various: pass logger.Logf through to more places

Updates #7537

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Id89acab70ea678c8c7ff0f44792d54c7223337c6

tsnet/tsnet.go

@@ -40,6 +40,7 @@ import (
 	"tailscale.com/logpolicy"
 	"tailscale.com/logtail"
 	"tailscale.com/logtail/filch"
+	"tailscale.com/net/dnsfallback"
 	"tailscale.com/net/memnet"
 	"tailscale.com/net/proxymux"
 	"tailscale.com/net/socks5"
@@ -619,6 +620,25 @@ func (s *Server) logf(format string, a ...interface{}) {
 	log.Printf(format, a...)
 }
 
+// ReplaceGlobalLoggers will replace any Tailscale-specific package-global
+// loggers with this Server's logger. It returns a function that, when called,
+// will undo any changes made.
+//
+// Note that calling this function from multiple Servers will result in the
+// last call taking all logs; logs are not duplicated.
+func (s *Server) ReplaceGlobalLoggers() (undo func()) {
+	var undos []func()
+
+	oldDnsFallback := dnsfallback.SetLogger(s.logf)
+	undos = append(undos, func() { dnsfallback.SetLogger(oldDnsFallback) })
+
+	return func() {
+		for _, fn := range undos {
+			fn()
+		}
+	}
+}
+
 // printAuthURLLoop loops once every few seconds while the server is still running and
 // is in NeedsLogin state, printing out the auth URL.
 func (s *Server) printAuthURLLoop() {