[feature] Hashtag federation (in/out), hashtag client API endpoints (#2032)

* update go-fed

* do the things

* remove unused columns from tags

* update to latest lingo from main

* further tag shenanigans

* serve stub page at tag endpoint

* we did it lads

* tests, oh tests, ohhh tests, oh tests (doo doo doo doo)

* swagger docs

* document hashtag usage + federation

* instanceGet

* don't bother parsing tag href

* rename whereStartsWith -> whereStartsLike

* remove GetOrCreateTag

* dont cache status tag timelineability

internal/processing/timeline/tag.go

@@ -0,0 +1,141 @@
+// 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 timeline
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+	"github.com/superseriousbusiness/gotosocial/internal/db"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+	"github.com/superseriousbusiness/gotosocial/internal/log"
+	"github.com/superseriousbusiness/gotosocial/internal/text"
+	"github.com/superseriousbusiness/gotosocial/internal/util"
+)
+
+// TagTimelineGet gets a pageable timeline for the given
+// tagName and given paging parameters. It will ensure
+// that each status in the timeline is actually visible
+// to requestingAcct before returning it.
+func (p *Processor) TagTimelineGet(
+	ctx context.Context,
+	requestingAcct *gtsmodel.Account,
+	tagName string,
+	maxID string,
+	sinceID string,
+	minID string,
+	limit int,
+) (*apimodel.PageableResponse, gtserror.WithCode) {
+	tag, errWithCode := p.getTag(ctx, tagName)
+	if errWithCode != nil {
+		return nil, errWithCode
+	}
+
+	if tag == nil || !*tag.Useable || !*tag.Listable {
+		// Obey mastodon API by returning 404 for this.
+		err := fmt.Errorf("tag was not found, or not useable/listable on this instance")
+		return nil, gtserror.NewErrorNotFound(err, err.Error())
+	}
+
+	statuses, err := p.state.DB.GetTagTimeline(ctx, tag.ID, maxID, sinceID, minID, limit)
+	if err != nil && !errors.Is(err, db.ErrNoEntries) {
+		err = gtserror.Newf("db error getting statuses: %w", err)
+		return nil, gtserror.NewErrorInternalError(err)
+	}
+
+	return p.packageTagResponse(
+		ctx,
+		requestingAcct,
+		statuses,
+		limit,
+		// Use API URL for tag.
+		"/api/v1/timelines/tag/"+tagName,
+	)
+}
+
+func (p *Processor) getTag(ctx context.Context, tagName string) (*gtsmodel.Tag, gtserror.WithCode) {
+	// Normalize + validate tag name.
+	tagNameNormal, ok := text.NormalizeHashtag(tagName)
+	if !ok {
+		err := gtserror.Newf("string '%s' could not be normalized to a valid hashtag", tagName)
+		return nil, gtserror.NewErrorBadRequest(err, err.Error())
+	}
+
+	// Ensure we have tag with this name in the db.
+	tag, err := p.state.DB.GetTagByName(ctx, tagNameNormal)
+	if err != nil && !errors.Is(err, db.ErrNoEntries) {
+		// Real db error.
+		err = gtserror.Newf("db error getting tag by name: %w", err)
+		return nil, gtserror.NewErrorInternalError(err)
+	}
+
+	return tag, nil
+}
+
+func (p *Processor) packageTagResponse(
+	ctx context.Context,
+	requestingAcct *gtsmodel.Account,
+	statuses []*gtsmodel.Status,
+	limit int,
+	requestPath string,
+) (*apimodel.PageableResponse, gtserror.WithCode) {
+	count := len(statuses)
+	if count == 0 {
+		return util.EmptyPageableResponse(), nil
+	}
+
+	var (
+		items = make([]interface{}, 0, count)
+
+		// Set next + prev values before filtering and API
+		// converting, so caller can still page properly.
+		nextMaxIDValue = statuses[count-1].ID
+		prevMinIDValue = statuses[0].ID
+	)
+
+	for _, s := range statuses {
+		timelineable, err := p.filter.StatusTagTimelineable(ctx, requestingAcct, s)
+		if err != nil {
+			log.Errorf(ctx, "error checking status visibility: %v", err)
+			continue
+		}
+
+		if !timelineable {
+			continue
+		}
+
+		apiStatus, err := p.tc.StatusToAPIStatus(ctx, s, requestingAcct)
+		if err != nil {
+			log.Errorf(ctx, "error converting to api status: %v", err)
+			continue
+		}
+
+		items = append(items, apiStatus)
+	}
+
+	return util.PackagePageableResponse(util.PageableResponseParams{
+		Items:          items,
+		Path:           requestPath,
+		NextMaxIDValue: nextMaxIDValue,
+		PrevMinIDValue: prevMinIDValue,
+		Limit:          limit,
+	})
+}