builder: dragonfly builder hangs on "building packages" step

[timdarbydotnet]

The builder hangs forever here:
Building Go cmd/dist using /usr/local/go.
Building Go toolchain1 using /usr/local/go.
Building Go bootstrap cmd/go (go_bootstrap) using Go toolchain1.
Building Go toolchain2 using go_bootstrap and Go toolchain1.
Building Go toolchain3 using go_bootstrap and Go toolchain2.
Building packages and commands for dragonfly/amd64.

I can't tell when this broke because I've had problems with the builder recently and it hasn't had a successful run in a while. These processes are all just sitting there idle:

tim 1670 0.0 0.0 4532 2608 1 I2+ 5:12PM 0:00.00 /usr/local/bin/bash /home/tim/gobuilder/builder.sh
tim 1671 0.0 0.4 94680 70368 1 S2+ 5:12PM 0:00.57 /home/tim/gobuilder/go/bin/buildlet -coordinator=farmer.golang.org -reverse-type=host-dragonfly-amd64-tdfbsd -reboot=false -hostname=dragonflybsd-amd64
tim 1700 0.0 0.0 4980 2700 1 I2+ 5:19PM 0:00.01 bash /tmp/workdir-host-dragonfly-amd64-tdfbsd/go/src/make.bash
tim 2372 0.0 0.1 110080 25388 1 S1+ 5:20PM 0:00.32 /tmp/workdir-host-dragonfly-amd64-tdfbsd/go/pkg/tool/dragonfly_amd64/go_bootstrap install -gcflags=all= -ldflags=all= std cmd
tim 2473 0.0 0.0 106480 4576 1 I0+ 5:21PM 0:00.00 /tmp/workdir-host-dragonfly-amd64-tdfbsd/go/pkg/tool/dragonfly_amd64/cgo -objdir /tmp/go-build237083079/b031/ -importpath os/user -- -I /tmp/go-build237083079/b031/ -g -O2 /tmp/workdir-host-dragonfly-amd64-tdfbsd/go/src/os/user/cgo_lookup_unix.go /tmp/workdir-host-dragonfly-amd64-tdfbsd/go/src/os/user/getgrouplist_unix.go /tmp/workdir-host-dragonfly-amd64-tdfbsd/go/src/os/user/listgroups_unix.go
tim 2476 0.0 0.0 6288 3404 1 I2+ 5:21PM 0:00.00 /usr/libexec/gcc50/gcc -w -Wno-error -o/tmp/go-build237083079/b031//cgo.o -gdwarf-2 -c -xc -I /tmp/go-build237083079/b031/ -g -m64 - -O0
tim 2477 0.0 0.1 24984 18404 1 I3+ 5:21PM 0:00.01 /usr/libexec/gcc50/cc1 -quiet -I /tmp/go-build237083079/b031/ - -quiet -dumpbase - -m64 -mtune=generic -auxbase-strip /tmp/go-build237083079/b031//cgo.o -gdwarf-2 -g -O0 -Wno-error -w -o /tmp//ccpLkZQG.s
tim 2491 0.0 0.0 105072 4496 1 I1+ 5:21PM 0:00.00 /tmp/workdir-host-dragonfly-amd64-tdfbsd/go/pkg/tool/dragonfly_amd64/cgo -objdir /tmp/go-build237083079/b078/ -importpath net -- -I /tmp/go-build237083079/b078/ -g -O2 /tmp/workdir-host-dragonfly-amd64-tdfbsd/go/src/net/cgo_bsd.go /tmp/workdir-host-dragonfly-amd64-tdfbsd/go/src/net/cgo_resold.go /tmp/workdir-host-dragonfly-amd64-tdfbsd/go/src/net/cgo_sockold.go /tmp/workdir-host-dragonfly-amd64-tdfbsd/go/src/net/cgo_unix.go
tim 2494 0.0 0.0 6288 3404 1 I2+ 5:21PM 0:00.00 /usr/libexec/gcc50/gcc -w -Wno-error -o/tmp/go-build237083079/b078//cgo.o -gdwarf-2 -c -xc -I /tmp/go-build237083079/b078/ -g -m64 - -O0
tim 2495 0.0 0.1 24984 18404 1 I3+ 5:21PM 0:00.01 /usr/libexec/gcc50/cc1 -quiet -I /tmp/go-build237083079/b078/ - -quiet -dumpbase - -m64 -mtune=generic -auxbase-strip /tmp/go-build237083079/b078//cgo.o -gdwarf-2 -g -O0 -Wno-error -w -o /tmp//cceX7Nl2.s
tim 2545 0.0 0.0 105072 4196 1 I0+ 5:21PM 0:00.00 /tmp/workdir-host-dragonfly-amd64-tdfbsd/go/pkg/tool/dragonfly_amd64/cgo -objdir /tmp/go-build237083079/b179/ -importpath os/signal/internal/pty -- -I /tmp/go-build237083079/b179/ -g -O2 /tmp/workdir-host-dragonfly-amd64-tdfbsd/go/src/os/signal/internal/pty/pty.go
tim 2548 0.0 0.0 6288 3404 1 I3+ 5:21PM 0:00.00 /usr/libexec/gcc50/gcc -w -Wno-error -o/tmp/go-build237083079/b179//cgo.o -gdwarf-2 -c -xc -I /tmp/go-build237083079/b179/ -g -m64 - -O0
tim 2549 0.0 0.1 24984 18404 1 I0+ 5:21PM 0:00.01 /usr/libexec/gcc50/cc1 -quiet -I /tmp/go-build237083079/b179/ - -quiet -dumpbase - -m64 -mtune=generic -auxbase-strip /tmp/go-build237083079/b179//cgo.o -gdwarf-2 -g -O0 -Wno-error -w -o /tmp//cc0zlVsu.s

[gopherbot]

Change https://golang.org/cl/131497 mentions this issue: Revert "syscall, os: use pipe2 syscall on DragonflyBSD instead of pipe"

[timdarbydotnet]

Is there a way to make the builder skip commits made prior to the revert? Every time it tries one of those, it's going to hang.