[feature] Implement the preferences client API (#1740)

This adds the preferences endpoint to our Mastodon Client API
implementation. It's a read-only endpoint that returns a number of
user preferences. Applications can query these settings when logging in
a user (for the first time) to configure themselves.

internal/processing/preferences.go

@@ -0,0 +1,57 @@
+// 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 processing
+
+import (
+	"context"
+
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+)
+
+func (p *Processor) PreferencesGet(ctx context.Context, accountID string) (*apimodel.Preferences, gtserror.WithCode) {
+	act, err := p.state.DB.GetAccountByID(ctx, accountID)
+	if err != nil {
+		return nil, gtserror.NewErrorInternalError(err)
+	}
+
+	return &apimodel.Preferences{
+		PostingDefaultVisibility: mastoPrefVisibility(act.Privacy),
+		PostingDefaultSensitive:  *act.Sensitive,
+		PostingDefaultLanguage:   act.Language,
+		// The Reading* preferences don't appear to actually be settable by the
+		// client, so forcing some sensible defaults here
+		ReadingExpandMedia:    "default",
+		ReadingExpandSpoilers: false,
+		ReadingAutoPlayGifs:   false,
+	}, nil
+}
+
+func mastoPrefVisibility(vis gtsmodel.Visibility) string {
+	switch vis {
+	case gtsmodel.VisibilityPublic, gtsmodel.VisibilityDirect:
+		return string(vis)
+	case gtsmodel.VisibilityUnlocked:
+		return "unlisted"
+	default:
+		// this will catch gtsmodel.VisibilityMutualsOnly and other types Mastodon doesn't
+		// have and map them to the most restrictive state
+		return "private"
+	}
+}