Add nodeinfo endpoint for federation purposes (#16953)

Nodeinfo is a way to expose certain metadata about a server for use of discovery regarding functionality of its federation capabilities.

Two endpoints are required:
1. `/.well-known/nodeinfo` which informs client where it can find the location of the location of its metadata (including which version of the schema is used)
2. the endpoint which exposes the metadata in json format according to schema.

Notes:
* `openRegistrations` is a required field, but I propose to set to false as default in case someone writes a crawler to discover "open" gitea instances
* to limit data leakage I also propose to not include the `usage` field (note it is required so it should be included, but left as empty).

More info:
https://github.com/jhass/nodeinfo
https://github.com/jhass/nodeinfo/tree/main/schemas/2.1
http://nodeinfo.diaspora.software/protocol.html

routers/api/v1/api.go

@@ -580,6 +580,9 @@ func Routes(sessioner func(http.Handler) http.Handler) *web.Route {
 			})
 		}
 		m.Get("/version", misc.Version)
+		if setting.Federation.Enabled {
+			m.Get("/nodeinfo", misc.NodeInfo)
+		}
 		m.Get("/signing-key.gpg", misc.SigningKey)
 		m.Post("/markdown", bind(api.MarkdownOption{}), misc.Markdown)
 		m.Post("/markdown/raw", misc.MarkdownRaw)

routers/api/v1/misc/nodeinfo.go

@@ -0,0 +1,45 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package misc
+
+import (
+	"net/http"
+
+	"code.gitea.io/gitea/modules/context"
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/structs"
+)
+
+// NodeInfo returns the NodeInfo for the Gitea instance to allow for federation
+func NodeInfo(ctx *context.APIContext) {
+	// swagger:operation GET /nodeinfo miscellaneous getNodeInfo
+	// ---
+	// summary: Returns the nodeinfo of the Gitea application
+	// produces:
+	// - application/json
+	// responses:
+	//   "200":
+	//     "$ref": "#/responses/NodeInfo"
+
+	nodeInfo := &structs.NodeInfo{
+		Version: "2.1",
+		Software: structs.NodeInfoSoftware{
+			Name:       "gitea",
+			Version:    setting.AppVer,
+			Repository: "https://github.com/go-gitea/gitea.git",
+			Homepage:   "https://gitea.io/",
+		},
+		Protocols: []string{"activitypub"},
+		Services: structs.NodeInfoServices{
+			Inbound:  []string{},
+			Outbound: []string{},
+		},
+		OpenRegistrations: setting.Service.ShowRegistrationButton,
+		Usage: structs.NodeInfoUsage{
+			Users: structs.NodeInfoUsageUsers{},
+		},
+	}
+	ctx.JSON(http.StatusOK, nodeInfo)
+}

routers/api/v1/swagger/nodeinfo.go

@@ -0,0 +1,16 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package swagger
+
+import (
+	api "code.gitea.io/gitea/modules/structs"
+)
+
+// NodeInfo
+// swagger:response NodeInfo
+type swaggerResponseNodeInfo struct {
+	// in:body
+	Body api.NodeInfo `json:"body"`
+}

routers/web/nodeinfo.go

@@ -0,0 +1,33 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package web
+
+import (
+	"fmt"
+	"net/http"
+
+	"code.gitea.io/gitea/modules/context"
+	"code.gitea.io/gitea/modules/setting"
+)
+
+type nodeInfoLinks struct {
+	Links []nodeInfoLink `json:"links"`
+}
+
+type nodeInfoLink struct {
+	Href string `json:"href"`
+	Rel  string `json:"rel"`
+}
+
+// NodeInfoLinks returns links to the node info endpoint
+func NodeInfoLinks(ctx *context.Context) {
+	nodeinfolinks := &nodeInfoLinks{
+		Links: []nodeInfoLink{{
+			fmt.Sprintf("%sapi/v1/nodeinfo", setting.AppURL),
+			"http://nodeinfo.diaspora.software/ns/schema/2.1",
+		}},
+	}
+	ctx.JSON(http.StatusOK, nodeinfolinks)
+}

routers/web/web.go

@@ -234,6 +234,9 @@ func RegisterRoutes(m *web.Route) {
 	// for health check
 	m.Get("/", Home)
 	m.Get("/.well-known/openid-configuration", user.OIDCWellKnown)
+	if setting.Federation.Enabled {
+		m.Get("/.well-known/nodeinfo", NodeInfoLinks)
+	}
 	m.Group("/explore", func() {
 		m.Get("", func(ctx *context.Context) {
 			ctx.Redirect(setting.AppSubURL + "/explore/repos")