Breaking change: runtime/debug.ReadBuildInfo no longer populates BuildInfo.Main in 1.18

[bufdev]

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

$ go version
go version go1.18 darwin/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
GO111MODULE="on"
GOARCH="amd64"
GOBIN="/Users/alice/.cache/buf/Darwin/x86_64/gobin"
GOCACHE="/Users/alice/.cache/buf/Darwin/x86_64/gocache"
GOENV="/Users/alice/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/alice/.cache/buf/Darwin/x86_64/go/pkg/mod"
GONOPROXY="github.com/bufbuild,github.com/bufbuild/buf"
GONOSUMDB="github.com/bufbuild,github.com/bufbuild/buf"
GOOS="darwin"
GOPATH="/Users/alice/.cache/buf/Darwin/x86_64/go"
GOPRIVATE="github.com/bufbuild,github.com/bufbuild/buf"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/alice/.cache/gimme/versions/go1.18.darwin.amd64"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/alice/.cache/gimme/versions/go1.18.darwin.amd64/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.18"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/alice/git/buf/go.mod"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/gl/rk7vtj000000000000gn/T/go-build0041845511=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

https://pkg.go.dev/runtime/debug#ReadBuildInfo no longer sets the Main field on https://pkg.go.dev/runtime/debug#BuildInfo. We use debug.ReadBuildInfo to make it so that code from https://github.com/bufbuild/buf/tree/main/private can only be imported by projects in github.com/bufbuild - our version of "organization-private". We do this via https://github.com/bufbuild/buf/blob/main/private/usage/usage.go. See bufbuild/homebrew-buf#8

mkdir tmp
cd ./tmp
cat <<EOF > go.mod
module foo

go 1.17
EOF
cat <<EOF > main.go
package main

import (
	"fmt"
	"runtime/debug"
)

func main() {
	buildInfo, _ := debug.ReadBuildInfo()
	fmt.Printf("%+v\n", buildInfo)
}
EOF
go build -o foo main.go
./foo

What did you expect to see?

A value for the Main.Path field of foo.

In 1.17.8 this is:

&{Path:command-line-arguments Main:{Path:foo Version:(devel) Sum: Replace:<nil>} Deps:[]}

What did you see instead?

In 1.18 this is:

go	go1.18
path	command-line-arguments
build	-compiler=gc
build	CGO_ENABLED=1
build	CGO_CFLAGS=
build	CGO_CPPFLAGS=
build	CGO_CXXFLAGS=
build	CGO_LDFLAGS=
build	GOARCH=amd64
build	GOOS=darwin
build	GOAMD64=v1

Formatting differences are expected based on the work in https://github.com/golang/go/commits/master/src/runtime/debug/mod.go, but no longer producing the Main field is a breaking change. I've verified that no fields are set as well, and that this is not just formatting, by specifically printing buildInfo.Main.Path from the code above (which is empty).

[seankhliao]

This is CL 339170 and working as intended.

[lrewega]

@michaelmatloob @seankhliao I'm rather surprised by this otherwise undocumented change in behaviour, I would expect at least some mention of this in the release notes. @ianlancetaylor can we amend the notes to help other folks from tripping over this change? Could you also please confirm that my understanding below is correct? I am not sure I fully understand the consequences of this change.

---

For future readers who stumble upon this: It looks like as part of the workspaces feature, some runtime/debug.BuildInfo details for the pseudo-package command-line-arguments, which represents file arguments passed to go commands, were removed. Using a package path (relative or otherwise) instead of file arguments (such as path/to/main.go) avoids the command-line-arguments package, and produces consistent results across versions. For example, changing the original example above from

go build -o foo main.go

to

go build -o foo .

produces the expected output:

go	go1.18rc1
path	foo
mod	foo	(devel)
build	-compiler=gc
build	CGO_ENABLED=1
build	CGO_CFLAGS=
build	CGO_CPPFLAGS=
build	CGO_CXXFLAGS=
build	CGO_LDFLAGS=
build	GOARCH=arm64
build	GOOS=darwin

Note the presence of mod foo (devel), representing buildInfo.Main.Path, which is absent when using file arguments.

[lrewega]

Related to #51279, due to changing command-line-arguments to not-a-module.

cc @bcmills