cmd/go: do not modify environment for 'go run'

[calmh]

For example compiling for dragonfly/amd64 on darwin/amd64:

(Via a build script, but what happens is just a setting of GOPATH for vendoring and some ldflags for version tagging being added)

jb@syno:~/src/github.com/syncthing/syncthing (go15) $ go run build.go -goos dragonfly
*** Unknown Go version "go version devel +b6ead9f Tue Jul 7 21:53:11 2015 +0000 darwin/amd64".
*** This isn't known to work, proceed on your own risk.
GOPATH=/Users/jb/src/github.com/syncthing/syncthing/Godeps/_workspace:/Users/jb
go install -v -ldflags -w -X main.Version=v0.11.15+6-gff03813 -X main.BuildStamp=1436877241 -X main.BuildUser=jb -X main.BuildHost=syno -X main.BuildEnv=default ./cmd/...
runtime/cgo
# runtime/cgo
/usr/local/go-exp/src/runtime/cgo/gcc_dragonfly_amd64.c:37:2: error: implicit declaration of function 'SIGFILLSET' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
/usr/local/go-exp/src/runtime/cgo/gcc_dragonfly_amd64.c:37:13: error: variable 'ign' is uninitialized when used here [-Werror,-Wuninitialized]
/usr/local/go-exp/src/runtime/cgo/gcc_dragonfly_amd64.c:32:14: note: initialize the variable 'ign' to silence this warning
exit status 2
exit status 1

I'm not sure what it's trying to do, but the error sounds like it's trying to compile stuff for dragonfly using the system compiler, which is obviously quite different from what would exist on actual dragonfly. Disabling cgo makes it work:

jb@syno:~/src/github.com/syncthing/syncthing (go15) $ CGO_ENABLED=0 go run build.go -goos dragonfly
*** Unknown Go version "go version devel +b6ead9f Tue Jul 7 21:53:11 2015 +0000 darwin/amd64".
*** This isn't known to work, proceed on your own risk.
GOPATH=/Users/jb/src/github.com/syncthing/syncthing/Godeps/_workspace:/Users/jb
go install -v -ldflags -w -X main.Version=v0.11.15+6-gff03813 -X main.BuildStamp=1436877241 -X main.BuildUser=unknown-user -X main.BuildHost=syno -X main.BuildEnv=default ./cmd/...

Is this expected? (cgo not working is obviously expected, what surprises me is that it seems to try to enable it by default.)

(I see similar but different errors on linux/amd64 -> dragonfly/amd64 so this one at least is not darwin specific)

[ianlancetaylor]

We need to know what your build.go program does.

Normally CGO_ENABLED is set to false for a cross-compiler. That code is in defaultContext in the go/build package, and it has not changed since 1.4. Apparently something has changed somewhere, but without knowing what build.go does I don't know what it is.

[calmh]

Yeah, I figured it out. The build script itself does nothing fancy apart from building the parameter lists above and then running "go" with them. However, when I go run build.go, the first instance of go adds CGO_ENABLED, GOGCCFLAGS and a few other variables to the environment. These are then inherited when the build script execs "go install ...". I can filter them out, by inspecting the environment set by go run and removing the stuff I don't want.

Something here seems to have changed, but it may as well just be the fact that packages are built on the fly for cross compilation rather than as it was before.

[ianlancetaylor]

I'll close this, but feel free to reopen if you find out what changed and think it should be changed back.

[calmh]

What changed is that "go run" now sets a number of environment variables it didn't before:

jb@syno:~ $ cat test.go 
package main

import (
    "os"
    "os/exec"
)

func main() {
    c := exec.Command("/bin/bash", "-c", "export")
    c.Stdout = os.Stdout
    c.Run()
}

# Go 1.5:
jb@syno:~ $ go run test.go | grep GO
declare -x CGO_ENABLED="1"
declare -x GOARCH="amd64"
declare -x GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fno-common"
declare -x GOHOSTARCH="amd64"
declare -x GOHOSTOS="darwin"
declare -x GOOS="darwin"
declare -x GOPATH="/Users/jb"
declare -x GOROOT="/usr/local/go-exp"
declare -x GOTOOLDIR="/usr/local/go-exp/pkg/tool/darwin_amd64"

# Go 1.4:
jb@syno:~ $ GOROOT=/usr/local/go1.4.2 /usr/local/go1.4.2/bin/go run test.go | grep GO
declare -x GOPATH="/Users/jb"
declare -x GOROOT="/usr/local/go1.4.2"
jb@syno:~ $ 

If this is intentional, I guess it's fine. Possibly it should be documented somewhere.

[ianlancetaylor]

Thanks for looking into it. The change was intentional: https://go-review.googlesource.com/6409 .

Reopening to document this.

[rsc]

The change was intentional for things like go tool compile. It was not meant to affect go run. That's a bug and should be fixed.

[gopherbot]

CL https://golang.org/cl/12483 mentions this issue.