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>

client/local/routecheck.go

@@ -0,0 +1,43 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ts_omit_routecheck
+
+package local
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net/http"
+
+	"tailscale.com/net/routecheck"
+)
+
+// ErrReportPending is returned by [Client.RouteCheck] and [Client.RouteCheckProbe]
+// when the report is pending.
+var ErrRouteCheckReportUnavailable = errors.New("report pending")
+
+// RouteCheckProbe performs a routecheck probe and waits for its report.
+func (lc *Client) RouteCheckProbe(ctx context.Context) (*routecheck.Report, error) {
+	body, err := lc.send(ctx, "POST", "/localapi/v0/routecheck?probe=true", http.StatusOK, nil)
+	if err != nil {
+		if hs, ok := errors.AsType[httpStatusError](err); ok && hs.HTTPStatus == http.StatusNoContent {
+			return nil, ErrRouteCheckReportUnavailable
+		}
+		return nil, fmt.Errorf("error %w: %s", err, body)
+	}
+	return decodeJSON[*routecheck.Report](body)
+}
+
+// RouteCheck requests the report compiled by the latest routecheck probe.
+func (lc *Client) RouteCheck(ctx context.Context) (*routecheck.Report, error) {
+	body, err := lc.send(ctx, "POST", "/localapi/v0/routecheck", http.StatusOK, nil)
+	if err != nil {
+		if hs, ok := errors.AsType[httpStatusError](err); ok && hs.HTTPStatus == http.StatusNoContent {
+			return nil, ErrRouteCheckReportUnavailable
+		}
+		return nil, fmt.Errorf("error %w: %s", err, body)
+	}
+	return decodeJSON[*routecheck.Report](body)
+}