runtime/race: external_cgo_thread passed unexpectedly in TestOutput

[cuonglm]

https://storage.googleapis.com/go-build-log/63849344/linux-amd64-race_657c252c.log

Output extracted from test log:

--- FAIL: TestOutput (9.68s)
    output_test.go:76: failed test case external_cgo_thread, expect:
        ==================
        WARNING: DATA RACE
        Read at 0x[0-9,a-f]+ by main goroutine:
          main\.main\(\)
              .*/main\.go:34 \+0x[0-9,a-f]+
        
        Previous write at 0x[0-9,a-f]+ by goroutine [0-9]:
          main\.goCallback\(\)
              .*/main\.go:27 \+0x[0-9,a-f]+
          main._cgoexpwrap_[0-9a-z]+_goCallback\(\)
              .*_cgo_gotypes\.go:[0-9]+ \+0x[0-9,a-f]+
        
        Goroutine [0-9] \(running\) created at:
          runtime\.newextram\(\)
              .*/runtime/proc.go:[0-9]+ \+0x[0-9,a-f]+
        ==================
        got:

[katiehockman]

Can you please provide more information on what action we should be taking with this issue report? It looks like this is just from a trybots run in https://go-review.googlesource.com/c/go/+/182561, and that the builders are passing for linux-amd64-race right now.

[cuonglm]

@katiehockman it's a data race, so it can happen this time but not that time.

[katiehockman]

/cc @hyangah