net: race detected on linux-amd64-race

[dsnet]

Recently see on try-bot:

==================
WARNING: DATA RACE
Write at 0x0000008ff0d0 by main goroutine:
  net.uninstallTestHooks()
      /tmp/workdir/go/src/net/main_unix_test.go:39 +0x7a
  sync.(*Once).Do()
      /tmp/workdir/go/src/sync/once.go:44 +0xe1
  net.TestMain()
      /tmp/workdir/go/src/net/main_test.go:52 +0x6c
  main.main()
      net/_test/_testmain.go:524 +0x1cd

Previous read at 0x0000008ff0d0 by goroutine 96:
  internal/poll.(*FD).destroy()
      /tmp/workdir/go/src/internal/poll/fd_unix.go:62 +0x52
  internal/poll.(*FD).decref()
      /tmp/workdir/go/src/internal/poll/fd_mutex.go:211 +0x55
  internal/poll.(*FD).Close()
      /tmp/workdir/go/src/internal/poll/fd_unix.go:81 +0xf5
  net.(*netFD).Close()
      /tmp/workdir/go/src/net/fd_unix.go:184 +0x60
  net.(*conn).Close()
      /tmp/workdir/go/src/net/net.go:200 +0x60
  net.(*UDPConn).Close()
      <autogenerated>:1 +0x43
  net.(*dnsPacketConn).Close()
      <autogenerated>:1 +0x5b
  net.(*Resolver).exchange()
      /tmp/workdir/go/src/net/dnsclient_unix.go:145 +0x57f
  net.(*Resolver).tryOneName()
      /tmp/workdir/go/src/net/dnsclient_unix.go:161 +0x1ef
  net.(*Resolver).goLookupIPCNAMEOrder.func1()
      /tmp/workdir/go/src/net/dnsclient_unix.go:483 +0xa5

Goroutine 96 (finished) created at:
  net.(*Resolver).goLookupIPCNAMEOrder()
      /tmp/workdir/go/src/net/dnsclient_unix.go:482 +0x253
  net.(*Resolver).lookupIP()
      /tmp/workdir/go/src/net/lookup_unix.go:101 +0xdf
  net.(*Resolver).(net.lookupIP)-fm()
      /tmp/workdir/go/src/net/lookup.go:187 +0x7f
  net.glob..func10()
      /tmp/workdir/go/src/net/hook.go:19 +0x6e
  net.(*Resolver).LookupIPAddr.func1()
      /tmp/workdir/go/src/net/lookup.go:193 +0xc9
  internal/singleflight.(*Group).doCall()
      /tmp/workdir/go/src/internal/singleflight/singleflight.go:93 +0x4c
==================

Full log: https://storage.googleapis.com/go-build-log/f1123583/linux-amd64-race_75b56ae6.log

[ianlancetaylor]

Dup of #21090?

[dsnet]

Seems like it. The stack traces are the same.