logpolicy,tsnet: remove syspolicy dependency

tsnet depends on logpolicy, which in turn depended on util/syspolicy
because of a single LogTarget policy setting it uses.

In this commit, we replace that dependency with a feature.Hook,
which only tailscaled or its platform-specific alternatives should set.

Updates #20031

Signed-off-by: Nick Khyl <nickk@tailscale.com>

logpolicy/logpolicy.go

@@ -54,13 +54,15 @@ import (
 	"tailscale.com/util/eventbus"
 	"tailscale.com/util/must"
 	"tailscale.com/util/racebuild"
-	"tailscale.com/util/syspolicy/pkey"
-	"tailscale.com/util/syspolicy/policyclient"
 	"tailscale.com/util/testenv"
 	"tailscale.com/version"
 	"tailscale.com/version/distro"
 )
 
+// GetLogTarget is an optional hook to register a function
+// that returns the log target URL to be used by logpolicy.
+var GetLogTarget feature.Hook[func() string]
+
 var getLogTargetOnce struct {
 	sync.Once
 	v string // URL of logs server, or empty for default
@@ -68,8 +70,12 @@ var getLogTargetOnce struct {
 
 func getLogTarget() string {
 	getLogTargetOnce.Do(func() {
-		envTarget, _ := os.LookupEnv("TS_LOG_TARGET")
-		getLogTargetOnce.v, _ = policyclient.Get().GetString(pkey.LogTarget, envTarget)
+		if f, ok := GetLogTarget.GetOk(); ok {
+			getLogTargetOnce.v = f()
+		}
+		if getLogTargetOnce.v == "" {
+			getLogTargetOnce.v, _ = os.LookupEnv("TS_LOG_TARGET")
+		}
 	})
 
 	return getLogTargetOnce.v

logpolicy/maybe_syspolicy.go

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