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

modules/setting/federation.go

@@ -0,0 +1,22 @@
+// 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 setting
+
+import "code.gitea.io/gitea/modules/log"
+
+// Federation settings
+var (
+	Federation = struct {
+		Enabled bool
+	}{
+		Enabled: true,
+	}
+)
+
+func newFederationService() {
+	if err := Cfg.Section("federation").MapTo(&Federation); err != nil {
+		log.Fatal("Failed to map Federation settings: %v", err)
+	}
+}

modules/setting/setting.go

@@ -1188,6 +1188,7 @@ func NewServices() {
 	NewQueueService()
 	newProject()
 	newMimeTypeMap()
+	newFederationService()
 }
 
 // NewServicesForInstall initializes the services for install

modules/structs/nodeinfo.go

@@ -0,0 +1,44 @@
+// 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 structs
+
+// NodeInfo contains standardized way of exposing metadata about a server running one of the distributed social networks
+type NodeInfo struct {
+	Version           string           `json:"version"`
+	Software          NodeInfoSoftware `json:"software"`
+	Protocols         []string         `json:"protocols"`
+	Services          NodeInfoServices `json:"services"`
+	OpenRegistrations bool             `json:"openRegistrations"`
+	Usage             NodeInfoUsage    `json:"usage"`
+	Metadata          struct{}         `json:"metadata"`
+}
+
+// NodeInfoSoftware contains Metadata about server software in use
+type NodeInfoSoftware struct {
+	Name       string `json:"name"`
+	Version    string `json:"version"`
+	Repository string `json:"repository"`
+	Homepage   string `json:"homepage"`
+}
+
+// NodeInfoServices contains the third party sites this server can connect to via their application API
+type NodeInfoServices struct {
+	Inbound  []string `json:"inbound"`
+	Outbound []string `json:"outbound"`
+}
+
+// NodeInfoUsage contains usage statistics for this server
+type NodeInfoUsage struct {
+	Users         NodeInfoUsageUsers `json:"users"`
+	LocalPosts    int                `json:"localPosts,omitempty"`
+	LocalComments int                `json:"localComments,omitempty"`
+}
+
+// NodeInfoUsageUsers contains statistics about the users of this server
+type NodeInfoUsageUsers struct {
+	Total          int `json:"total,omitempty"`
+	ActiveHalfyear int `json:"activeHalfyear,omitempty"`
+	ActiveMonth    int `json:"activeMonth,omitempty"`
+}