cmd/go, cmd/cgo: repeatable builds with Clang 3.8

[EtienneBruines]

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?
   go version devel +f8fc371 Thu Apr 21 17:04:01 2016 +0000 linux/amd64
   (but GOROOT_BOOTSTRAP is set to an installation of go1.6.2)
2. What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/etiennebruines/workspaces/go"
GORACE=""
GOROOT="/home/etiennebruines/workspaces/go-master"
GOTOOLDIR="/home/etiennebruines/workspaces/go-master/pkg/tool/linux_amd64"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build179019630=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"

(Tested different versions of `clang` (with different results!), more info down below. )

1. What did you do?
   Attempted to build master branch of Go (currently HEAD is at 40f1d0c )
   (more specifically: ./all.bash within the src directory)
2. What did you expect to see?
   No errors.
3. What did you see instead?
   An error within the tests, namely:

--- FAIL: TestCgoConsistentResults (1.10s)
    go_test.go:244: running testgo [build -o /tmp/gotest260338695/cgotest1 cgotest]
    go_test.go:244: running testgo [build -x -o /tmp/gotest260338695/cgotest2 cgotest]
    go_test.go:263: standard error:
    go_test.go:264: WORK=/tmp/go-build737947577
        mkdir -p $WORK/cgotest/_obj/
        mkdir -p $WORK/
        cd /home/etiennebruines/workspaces/go-master/src/cmd/go/testdata/src/cgotest
        CGO_LDFLAGS="-g" "-O2" /home/etiennebruines/workspaces/go-master/pkg/tool/linux_amd64/cgo -objdir $WORK/cgotest/_obj/ -importpath cgotest -- -I $WORK/cgotest/_obj/ m.go
        cd $WORK
        clang-3.8 -fdebug-prefix-map=a=b -c trivial.c
        clang-3.8 -gno-record-gcc-switches -c trivial.c
        cd /home/etiennebruines/workspaces/go-master/src/cmd/go/testdata/src/cgotest
        clang-3.8 -I . -fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=$WORK=/tmp/go-build -gno-record-gcc-switches -I $WORK/cgotest/_obj/ -g -O2 -o $WORK/cgotest/_obj/_cgo_main.o -c $WORK/cgotest/_obj/_cgo_main.c
        clang-3.8 -I . -fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=$WORK=/tmp/go-build -gno-record-gcc-switches -I $WORK/cgotest/_obj/ -g -O2 -o $WORK/cgotest/_obj/_cgo_export.o -c $WORK/cgotest/_obj/_cgo_export.c
        clang-3.8 -I . -fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=$WORK=/tmp/go-build -gno-record-gcc-switches -I $WORK/cgotest/_obj/ -g -O2 -o $WORK/cgotest/_obj/m.cgo2.o -c $WORK/cgotest/_obj/m.cgo2.c
        clang-3.8 -I . -fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=$WORK=/tmp/go-build -gno-record-gcc-switches -o $WORK/cgotest/_obj/_cgo_.o $WORK/cgotest/_obj/_cgo_main.o $WORK/cgotest/_obj/_cgo_export.o $WORK/cgotest/_obj/m.cgo2.o -g -O2
        /home/etiennebruines/workspaces/go-master/pkg/tool/linux_amd64/cgo -objdir $WORK/cgotest/_obj/ -dynpackage cgotest -dynimport $WORK/cgotest/_obj/_cgo_.o -dynout $WORK/cgotest/_obj/_cgo_import.go
        cd $WORK
        clang-3.8 -no-pie -c trivial.c
        cd /home/etiennebruines/workspaces/go-master/src/cmd/go/testdata/src/cgotest
        clang-3.8 -I . -fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=$WORK=/tmp/go-build -gno-record-gcc-switches -o $WORK/cgotest/_obj/_all.o $WORK/cgotest/_obj/_cgo_export.o $WORK/cgotest/_obj/m.cgo2.o -g -O2 -Wl,-r -nostdlib -Wl,--build-id=none
        /home/etiennebruines/workspaces/go-master/pkg/tool/linux_amd64/compile -o $WORK/cgotest.a -trimpath $WORK -p cgotest -buildid 1491fc40a2d4f2376923084c60120b2fd293fe0e -D _/home/etiennebruines/workspaces/go-master/src/cmd/go/testdata/src/cgotest -I $WORK -pack $WORK/cgotest/_obj/_cgo_gotypes.go $WORK/cgotest/_obj/m.cgo1.go $WORK/cgotest/_obj/_cgo_import.go
        pack r $WORK/cgotest.a $WORK/cgotest/_obj/_all.o # internal
        mkdir -p /tmp/gotest260338695/
        mv $WORK/cgotest.a /tmp/gotest260338695/cgotest2

    go_test.go:2775: building cgotest twice did not produce the same output
FAIL
FAIL    cmd/go  29.068s

Notes

• Without clang, this error is not thrown (fails at cmd/cgo: ThreadSanitizer barks about data races in all.bash #14602, so not sure if issue is present)
• With clang-3.6, this error is not thrown (not sure why not, but some things are disabled with older clang and newer kernel (4.3))
• With clang-3.8, this error is thrown (but cmd/cgo: ThreadSanitizer barks about data races in all.bash #14602 is "fixed").

When running CGO_ENABLED=0, all errors go away (makes sense).

[rsc]

This is unfortunate but not critical for the release.

[ianlancetaylor]

Postponing to 1.8.

[rsc]

It sounds like we just need to wait for clang and gcc to fix their reproducible build support.

clang

Support for -fdebug-prefix-map was added Oct 2015. Clang 3.7.0 was released Sep 2015, Clang 3.7.1 was released Jan 2016, and Clang 3.8 was released Mar 2016. My guess is the change may have missed the Clang 3.7 point release but was likely in Clang 3.8. Without detecting support for the flag, the test is skipped. This is why we don't see failures before Clang 3.8.

The linux-amd64-clang gomote image is using Clang 3.5, so we don't test this case on the builder.

@bradfitz, is it possible to update to a newer clang? Latest is 3.9.0 released Sep 2 2016. http://llvm.org/releases/

gcc

Support for -fdebug-prefix-map is more widespread, but some versions don't quite work. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69821 shows that while -fdebug-prefix-map did the right thing to the debug symbols, gcc then included that option in the DW_AT_producer field of the dwarf. If the goal of -fdebug-prefix-map=old=new is to eliminate the string "old" from the binary, writing down that option into the binary defeats the purpose. The bug entry says the target milestone is gcc 6 and the discussion there sounds like the decision was not to backport to gcc 5. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=819176, the Debian entry for this bug, suggests it is fixed in Debian's gcc 5.4.0. It is unclear whether that means the fix is only in Debian's copy or is upstream. It's unclear to me when DW_AT_producer first started recording the whole command line.

Obviously the tests do pass on our gcc builders, and I confirmed that (unlike the clang builder) the gcc on the builder does support the flag, so the test should be running. Perhaps gcc 4.3 had a related problem.