[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

internal/api/client.go

@@ -32,6 +32,7 @@ import (
 	"github.com/superseriousbusiness/gotosocial/internal/api/client/featuredtags"
 	filtersV1 "github.com/superseriousbusiness/gotosocial/internal/api/client/filters/v1"
 	filtersV2 "github.com/superseriousbusiness/gotosocial/internal/api/client/filters/v2"
+	"github.com/superseriousbusiness/gotosocial/internal/api/client/followedtags"
 	"github.com/superseriousbusiness/gotosocial/internal/api/client/followrequests"
 	"github.com/superseriousbusiness/gotosocial/internal/api/client/instance"
 	"github.com/superseriousbusiness/gotosocial/internal/api/client/interactionpolicies"
@@ -46,6 +47,7 @@ import (
 	"github.com/superseriousbusiness/gotosocial/internal/api/client/search"
 	"github.com/superseriousbusiness/gotosocial/internal/api/client/statuses"
 	"github.com/superseriousbusiness/gotosocial/internal/api/client/streaming"
+	"github.com/superseriousbusiness/gotosocial/internal/api/client/tags"
 	"github.com/superseriousbusiness/gotosocial/internal/api/client/timelines"
 	"github.com/superseriousbusiness/gotosocial/internal/api/client/user"
 	"github.com/superseriousbusiness/gotosocial/internal/db"
@@ -59,7 +61,7 @@ type Client struct {
 	processor *processing.Processor
 	db        db.DB
 
-	accounts            *accounts.Module            // api/v1/accounts
+	accounts            *accounts.Module            // api/v1/accounts, api/v1/profile
 	admin               *admin.Module               // api/v1/admin
 	apps                *apps.Module                // api/v1/apps
 	blocks              *blocks.Module              // api/v1/blocks
@@ -71,6 +73,7 @@ type Client struct {
 	filtersV1           *filtersV1.Module           // api/v1/filters
 	filtersV2           *filtersV2.Module           // api/v2/filters
 	followRequests      *followrequests.Module      // api/v1/follow_requests
+	followedTags        *followedtags.Module        // api/v1/followed_tags
 	instance            *instance.Module            // api/v1/instance
 	interactionPolicies *interactionpolicies.Module // api/v1/interaction_policies
 	lists               *lists.Module               // api/v1/lists
@@ -84,6 +87,7 @@ type Client struct {
 	search              *search.Module              // api/v1/search, api/v2/search
 	statuses            *statuses.Module            // api/v1/statuses
 	streaming           *streaming.Module           // api/v1/streaming
+	tags                *tags.Module                // api/v1/tags
 	timelines           *timelines.Module           // api/v1/timelines
 	user                *user.Module                // api/v1/user
 }
@@ -117,6 +121,7 @@ func (c *Client) Route(r *router.Router, m ...gin.HandlerFunc) {
 	c.filtersV1.Route(h)
 	c.filtersV2.Route(h)
 	c.followRequests.Route(h)
+	c.followedTags.Route(h)
 	c.instance.Route(h)
 	c.interactionPolicies.Route(h)
 	c.lists.Route(h)
@@ -130,6 +135,7 @@ func (c *Client) Route(r *router.Router, m ...gin.HandlerFunc) {
 	c.search.Route(h)
 	c.statuses.Route(h)
 	c.streaming.Route(h)
+	c.tags.Route(h)
 	c.timelines.Route(h)
 	c.user.Route(h)
 }
@@ -151,6 +157,7 @@ func NewClient(state *state.State, p *processing.Processor) *Client {
 		filtersV1:           filtersV1.New(p),
 		filtersV2:           filtersV2.New(p),
 		followRequests:      followrequests.New(p),
+		followedTags:        followedtags.New(p),
 		instance:            instance.New(p),
 		interactionPolicies: interactionpolicies.New(p),
 		lists:               lists.New(p),
@@ -164,6 +171,7 @@ func NewClient(state *state.State, p *processing.Processor) *Client {
 		search:              search.New(p),
 		statuses:            statuses.New(p),
 		streaming:           streaming.New(p, time.Second*30, 4096),
+		tags:                tags.New(p),
 		timelines:           timelines.New(p),
 		user:                user.New(p),
 	}

internal/api/client/featuredtags/get.go

@@ -34,7 +34,7 @@ import (
 //
 //	---
 //	tags:
-//	- featured_tags
+//	- tags
 //
 //	produces:
 //	- application/json

internal/api/client/followedtags/followedtags.go

@@ -0,0 +1,43 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package followedtags
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	"github.com/superseriousbusiness/gotosocial/internal/processing"
+)
+
+const (
+	BasePath = "/v1/followed_tags"
+)
+
+type Module struct {
+	processor *processing.Processor
+}
+
+func New(processor *processing.Processor) *Module {
+	return &Module{
+		processor: processor,
+	}
+}
+
+func (m *Module) Route(attachHandler func(method string, path string, f ...gin.HandlerFunc) gin.IRoutes) {
+	attachHandler(http.MethodGet, BasePath, m.FollowedTagsGETHandler)
+}

internal/api/client/followedtags/followedtags_test.go

@@ -0,0 +1,104 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package followedtags_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/suite"
+	"github.com/superseriousbusiness/gotosocial/internal/api/client/followedtags"
+	"github.com/superseriousbusiness/gotosocial/internal/config"
+	"github.com/superseriousbusiness/gotosocial/internal/db"
+	"github.com/superseriousbusiness/gotosocial/internal/email"
+	"github.com/superseriousbusiness/gotosocial/internal/federation"
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+	"github.com/superseriousbusiness/gotosocial/internal/media"
+	"github.com/superseriousbusiness/gotosocial/internal/processing"
+	"github.com/superseriousbusiness/gotosocial/internal/state"
+	"github.com/superseriousbusiness/gotosocial/internal/storage"
+	"github.com/superseriousbusiness/gotosocial/testrig"
+)
+
+type FollowedTagsTestSuite struct {
+	suite.Suite
+	db           db.DB
+	storage      *storage.Driver
+	mediaManager *media.Manager
+	federator    *federation.Federator
+	processor    *processing.Processor
+	emailSender  email.Sender
+	sentEmails   map[string]string
+	state        state.State
+
+	// standard suite models
+	testTokens       map[string]*gtsmodel.Token
+	testClients      map[string]*gtsmodel.Client
+	testApplications map[string]*gtsmodel.Application
+	testUsers        map[string]*gtsmodel.User
+	testAccounts     map[string]*gtsmodel.Account
+	testTags         map[string]*gtsmodel.Tag
+
+	// module being tested
+	followedTagsModule *followedtags.Module
+}
+
+func (suite *FollowedTagsTestSuite) SetupSuite() {
+	suite.testTokens = testrig.NewTestTokens()
+	suite.testClients = testrig.NewTestClients()
+	suite.testApplications = testrig.NewTestApplications()
+	suite.testUsers = testrig.NewTestUsers()
+	suite.testAccounts = testrig.NewTestAccounts()
+	suite.testTags = testrig.NewTestTags()
+}
+
+func (suite *FollowedTagsTestSuite) SetupTest() {
+	suite.state.Caches.Init()
+	testrig.StartNoopWorkers(&suite.state)
+
+	testrig.InitTestConfig()
+	config.Config(func(cfg *config.Configuration) {
+		cfg.WebAssetBaseDir = "../../../../web/assets/"
+		cfg.WebTemplateBaseDir = "../../../../web/templates/"
+	})
+	testrig.InitTestLog()
+
+	suite.db = testrig.NewTestDB(&suite.state)
+	suite.state.DB = suite.db
+	suite.storage = testrig.NewInMemoryStorage()
+	suite.state.Storage = suite.storage
+
+	suite.mediaManager = testrig.NewTestMediaManager(&suite.state)
+	suite.federator = testrig.NewTestFederator(&suite.state, testrig.NewTestTransportController(&suite.state, testrig.NewMockHTTPClient(nil, "../../../../testrig/media")), suite.mediaManager)
+	suite.sentEmails = make(map[string]string)
+	suite.emailSender = testrig.NewEmailSender("../../../../web/template/", suite.sentEmails)
+	suite.processor = testrig.NewTestProcessor(&suite.state, suite.federator, suite.emailSender, suite.mediaManager)
+	suite.followedTagsModule = followedtags.New(suite.processor)
+
+	testrig.StandardDBSetup(suite.db, nil)
+	testrig.StandardStorageSetup(suite.storage, "../../../../testrig/media")
+}
+
+func (suite *FollowedTagsTestSuite) TearDownTest() {
+	testrig.StandardDBTeardown(suite.db)
+	testrig.StandardStorageTeardown(suite.storage)
+	testrig.StopWorkers(&suite.state)
+}
+
+func TestFollowedTagsTestSuite(t *testing.T) {
+	suite.Run(t, new(FollowedTagsTestSuite))
+}

internal/api/client/followedtags/get.go

@@ -0,0 +1,139 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package followedtags
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	apiutil "github.com/superseriousbusiness/gotosocial/internal/api/util"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/oauth"
+	"github.com/superseriousbusiness/gotosocial/internal/paging"
+)
+
+// FollowedTagsGETHandler swagger:operation GET /api/v1/followed_tags getFollowedTags
+//
+// Get an array of all hashtags that you currently follow.
+//
+//	---
+//	tags:
+//	- tags
+//
+//	produces:
+//	- application/json
+//
+//	security:
+//	- OAuth2 Bearer:
+//		- read:follows
+//
+//	parameters:
+//	-
+//		name: max_id
+//		type: string
+//		description: >-
+//			Return only followed tags *OLDER* than the given max ID.
+//			The followed tag with the specified ID will not be included in the response.
+//			NOTE: the ID is of the internal followed tag, NOT a tag name.
+//		in: query
+//		required: false
+//	-
+//		name: since_id
+//		type: string
+//		description: >-
+//			Return only followed tags *NEWER* than the given since ID.
+//			The followed tag with the specified ID will not be included in the response.
+//			NOTE: the ID is of the internal followed tag, NOT a tag name.
+//		in: query
+//	-
+//		name: min_id
+//		type: string
+//		description: >-
+//			Return only followed tags *IMMEDIATELY NEWER* than the given min ID.
+//			The followed tag with the specified ID will not be included in the response.
+//			NOTE: the ID is of the internal followed tag, NOT a tag name.
+//		in: query
+//		required: false
+//	-
+//		name: limit
+//		type: integer
+//		description: Number of followed tags to return.
+//		default: 100
+//		minimum: 1
+//		maximum: 200
+//		in: query
+//		required: false
+//
+//	responses:
+//		'200':
+//			headers:
+//				Link:
+//					type: string
+//					description: Links to the next and previous queries.
+//			schema:
+//				type: array
+//				items:
+//					"$ref": "#/definitions/tag"
+//		'400':
+//			description: bad request
+//		'401':
+//			description: unauthorized
+//		'404':
+//			description: not found
+//		'406':
+//			description: not acceptable
+//		'500':
+//			description: internal server error
+func (m *Module) FollowedTagsGETHandler(c *gin.Context) {
+	authed, err := oauth.Authed(c, true, true, true, true)
+	if err != nil {
+		apiutil.ErrorHandler(c, gtserror.NewErrorUnauthorized(err, err.Error()), m.processor.InstanceGetV1)
+		return
+	}
+
+	if _, err := apiutil.NegotiateAccept(c, apiutil.JSONAcceptHeaders...); err != nil {
+		apiutil.ErrorHandler(c, gtserror.NewErrorNotAcceptable(err, err.Error()), m.processor.InstanceGetV1)
+		return
+	}
+
+	page, errWithCode := paging.ParseIDPage(c,
+		1,   // min limit
+		200, // max limit
+		100, // default limit
+	)
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	resp, errWithCode := m.processor.Tags().Followed(
+		c.Request.Context(),
+		authed.Account.ID,
+		page,
+	)
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	if resp.LinkHeader != "" {
+		c.Header("Link", resp.LinkHeader)
+	}
+
+	apiutil.JSON(c, http.StatusOK, resp.Items)
+}

internal/api/client/followedtags/get_test.go

@@ -0,0 +1,125 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package followedtags_test
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+
+	"github.com/superseriousbusiness/gotosocial/internal/api/client/followedtags"
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+	"github.com/superseriousbusiness/gotosocial/internal/config"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/oauth"
+	"github.com/superseriousbusiness/gotosocial/testrig"
+)
+
+func (suite *FollowedTagsTestSuite) getFollowedTags(
+	accountFixtureName string,
+	expectedHTTPStatus int,
+	expectedBody string,
+) ([]apimodel.Tag, error) {
+	// instantiate recorder + test context
+	recorder := httptest.NewRecorder()
+	ctx, _ := testrig.CreateGinTestContext(recorder, nil)
+	ctx.Set(oauth.SessionAuthorizedAccount, suite.testAccounts[accountFixtureName])
+	ctx.Set(oauth.SessionAuthorizedToken, oauth.DBTokenToToken(suite.testTokens[accountFixtureName]))
+	ctx.Set(oauth.SessionAuthorizedApplication, suite.testApplications["application_1"])
+	ctx.Set(oauth.SessionAuthorizedUser, suite.testUsers[accountFixtureName])
+
+	// create the request
+	ctx.Request = httptest.NewRequest(http.MethodGet, config.GetProtocol()+"://"+config.GetHost()+"/api/"+followedtags.BasePath, nil)
+	ctx.Request.Header.Set("accept", "application/json")
+
+	// trigger the handler
+	suite.followedTagsModule.FollowedTagsGETHandler(ctx)
+
+	// read the response
+	result := recorder.Result()
+	defer result.Body.Close()
+
+	b, err := io.ReadAll(result.Body)
+	if err != nil {
+		return nil, err
+	}
+
+	errs := gtserror.NewMultiError(2)
+
+	// check code + body
+	if resultCode := recorder.Code; expectedHTTPStatus != resultCode {
+		errs.Appendf("expected %d got %d", expectedHTTPStatus, resultCode)
+		if expectedBody == "" {
+			return nil, errs.Combine()
+		}
+	}
+
+	// if we got an expected body, return early
+	if expectedBody != "" {
+		if string(b) != expectedBody {
+			errs.Appendf("expected %s got %s", expectedBody, string(b))
+		}
+		return nil, errs.Combine()
+	}
+
+	resp := []apimodel.Tag{}
+	if err := json.Unmarshal(b, &resp); err != nil {
+		return nil, err
+	}
+
+	return resp, nil
+}
+
+// Test that we can list a user's followed tags.
+func (suite *FollowedTagsTestSuite) TestGet() {
+	accountFixtureName := "local_account_2"
+	testAccount := suite.testAccounts[accountFixtureName]
+	testTag := suite.testTags["welcome"]
+
+	// Follow an existing tag.
+	if err := suite.db.PutFollowedTag(context.Background(), testAccount.ID, testTag.ID); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	followedTags, err := suite.getFollowedTags(accountFixtureName, http.StatusOK, "")
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	if suite.Len(followedTags, 1) {
+		followedTag := followedTags[0]
+		suite.Equal(testTag.Name, followedTag.Name)
+		if suite.NotNil(followedTag.Following) {
+			suite.True(*followedTag.Following)
+		}
+	}
+}
+
+// Test that we can list a user's followed tags even if they don't have any.
+func (suite *FollowedTagsTestSuite) TestGetEmpty() {
+	accountFixtureName := "local_account_1"
+
+	followedTags, err := suite.getFollowedTags(accountFixtureName, http.StatusOK, "")
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	suite.Len(followedTags, 0)
+}

internal/api/client/tags/follow.go

@@ -0,0 +1,92 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package tags
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	apiutil "github.com/superseriousbusiness/gotosocial/internal/api/util"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/oauth"
+)
+
+// FollowTagPOSTHandler swagger:operation POST /api/v1/tags/{tag_name}/follow followTag
+//
+// Follow a hashtag.
+//
+// Idempotent: if you are already following the tag, this call will still succeed.
+//
+//	---
+//	tags:
+//	- tags
+//
+//	produces:
+//	- application/json
+//
+//	security:
+//	- OAuth2 Bearer:
+//		- write:follows
+//
+//	parameters:
+//	-
+//		name: tag_name
+//		type: string
+//		description: Name of the tag (no leading `#`)
+//		in: path
+//		required: true
+//
+//	responses:
+//		'200':
+//			description: "Info about the tag."
+//			schema:
+//				"$ref": "#/definitions/tag"
+//		'400':
+//			description: bad request
+//		'401':
+//			description: unauthorized
+//		'403':
+//			description: forbidden
+//		'500':
+//			description: internal server error
+func (m *Module) FollowTagPOSTHandler(c *gin.Context) {
+	authed, err := oauth.Authed(c, true, true, true, true)
+	if err != nil {
+		apiutil.ErrorHandler(c, gtserror.NewErrorUnauthorized(err, err.Error()), m.processor.InstanceGetV1)
+		return
+	}
+
+	if authed.Account.IsMoving() {
+		apiutil.ForbiddenAfterMove(c)
+		return
+	}
+
+	name, errWithCode := apiutil.ParseTagName(c.Param(apiutil.TagNameKey))
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	apiTag, errWithCode := m.processor.Tags().Follow(c.Request.Context(), authed.Account, name)
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	apiutil.JSON(c, http.StatusOK, apiTag)
+}

internal/api/client/tags/follow_test.go

@@ -0,0 +1,82 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package tags_test
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/superseriousbusiness/gotosocial/internal/api/client/tags"
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+)
+
+func (suite *TagsTestSuite) follow(
+	accountFixtureName string,
+	tagName string,
+	expectedHTTPStatus int,
+	expectedBody string,
+) (*apimodel.Tag, error) {
+	return suite.tagAction(
+		accountFixtureName,
+		tagName,
+		http.MethodPost,
+		tags.FollowPath,
+		suite.tagsModule.FollowTagPOSTHandler,
+		expectedHTTPStatus,
+		expectedBody,
+	)
+}
+
+// Follow a tag we don't already follow.
+func (suite *TagsTestSuite) TestFollow() {
+	accountFixtureName := "local_account_2"
+	testTag := suite.testTags["welcome"]
+
+	apiTag, err := suite.follow(accountFixtureName, testTag.Name, http.StatusOK, "")
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	suite.Equal(testTag.Name, apiTag.Name)
+	if suite.NotNil(apiTag.Following) {
+		suite.True(*apiTag.Following)
+	}
+}
+
+// When we follow a tag already followed by the account, it should succeed.
+func (suite *TagsTestSuite) TestFollowIdempotent() {
+	accountFixtureName := "local_account_2"
+	testAccount := suite.testAccounts[accountFixtureName]
+	testTag := suite.testTags["welcome"]
+
+	// Setup: follow an existing tag.
+	if err := suite.db.PutFollowedTag(context.Background(), testAccount.ID, testTag.ID); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Follow it again through the API.
+	apiTag, err := suite.follow(accountFixtureName, testTag.Name, http.StatusOK, "")
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	suite.Equal(testTag.Name, apiTag.Name)
+	if suite.NotNil(apiTag.Following) {
+		suite.True(*apiTag.Following)
+	}
+}

internal/api/client/tags/get.go

@@ -0,0 +1,89 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package tags
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	apiutil "github.com/superseriousbusiness/gotosocial/internal/api/util"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/oauth"
+)
+
+// TagGETHandler swagger:operation GET /api/v1/tags/{tag_name} getTag
+//
+// Get details for a hashtag, including whether you currently follow it.
+//
+// If the tag does not exist, this method will not create it in the database.
+//
+//	---
+//	tags:
+//	- tags
+//
+//	produces:
+//	- application/json
+//
+//	security:
+//	- OAuth2 Bearer:
+//		- read:follows
+//
+//	parameters:
+//	-
+//		name: tag_name
+//		type: string
+//		description: Name of the tag (no leading `#`)
+//		in: path
+//		required: true
+//
+//	responses:
+//		'200':
+//			description: "Info about the tag."
+//			schema:
+//				"$ref": "#/definitions/tag"
+//		'400':
+//			description: bad request
+//		'401':
+//			description: unauthorized
+//		'404':
+//			description: not found
+//		'406':
+//			description: not acceptable
+//		'500':
+//			description: internal server error
+func (m *Module) TagGETHandler(c *gin.Context) {
+	authed, err := oauth.Authed(c, true, true, true, true)
+	if err != nil {
+		apiutil.ErrorHandler(c, gtserror.NewErrorUnauthorized(err, err.Error()), m.processor.InstanceGetV1)
+		return
+	}
+
+	name, errWithCode := apiutil.ParseTagName(c.Param(apiutil.TagNameKey))
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	apiTag, errWithCode := m.processor.Tags().Get(c.Request.Context(), authed.Account, name)
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	apiutil.JSON(c, http.StatusOK, apiTag)
+}

internal/api/client/tags/get_test.go

@@ -0,0 +1,93 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package tags_test
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/superseriousbusiness/gotosocial/internal/api/client/tags"
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+)
+
+// tagAction follows or unfollows a tag.
+func (suite *TagsTestSuite) get(
+	accountFixtureName string,
+	tagName string,
+	expectedHTTPStatus int,
+	expectedBody string,
+) (*apimodel.Tag, error) {
+	return suite.tagAction(
+		accountFixtureName,
+		tagName,
+		http.MethodGet,
+		tags.TagPath,
+		suite.tagsModule.TagGETHandler,
+		expectedHTTPStatus,
+		expectedBody,
+	)
+}
+
+// Get a tag followed by the account.
+func (suite *TagsTestSuite) TestGetFollowed() {
+	accountFixtureName := "local_account_2"
+	testAccount := suite.testAccounts[accountFixtureName]
+	testTag := suite.testTags["welcome"]
+
+	// Setup: follow an existing tag.
+	if err := suite.db.PutFollowedTag(context.Background(), testAccount.ID, testTag.ID); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Get it through the API.
+	apiTag, err := suite.get(accountFixtureName, testTag.Name, http.StatusOK, "")
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	suite.Equal(testTag.Name, apiTag.Name)
+	if suite.NotNil(apiTag.Following) {
+		suite.True(*apiTag.Following)
+	}
+}
+
+// Get a tag not followed by the account.
+func (suite *TagsTestSuite) TestGetUnfollowed() {
+	accountFixtureName := "local_account_2"
+	testTag := suite.testTags["Hashtag"]
+
+	apiTag, err := suite.get(accountFixtureName, testTag.Name, http.StatusOK, "")
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	suite.Equal(testTag.Name, apiTag.Name)
+	if suite.NotNil(apiTag.Following) {
+		suite.False(*apiTag.Following)
+	}
+}
+
+// Get a tag that does not exist, which should result in a 404.
+func (suite *TagsTestSuite) TestGetNotFound() {
+	accountFixtureName := "local_account_2"
+
+	_, err := suite.get(accountFixtureName, "THIS_TAG_DOES_NOT_EXIST", http.StatusNotFound, "")
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+}

internal/api/client/tags/tags.go

@@ -0,0 +1,49 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package tags
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	apiutil "github.com/superseriousbusiness/gotosocial/internal/api/util"
+	"github.com/superseriousbusiness/gotosocial/internal/processing"
+)
+
+const (
+	BasePath     = "/v1/tags"
+	TagPath      = BasePath + "/:" + apiutil.TagNameKey
+	FollowPath   = TagPath + "/follow"
+	UnfollowPath = TagPath + "/unfollow"
+)
+
+type Module struct {
+	processor *processing.Processor
+}
+
+func New(processor *processing.Processor) *Module {
+	return &Module{
+		processor: processor,
+	}
+}
+
+func (m *Module) Route(attachHandler func(method string, path string, f ...gin.HandlerFunc) gin.IRoutes) {
+	attachHandler(http.MethodGet, TagPath, m.TagGETHandler)
+	attachHandler(http.MethodPost, FollowPath, m.FollowTagPOSTHandler)
+	attachHandler(http.MethodPost, UnfollowPath, m.UnfollowTagPOSTHandler)
+}

internal/api/client/tags/tags_test.go

@@ -0,0 +1,179 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package tags_test
+
+import (
+	"encoding/json"
+	"io"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/suite"
+	"github.com/superseriousbusiness/gotosocial/internal/api/client/tags"
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+	"github.com/superseriousbusiness/gotosocial/internal/config"
+	"github.com/superseriousbusiness/gotosocial/internal/db"
+	"github.com/superseriousbusiness/gotosocial/internal/email"
+	"github.com/superseriousbusiness/gotosocial/internal/federation"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+	"github.com/superseriousbusiness/gotosocial/internal/media"
+	"github.com/superseriousbusiness/gotosocial/internal/oauth"
+	"github.com/superseriousbusiness/gotosocial/internal/processing"
+	"github.com/superseriousbusiness/gotosocial/internal/state"
+	"github.com/superseriousbusiness/gotosocial/internal/storage"
+	"github.com/superseriousbusiness/gotosocial/testrig"
+)
+
+type TagsTestSuite struct {
+	suite.Suite
+	db           db.DB
+	storage      *storage.Driver
+	mediaManager *media.Manager
+	federator    *federation.Federator
+	processor    *processing.Processor
+	emailSender  email.Sender
+	sentEmails   map[string]string
+	state        state.State
+
+	// standard suite models
+	testTokens       map[string]*gtsmodel.Token
+	testClients      map[string]*gtsmodel.Client
+	testApplications map[string]*gtsmodel.Application
+	testUsers        map[string]*gtsmodel.User
+	testAccounts     map[string]*gtsmodel.Account
+	testTags         map[string]*gtsmodel.Tag
+
+	// module being tested
+	tagsModule *tags.Module
+}
+
+func (suite *TagsTestSuite) SetupSuite() {
+	suite.testTokens = testrig.NewTestTokens()
+	suite.testClients = testrig.NewTestClients()
+	suite.testApplications = testrig.NewTestApplications()
+	suite.testUsers = testrig.NewTestUsers()
+	suite.testAccounts = testrig.NewTestAccounts()
+	suite.testTags = testrig.NewTestTags()
+}
+
+func (suite *TagsTestSuite) SetupTest() {
+	suite.state.Caches.Init()
+	testrig.StartNoopWorkers(&suite.state)
+
+	testrig.InitTestConfig()
+	config.Config(func(cfg *config.Configuration) {
+		cfg.WebAssetBaseDir = "../../../../web/assets/"
+		cfg.WebTemplateBaseDir = "../../../../web/templates/"
+	})
+	testrig.InitTestLog()
+
+	suite.db = testrig.NewTestDB(&suite.state)
+	suite.state.DB = suite.db
+	suite.storage = testrig.NewInMemoryStorage()
+	suite.state.Storage = suite.storage
+
+	suite.mediaManager = testrig.NewTestMediaManager(&suite.state)
+	suite.federator = testrig.NewTestFederator(&suite.state, testrig.NewTestTransportController(&suite.state, testrig.NewMockHTTPClient(nil, "../../../../testrig/media")), suite.mediaManager)
+	suite.sentEmails = make(map[string]string)
+	suite.emailSender = testrig.NewEmailSender("../../../../web/template/", suite.sentEmails)
+	suite.processor = testrig.NewTestProcessor(&suite.state, suite.federator, suite.emailSender, suite.mediaManager)
+	suite.tagsModule = tags.New(suite.processor)
+
+	testrig.StandardDBSetup(suite.db, nil)
+	testrig.StandardStorageSetup(suite.storage, "../../../../testrig/media")
+}
+
+func (suite *TagsTestSuite) TearDownTest() {
+	testrig.StandardDBTeardown(suite.db)
+	testrig.StandardStorageTeardown(suite.storage)
+	testrig.StopWorkers(&suite.state)
+}
+
+// tagAction gets, follows, or unfollows a tag, returning the tag.
+func (suite *TagsTestSuite) tagAction(
+	accountFixtureName string,
+	tagName string,
+	method string,
+	path string,
+	handler func(c *gin.Context),
+	expectedHTTPStatus int,
+	expectedBody string,
+) (*apimodel.Tag, error) {
+	// instantiate recorder + test context
+	recorder := httptest.NewRecorder()
+	ctx, _ := testrig.CreateGinTestContext(recorder, nil)
+	ctx.Set(oauth.SessionAuthorizedAccount, suite.testAccounts[accountFixtureName])
+	ctx.Set(oauth.SessionAuthorizedToken, oauth.DBTokenToToken(suite.testTokens[accountFixtureName]))
+	ctx.Set(oauth.SessionAuthorizedApplication, suite.testApplications["application_1"])
+	ctx.Set(oauth.SessionAuthorizedUser, suite.testUsers[accountFixtureName])
+
+	// create the request
+	url := config.GetProtocol() + "://" + config.GetHost() + "/api/" + path
+	ctx.Request = httptest.NewRequest(
+		method,
+		strings.Replace(url, ":tag_name", tagName, 1),
+		nil,
+	)
+	ctx.Request.Header.Set("accept", "application/json")
+
+	ctx.AddParam("tag_name", tagName)
+
+	// trigger the handler
+	handler(ctx)
+
+	// read the response
+	result := recorder.Result()
+	defer result.Body.Close()
+
+	b, err := io.ReadAll(result.Body)
+	if err != nil {
+		return nil, err
+	}
+
+	errs := gtserror.NewMultiError(2)
+
+	// check code + body
+	if resultCode := recorder.Code; expectedHTTPStatus != resultCode {
+		errs.Appendf("expected %d got %d", expectedHTTPStatus, resultCode)
+		if expectedBody == "" {
+			return nil, errs.Combine()
+		}
+	}
+
+	// if we got an expected body, return early
+	if expectedBody != "" {
+		if string(b) != expectedBody {
+			errs.Appendf("expected %s got %s", expectedBody, string(b))
+		}
+		return nil, errs.Combine()
+	}
+
+	resp := &apimodel.Tag{}
+	if err := json.Unmarshal(b, resp); err != nil {
+		return nil, err
+	}
+
+	return resp, nil
+}
+
+func TestTagsTestSuite(t *testing.T) {
+	suite.Run(t, new(TagsTestSuite))
+}

internal/api/client/tags/unfollow.go

@@ -0,0 +1,94 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package tags
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	apiutil "github.com/superseriousbusiness/gotosocial/internal/api/util"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/oauth"
+)
+
+// UnfollowTagPOSTHandler swagger:operation POST /api/v1/tags/{tag_name}/unfollow unfollowTag
+//
+// Unfollow a hashtag.
+//
+// Idempotent: if you are not following the tag, this call will still succeed.
+//
+//	---
+//	tags:
+//	- tags
+//
+//	produces:
+//	- application/json
+//
+//	security:
+//	- OAuth2 Bearer:
+//		- write:follows
+//
+//	parameters:
+//	-
+//		name: tag_name
+//		type: string
+//		description: Name of the tag (no leading `#`)
+//		in: path
+//		required: true
+//
+//	responses:
+//		'200':
+//			description: "Info about the tag."
+//			schema:
+//				"$ref": "#/definitions/tag"
+//		'400':
+//			description: bad request
+//		'401':
+//			description: unauthorized
+//		'403':
+//			description: forbidden
+//		'404':
+//			description: unauthorized
+//		'500':
+//			description: internal server error
+func (m *Module) UnfollowTagPOSTHandler(c *gin.Context) {
+	authed, err := oauth.Authed(c, true, true, true, true)
+	if err != nil {
+		apiutil.ErrorHandler(c, gtserror.NewErrorUnauthorized(err, err.Error()), m.processor.InstanceGetV1)
+		return
+	}
+
+	if authed.Account.IsMoving() {
+		apiutil.ForbiddenAfterMove(c)
+		return
+	}
+
+	name, errWithCode := apiutil.ParseTagName(c.Param(apiutil.TagNameKey))
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	apiTag, errWithCode := m.processor.Tags().Unfollow(c.Request.Context(), authed.Account, name)
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	apiutil.JSON(c, http.StatusOK, apiTag)
+}

internal/api/client/tags/unfollow_test.go

@@ -0,0 +1,82 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package tags_test
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/superseriousbusiness/gotosocial/internal/api/client/tags"
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+)
+
+func (suite *TagsTestSuite) unfollow(
+	accountFixtureName string,
+	tagName string,
+	expectedHTTPStatus int,
+	expectedBody string,
+) (*apimodel.Tag, error) {
+	return suite.tagAction(
+		accountFixtureName,
+		tagName,
+		http.MethodPost,
+		tags.UnfollowPath,
+		suite.tagsModule.UnfollowTagPOSTHandler,
+		expectedHTTPStatus,
+		expectedBody,
+	)
+}
+
+// Unfollow a tag that we follow.
+func (suite *TagsTestSuite) TestUnfollow() {
+	accountFixtureName := "local_account_2"
+	testAccount := suite.testAccounts[accountFixtureName]
+	testTag := suite.testTags["welcome"]
+
+	// Setup: follow an existing tag.
+	if err := suite.db.PutFollowedTag(context.Background(), testAccount.ID, testTag.ID); err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Unfollow it through the API.
+	apiTag, err := suite.unfollow(accountFixtureName, testTag.Name, http.StatusOK, "")
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	suite.Equal(testTag.Name, apiTag.Name)
+	if suite.NotNil(apiTag.Following) {
+		suite.False(*apiTag.Following)
+	}
+}
+
+// When we unfollow a tag not followed by the account, it should succeed.
+func (suite *TagsTestSuite) TestUnfollowIdempotent() {
+	accountFixtureName := "local_account_2"
+	testTag := suite.testTags["Hashtag"]
+
+	apiTag, err := suite.unfollow(accountFixtureName, testTag.Name, http.StatusOK, "")
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	suite.Equal(testTag.Name, apiTag.Name)
+	if suite.NotNil(apiTag.Following) {
+		suite.False(*apiTag.Following)
+	}
+}

internal/api/model/tag.go

@@ -31,4 +31,7 @@ type Tag struct {
 	// Currently just a stub, if provided will always be an empty array.
 	// example: []
 	History *[]any `json:"history,omitempty"`
+	// Following is true if the user is following this tag, false if they're not,
+	// and not present if there is no currently authenticated user.
+	Following *bool `json:"following,omitempty"`
 }