client/local,ipn/localapi: add /localapi/v0/routecheck endpoint (#19640)

In order to support a `tailscale routecheck` command, we introduce the
`/localapi/v0/routecheck` endpoint to the local API. This endpoint
returns the most recent report collected by the routecheck client.
If `force=true` is an argument in the query string, then this endpoint
will actively probe before returning the report.

Updates #17366
Updates tailscale/corp#33033

Signed-off-by: Simon Law <sfllaw@tailscale.com>

feature/routecheck/ipn.go

@@ -5,9 +5,23 @@ package routecheck
 
 import (
 	"tailscale.com/ipn/ipnext"
+	"tailscale.com/ipn/ipnlocal"
 	"tailscale.com/net/routecheck"
 )
 
+// ClientFor returns the [routecheck.Client] for a given backend,
+// or nil if route checking is not available for that backend.
+func ClientFor(b *ipnlocal.LocalBackend) *routecheck.Client {
+	e, ok := ipnlocal.GetExt[*Extension](b)
+	if e == nil || !ok {
+		return nil
+	}
+	return e.Client
+}
+
+// Report contains the result of a single routecheck.
+type Report = routecheck.Report
+
 // NodeBackender is a shim between [ipnext.Host] and [routecheck.NodeBackender].
 type nodeBackender struct{ ipnext.Host }
 

feature/routecheck/localapi.go

@@ -0,0 +1,85 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+package routecheck
+
+import (
+	"net/http"
+	"strconv"
+	"time"
+
+	jsonv2 "github.com/go-json-experiment/json"
+	jsonv1 "github.com/go-json-experiment/json/v1"
+
+	"tailscale.com/ipn/localapi"
+	"tailscale.com/net/routecheck"
+	"tailscale.com/util/httpm"
+)
+
+func init() {
+	localapi.Register("routecheck", serveRouteCheck)
+}
+
+// ServeRouteCheck handles the API endpoint that serves the routecheck Report.
+// If the probe form field is true, then this handler will refresh the Report
+// before serving it.
+// If the timeout form field is a valid duration, the probe will consider a node
+// to be unreachable if it doesn’t respond before the timeout expires.
+func serveRouteCheck(h *localapi.Handler, w http.ResponseWriter, r *http.Request) {
+	rc := ClientFor(h.LocalBackend())
+	if rc == nil {
+		http.Error(w, "routecheck is not enabled", http.StatusServiceUnavailable)
+		return
+	}
+
+	if r.Method != httpm.POST {
+		http.Error(w, "want POST", http.StatusMethodNotAllowed)
+		return
+	}
+
+	var err error
+	var report *routecheck.Report
+	if defBool(r.FormValue("probe"), false) {
+		timeout := defDuration(r.FormValue("timeout"), routecheck.DefaultTimeout)
+		timeout = min(max(0, timeout), 60*time.Second) // clamp to [0s, 60s]
+		report, err = rc.Refresh(r.Context(), timeout)
+	} else {
+		report = rc.Report()
+	}
+	if err != nil {
+		localapi.WriteErrorJSON(w, err)
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	if report == nil {
+		w.WriteHeader(http.StatusNoContent)
+		return
+	}
+
+	// TODO(sfllaw): Since ipn/localapi is still using encoding/json
+	// with its default options, marshal with DefaultOptionsV1.
+	jsonv2.MarshalWrite(w, report, jsonv1.DefaultOptionsV1())
+}
+
+func defBool(a string, def bool) bool {
+	if a == "" {
+		return def
+	}
+	v, err := strconv.ParseBool(a)
+	if err != nil {
+		return def
+	}
+	return v
+}
+
+func defDuration(a string, def time.Duration) time.Duration {
+	if a == "" {
+		return def
+	}
+	v, err := time.ParseDuration(a)
+	if err != nil {
+		return def
+	}
+	return v
+}