appc,feature: add the start of new conn25 app connector

When peers request an IP address mapping to be stored, the connector
stores it in memory.

Fixes tailscale/corp#34251
Signed-off-by: Fran Bull <fran@tailscale.com>

feature/condregister/maybe_conn25.go

@@ -0,0 +1,8 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ts_omit_conn25
+
+package condregister
+
+import _ "tailscale.com/feature/conn25"

feature/conn25/conn25.go

@@ -0,0 +1,84 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package conn25 registers the conn25 feature and implements its associated ipnext.Extension.
+package conn25
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"tailscale.com/appc"
+	"tailscale.com/feature"
+	"tailscale.com/ipn/ipnext"
+	"tailscale.com/ipn/ipnlocal"
+	"tailscale.com/types/logger"
+)
+
+// featureName is the name of the feature implemented by this package.
+// It is also the [extension] name and the log prefix.
+const featureName = "conn25"
+
+func init() {
+	feature.Register(featureName)
+	newExtension := func(logf logger.Logf, sb ipnext.SafeBackend) (ipnext.Extension, error) {
+		e := &extension{
+			conn: &appc.Conn25{},
+		}
+		return e, nil
+	}
+	ipnext.RegisterExtension(featureName, newExtension)
+	ipnlocal.RegisterPeerAPIHandler("/v0/connector/transit-ip", handleConnectorTransitIP)
+}
+
+func handleConnectorTransitIP(h ipnlocal.PeerAPIHandler, w http.ResponseWriter, r *http.Request) {
+	e, ok := ipnlocal.GetExt[*extension](h.LocalBackend())
+	if !ok {
+		http.Error(w, "miswired", http.StatusInternalServerError)
+		return
+	}
+	e.handleConnectorTransitIP(h, w, r)
+}
+
+// extension is an [ipnext.Extension] managing the connector on platforms
+// that import this package.
+type extension struct {
+	conn *appc.Conn25
+}
+
+// Name implements [ipnext.Extension].
+func (e *extension) Name() string {
+	return featureName
+}
+
+// Init implements [ipnext.Extension].
+func (e *extension) Init(host ipnext.Host) error {
+	return nil
+}
+
+// Shutdown implements [ipnlocal.Extension].
+func (e *extension) Shutdown() error {
+	return nil
+}
+
+func (e *extension) handleConnectorTransitIP(h ipnlocal.PeerAPIHandler, w http.ResponseWriter, r *http.Request) {
+	const maxBodyBytes = 1024 * 1024
+	defer r.Body.Close()
+	if r.Method != "POST" {
+		http.Error(w, "Method should be POST", http.StatusMethodNotAllowed)
+		return
+	}
+	var req appc.ConnectorTransitIPRequest
+	err := json.NewDecoder(http.MaxBytesReader(w, r.Body, maxBodyBytes+1)).Decode(&req)
+	if err != nil {
+		http.Error(w, "Error decoding JSON", http.StatusBadRequest)
+		return
+	}
+	resp := e.conn.HandleConnectorTransitIPRequest(h.Peer().ID(), req)
+	bs, err := json.Marshal(resp)
+	if err != nil {
+		http.Error(w, "Error encoding JSON", http.StatusInternalServerError)
+		return
+	}
+	w.Write(bs)
+}