context.Context, statusID st
 }
 
 // timelineStatusUpdate looks up HOME and LIST timelines of accounts
-// that follow the the status author and pushes edit messages into any
+// that follow the the status author or tags and pushes edit messages into any
 // active streams.
 // Note that calling invalidateStatusFromTimelines takes care of the
 // state in general, we just need to do this for any streams that are
@@ -454,10 +609,15 @@ func (s *Surface) timelineStatusUpdate(ctx context.Context, status *gtsmodel.Sta
 	}
 
 	// Push to streams for each local follower of this account.
-	if err := s.timelineStatusUpdateForFollowers(ctx, status, follows); err != nil {
+	homeTimelinedAccountIDs, err := s.timelineStatusUpdateForFollowers(ctx, status, follows)
+	if err != nil {
 		return gtserror.Newf("error timelining status %s for followers: %w", status.ID, err)
 	}
 
+	if err := s.timelineStatusUpdateForTagFollowers(ctx, status, homeTimelinedAccountIDs); err != nil {
+		return gtserror.Newf("error timelining status %s for tag followers: %w", status.ID, err)
+	}
+
 	return nil
 }
 
@@ -465,13 +625,16 @@ func (s *Surface) timelineStatusUpdate(ctx context.Context, status *gtsmodel.Sta
 // slice of followers of the account that posted the given status,
 // pushing update messages into open list/home streams of each
 // follower.
+//
+// Returns a list of accounts which had this status updated in their home timelines.
 func (s *Surface) timelineStatusUpdateForFollowers(
 	ctx context.Context,
 	status *gtsmodel.Status,
 	follows []*gtsmodel.Follow,
-) error {
+) ([]string, error) {
 	var (
-		errs gtserror.MultiError
+		errs                    gtserror.MultiError
+		homeTimelinedAccountIDs = []string{}
 	)
 
 	for _, follow := range follows {
@@ -495,16 +658,11 @@ func (s *Surface) timelineStatusUpdateForFollowers(
 			continue
 		}
 
-		filters, err := s.State.DB.GetFiltersForAccountID(ctx, follow.AccountID)
-		if err != nil {
-			return gtserror.Newf("couldn't retrieve filters for account %s: %w", follow.AccountID, err)
-		}
-
-		mutes, err := s.State.DB.GetAccountMutes(gtscontext.SetBarebones(ctx), follow.AccountID, nil)
+		filters, mutes, err := s.getFiltersAndMutes(ctx, follow.AccountID)
 		if err != nil {
-			return gtserror.Newf("couldn't retrieve mutes for account %s: %w", follow.AccountID, err)
+			errs.Append(err)
+			continue
 		}
-		compiledMutes := usermute.NewCompiledUserMuteList(mutes)
 
 		// Add status to any relevant lists
 		// for this follow, if applicable.
@@ -514,26 +672,30 @@ func (s *Surface) timelineStatusUpdateForFollowers(
 			follow,
 			&errs,
 			filters,
-			compiledMutes,
+			mutes,
 		)
 
 		// Add status to home timeline for owner
 		// of this follow, if applicable.
-		err = s.timelineStreamStatusUpdate(
+		homeTimelined, err := s.timelineStreamStatusUpdate(
 			ctx,
 			follow.Account,
 			status,
 			stream.TimelineHome,
 			filters,
-			compiledMutes,
+			mutes,
 		)
 		if err != nil {
 			errs.Appendf("error home timelining status: %w", err)
 			continue
 		}
+
+		if homeTimelined {
+			homeTimelinedAccountIDs = append(homeTimelinedAccountIDs, follow.AccountID)
+		}
 	}
 
-	return errs.Combine()
+	return homeTimelinedAccountIDs, errs.Combine()
 }
 
 // listTimelineStatusUpdateForFollow pushes edits of the given status
@@ -580,7 +742,7 @@ func (s *Surface) listTimelineStatusUpdateForFollow(
 		// At this point we are certain this status
 		// should be included in the timeline of the
 		// list that this list entry belongs to.
-		if err := s.timelineStreamStatusUpdate(
+		if _, err := s.timelineStreamStatusUpdate(
 			ctx,
 			follow.Account,
 			status,
@@ -596,6 +758,8 @@ func (s *Surface) listTimelineStatusUpdateForFollow(
 
 // timelineStatusUpdate streams the edited status to the user using the
 // given streamType.
+//
+// Returns whether it was actually streamed.
 func (s *Surface) timelineStreamStatusUpdate(
 	ctx context.Context,
 	account *gtsmodel.Account,
@@ -603,16 +767,62 @@ func (s *Surface) timelineStreamStatusUpdate(
 	streamType string,
 	filters []*gtsmodel.Filter,
 	mutes *usermute.CompiledUserMuteList,
-) error {
+) (bool, error) {
 	apiStatus, err := s.Converter.StatusToAPIStatus(ctx, status, account, statusfilter.FilterContextHome, filters, mutes)
 	if errors.Is(err, statusfilter.ErrHideStatus) {
 		// Don't put this status in the stream.
-		return nil
+		return false, nil
 	}
 	if err != nil {
 		err = gtserror.Newf("error converting status %s to frontend representation: %w", status.ID, err)
-		return err
+		return false, err
 	}
 	s.Stream.StatusUpdate(ctx, account, apiStatus, streamType)
-	return nil
+	return true, nil
+}
+
+// timelineStatusUpdateForTagFollowers streams update notifications to the
+// home timeline of each local account which follows a tag used by the status,
+// skipping accounts for which it would have already been streamed.
+func (s *Surface) timelineStatusUpdateForTagFollowers(
+	ctx context.Context,
+	status *gtsmodel.Status,
+	alreadyHomeTimelinedAccountIDs []string,
+) error {
+	tagFollowerAccounts, err := s.tagFollowersForStatus(ctx, status, alreadyHomeTimelinedAccountIDs)
+	if err != nil {
+		return err
+	}
+
+	if status.BoostOf != nil {
+		// Unwrap boost and work with the original status.
+		status = status.BoostOf
+	}
+
+	// Stream the update to the home timeline of each tag follower.
+	errs := gtserror.MultiError{}
+	for _, tagFollowerAccount := range tagFollowerAccounts {
+		filters, mutes, err := s.getFiltersAndMutes(ctx, tagFollowerAccount.ID)
+		if err != nil {
+			errs.Append(err)
+			continue
+		}
+
+		if _, err := s.timelineStreamStatusUpdate(
+			ctx,
+			tagFollowerAccount,
+			status,
+			stream.TimelineHome,
+			filters,
+			mutes,
+		); err != nil {
+			errs.Appendf(
+				"error updating status %s on home timeline for account %s: %w",
+				status.ID,
+				tagFollowerAccount.ID,
+				err,
+			)
+		}
+	}
+	return errs.Combine()
 }