[feature] Implement /api/v1/instance/peers endpoint (#660)

* add missing license headers

* start adding instance peers get

* rename domainblock.go

* embed domain in domainblock so it can be reused

* update swagger docs

* add test instances to db

* update tests

* add/update instancepeersget

* update domain model

* add getinstancepeers to db

* instance-expose-peers, instance-expose-suspended

* add auth checks for both current filters

* attach endpoint to router

* include public comment

* obfuscate domain if required

* go mod tidy

* update swagger docs

* remove unnecessary comment

* return 'flat' peerlist if no query params provided

internal/api/client/instance/instancepeersget.go

@@ -0,0 +1,136 @@
+/*
+   GoToSocial
+   Copyright (C) 2021-2022 GoToSocial Authors admin@gotosocial.org
+
+   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 instance
+
+import (
+	"fmt"
+	"net/http"
+	"strings"
+
+	"github.com/superseriousbusiness/gotosocial/internal/api"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/oauth"
+
+	"github.com/gin-gonic/gin"
+)
+
+// InstancePeersGETHandler swagger:operation GET /api/v1/instance/peers instancePeersGet
+//
+// ---
+// tags:
+// - instance
+//
+// produces:
+// - application/json
+//
+// parameters:
+// - name: filter
+//   type: string
+//   description: |-
+//     Comma-separated list of filters to apply to results. Recognized values are:
+//     'open' -- include peers that are not suspended or silenced
+//     'suspended' -- include peers that have been suspended.
+//     If filter is 'open', only instances that haven't been suspended or silenced will be returned.
+//     If filter is 'suspended', only suspended instances will be shown.
+//     If filter is 'open,suspended', then all known instances will be returned.
+//     If filter is an empty string or not set, then 'open' will be assumed as the default.
+//   in: query
+//   required: false
+//
+// responses:
+//   '200':
+//     description: |-
+//       If no filter parameter is provided, or filter is empty, then a legacy,
+//       Mastodon-API compatible response will be returned. This will consist of
+//       just a 'flat' array of strings like `["example.com", "example.org"]`.
+//
+//       If a filter parameter is provided, then an array of objects with at least
+//       a `domain` key set on each object will be returned.
+//
+//       Domains that are silenced or suspended will also have a key
+//       'suspended_at' or 'silenced_at' that contains an iso8601 date string.
+//       If one of these keys is not present on the domain object, it is open.
+//       Suspended instances may in some cases be obfuscated, which means they
+//       will have some letters replaced by '*' to make it more difficult for
+//       bad actors to target instances with harassment.
+//
+//       Whether a flat response or a more detailed response is returned, domains
+//       will be sorted alphabetically by hostname.
+//     schema:
+//       type: array
+//       items:
+//         "$ref": "#/definitions/domain"
+//   '400':
+//      description: bad request
+//   '401':
+//      description: unauthorized
+//   '403':
+//      description: forbidden
+//   '404':
+//      description: not found
+//   '406':
+//      description: not acceptable
+//   '500':
+//      description: internal server error
+func (m *Module) InstancePeersGETHandler(c *gin.Context) {
+	authed, err := oauth.Authed(c, false, false, false, false)
+	if err != nil {
+		api.ErrorHandler(c, gtserror.NewErrorUnauthorized(err, err.Error()), m.processor.InstanceGet)
+		return
+	}
+
+	if _, err := api.NegotiateAccept(c, api.JSONAcceptHeaders...); err != nil {
+		api.ErrorHandler(c, gtserror.NewErrorNotAcceptable(err, err.Error()), m.processor.InstanceGet)
+		return
+	}
+
+	var includeSuspended bool
+	var includeOpen bool
+	var flat bool
+	if filterParam := c.Query(PeersFilterKey); filterParam != "" {
+		filters := strings.Split(filterParam, ",")
+		for _, f := range filters {
+			trimmed := strings.TrimSpace(f)
+			switch {
+			case strings.EqualFold(trimmed, "suspended"):
+				includeSuspended = true
+			case strings.EqualFold(trimmed, "open"):
+				includeOpen = true
+			default:
+				err := fmt.Errorf("filter %s not recognized; accepted values are 'open', 'suspended'", trimmed)
+				api.ErrorHandler(c, gtserror.NewErrorBadRequest(err, err.Error()), m.processor.InstanceGet)
+				return
+			}
+		}
+	} else {
+		// default is to only include open domains, and present
+		// them in a 'flat' manner (just an array of strings),
+		// to maintain compatibility with mastodon API
+		includeOpen = true
+		flat = true
+	}
+
+	data, errWithCode := m.processor.InstancePeersGet(c.Request.Context(), authed, includeSuspended, includeOpen, flat)
+	if errWithCode != nil {
+		api.ErrorHandler(c, errWithCode, m.processor.InstanceGet)
+		return
+	}
+
+	c.JSON(http.StatusOK, data)
+}