Compiling with modules and -trimpath records temp path in binary

[fd0]

What version of Go are you using (go version)?

go version go1.11.1 linux/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOCACHE="/home/fd0/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/fd0/shared/work/go"
GOPROXY=""
GORACE=""
GOROOT="/home/fd0/sdk/go1.11.1"
GOTMPDIR=""
GOTOOLDIR="/home/fd0/sdk/go1.11.1/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build079176254=/tmp/go-build -gno-record-gcc-switches"

What did you do?

For restic we're trying hard to provide reproducible builds. Among others, we strip the path to the directory with the source code using -trimpath, this worked great for Go 1.10. Unfortunately, building restic with Go 1.11 with Modules leaves a rather strange temporary path to _gomod_.go (which seems to record the build versions) in the binary. I did not find a way to strip or remove this path, resulting in different binaries for most builds. The temporary path is also cached in the build cache, I'm not sure if that's a good or bad thing, as consecutive builds without changing the code embeds the same temporary path.

This issue is related to #16860

Minimal example:

unset GOPATH
export GO111MODULE=on

cd $(mktemp -d /tmp/go-temp-path-XXXXXX)

cat <<EOF >main.go
package main

import "fmt"

func main() {
       fmt.Println("example")
}
EOF

go mod init example

The resulting binary example contains the path to the source code, and there's a reference to _gomod_.go but without a path:

go build

strings example | egrep '(/tmp|_gomod_)'
_gomod_.go
/tmp/go-temp-path-yN4rfF/main.go

What did you expect to see?

Using -trimpath should remove the path to the code and not embed any new temporary paths:

go build -gcflags -trimpath=$PWD

strings example | grep /tmp

What did you see instead?

Using -trimpath removes the path to the code, but now the _gomod_.go is referenced with a temporary path:

go build -gcflags -trimpath=$PWD

strings example | grep /tmp
/tmp/go-build222692051/b001/_gomod_.go

Additional remarks

It does not matter which path is removed via -trimpath, the temporary path to _gomod_.go is embedded as soon as -trimpath is used, e.g.:

go build -gcflags -trimpath=/x

strings example | grep /tmp
/tmp/go-build425085084/b001/_gomod_.go

[rsc]

This is a duplicate of #16860.

[fd0]

For the record, when building without modules, I don't observe this behavior, i.e. trimpath works as it should.