cmd/go: work around race in fmtCmd

CL 529219 made an existing race with accessing Builder.scriptDir from
Builder.fmtcmd (and now also Builder.Showcmd) much more likely by
dropping a theoretically unnecessary condition from the call from
Builder.run to Builder.fmtcmd.

For an example race report, see
https://build.golang.org/log/c3cad62d0fc33a8381d2091661c685ea1fc525c4

The race is between

  (*Builder).cover2() -> (*Builder).run() -> (*Builder).fmtcmd()

and various other call paths of the form

  (*Builder).build() -> (*gcToolchain).*  (*Builder).Showcmd() -> (*Builder).fmtcmd()

The race can be reproduced with

  go install -race cmd/go
  stress -p 1 go test -x -cover -a log

Return this race to its existing likelihood by putting the condition
back. This isn't a "correct" solution because the race could still
happen if the "cover" tool invoked by Builder.cover2 emits output. But
this will do for a temporary fix.

Change-Id: Ifd811dea07f05e1422fd02b63cd958627727aa12
Reviewed-on: https://go-review.googlesource.com/c/go/+/536355
Auto-Submit: Austin Clements <austin@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

0 files changed, 0 insertions, 0 deletions