feature/routecheck: add ts_omit_routecheck feature flag (#19638)

RouteCheck, which checks that overlapping routers are reachable, is
enabled by default for both tailscaled and tsnet.

Updates #17366
Updates tailscale/corp#33033

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

feature/buildfeatures/feature_routecheck_disabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build ts_omit_routecheck
+
+package buildfeatures
+
+// HasRouteCheck is whether the binary was built with support for modular feature "Support checking the reachability of overlapping routers, for choosing between multiple network paths to the same IP address".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_routecheck" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasRouteCheck = false

feature/buildfeatures/feature_routecheck_enabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build !ts_omit_routecheck
+
+package buildfeatures
+
+// HasRouteCheck is whether the binary was built with support for modular feature "Support checking the reachability of overlapping routers, for choosing between multiple network paths to the same IP address".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_routecheck" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasRouteCheck = true

feature/condregister/maybe_routecheck.go

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

feature/featuretags/featuretags.go

@@ -235,6 +235,10 @@ var Features = map[FeatureTag]FeatureMeta{
 		Desc: "Linux systemd-resolved integration",
 		Deps: []FeatureTag{"dbus"},
 	},
+	"routecheck": {
+		Sym:  "RouteCheck",
+		Desc: "Support checking the reachability of overlapping routers, for choosing between multiple network paths to the same IP address",
+	},
 	"sdnotify": {
 		Sym:  "SDNotify",
 		Desc: "systemd notification support",

feature/routecheck/routecheck.go

@@ -0,0 +1,17 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package routecheck registers support for RouteCheck,
+// which checks the reachability of overlapping routers.
+//
+// When there are multiple network paths to an IP address, it is being routed by
+// overlapping routers. The client uses reachability to pick between those
+// paths: either sticking with an active WireGuard session or choosing from the
+// peers that it has determined it can reach. It doesn’t need reachability for
+// IP addresses that have only one network path, since it can naively attempt to
+// establish a WireGuard session.
+package routecheck
+
+func init() {
+	// TODO(sfllaw): Initialize the new routecheck package.
+}