cmd/go: module info (from runtime/debug.modinfo) dropped from final binary if unused #28753

hyangah · 2018-11-12T23:53:34Z

Dependency on submodule (in the same repo) sometimes doesn't appear in the debug modinfo.

Tested with Go built from the tip today

$ go version
go version devel +f58b02a29c Sun Nov 11 18:49:40 2018 +0000 linux/amd64

How to reproduce:

$ go get rsc.io/goversion

$ git clone https://github.com/hyangah/modtest7.git
$ cd modtest7
$ tree
.
├── go.mod
├── go.sum
├── hello
│   ├── go.mod
│   └── hello.go
└── main.go
$ cat main.go
package main

import (
	"github.com/hyangah/modtest7/hello"
)

func main() {
	hello.Hi()
}

$ go build
$ goversion -m modtest7
modtest7 devel +f58b02a29c Sun Nov 11 18:49:40 2018 +0000
	path  github.com/hyangah/modtest7
	mod   github.com/hyangah/modtest7  (devel)

I expected the module github.com/hyangah/modtest7/hello is listed as a dependency, but it didn't appear.

go build -x -v output

$ go build -x -v
WORK=/tmp/go-build666928546
mkdir -p /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24 # git2 https://github.com/hyangah/modtest7
cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git init --bare
0.005s # cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git init --bare
cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git remote add origin https://github.com/hyangah/modtest7
0.003s # cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git remote add origin https://github.com/hyangah/modtest7
go: finding github.com/hyangah/modtest7/hello v0.0.0-20181112233247-afc59526b1e9
cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git -c log.showsignature=false log -n1 '--format=format:%H %ct %D' afc59526b1e9
0.003s # cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git -c log.showsignature=false log -n1 '--format=format:%H %ct %D' afc59526b1e9
cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git tag -l
0.002s # cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git tag -l
cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git ls-remote -q origin
0.244s # cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git ls-remote -q origin
cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git fetch -f origin 'refs/heads/*:refs/heads/*' 'refs/tags/*:refs/tags/*'
0.293s # cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git fetch -f origin 'refs/heads/*:refs/heads/*' 'refs/tags/*:refs/tags/*'
cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git -c log.showsignature=false log -n1 '--format=format:%H %ct %D' afc59526b1e9
0.003s # cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git -c log.showsignature=false log -n1 '--format=format:%H %ct %D' afc59526b1e9
cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git cat-file blob afc59526b1e9357dfd74c2b55fedf7c09461e2e9:hello/go.mod
0.003s # cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git cat-file blob afc59526b1e9357dfd74c2b55fedf7c09461e2e9:hello/go.mod
cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git cat-file blob afc59526b1e9357dfd74c2b55fedf7c09461e2e9:hello/go.mod
0.003s # cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git cat-file blob afc59526b1e9357dfd74c2b55fedf7c09461e2e9:hello/go.mod
go: downloading github.com/hyangah/modtest7/hello v0.0.0-20181112233247-afc59526b1e9
cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git cat-file blob afc59526b1e9357dfd74c2b55fedf7c09461e2e9:hello/go.mod
0.003s # cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git cat-file blob afc59526b1e9357dfd74c2b55fedf7c09461e2e9:hello/go.mod
cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git -c core.autocrlf=input -c core.eol=lf archive --format=zip --prefix=prefix/ afc59526b1e9357dfd74c2b55fedf7c09461e2e9 -- hello
0.004s # cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git -c core.autocrlf=input -c core.eol=lf archive --format=zip --prefix=prefix/ afc59526b1e9357dfd74c2b55fedf7c09461e2e9 -- hello
cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git cat-file blob afc59526b1e9357dfd74c2b55fedf7c09461e2e9:LICENSE
0.003s # cd /usr/local/google/home/hakim/go/pkg/mod/cache/vcs/690aa3d58c3a1743e72ae301f73382e197623d579a5ad3ddc35eca733d606c24; git cat-file blob afc59526b1e9357dfd74c2b55fedf7c09461e2e9:LICENSE
mkdir -p $WORK/b001/
cat >$WORK/b001/importcfg.link << 'EOF' # internal
packagefile github.com/hyangah/modtest7=/usr/local/google/home/hakim/.cache/go-build/f3/f3364a757515e565a17db8ce81386ba3ce3cfc3d411b3d94838c597c3de137e7-d
packagefile github.com/hyangah/modtest7/hello=/usr/local/google/home/hakim/.cache/go-build/27/2763495febf1ae14d202f8e098930fc5d5329828aa73caf78f1d3517f3b8e2e3-d
packagefile runtime=/usr/local/google/home/hakim/.cache/go-build/fa/fadd0857eaf0d83dbe2ea7918b5b8c1b7849ff105ff155d26c29341dfb479130-d
packagefile internal/bytealg=/usr/local/google/home/hakim/.cache/go-build/b9/b9aa84a005f37674bb1d54facbba420cac72f89554ec18744e4fc10e15b1a573-d
packagefile internal/cpu=/usr/local/google/home/hakim/golang/go/pkg/linux_amd64/internal/cpu.a
packagefile runtime/internal/atomic=/usr/local/google/home/hakim/golang/go/pkg/linux_amd64/runtime/internal/atomic.a
packagefile runtime/internal/math=/usr/local/google/home/hakim/golang/go/pkg/linux_amd64/runtime/internal/math.a
packagefile runtime/internal/sys=/usr/local/google/home/hakim/golang/go/pkg/linux_amd64/runtime/internal/sys.a
EOF
mkdir -p $WORK/b001/exe/
cd .
/usr/local/google/home/hakim/golang/go/pkg/tool/linux_amd64/link -o $WORK/b001/exe/a.out -importcfg $WORK/b001/importcfg.link -buildmode=exe -buildid=yV_6u0tuDNnxPReT53qd/iEGjVHL_ChZAAZfv5eKJ/JVIBetcUg4mrkMBZzVD2/yV_6u0tuDNnxPReT53qd -extld=gcc /usr/local/google/home/hakim/.cache/go-build/f3/f3364a757515e565a17db8ce81386ba3ce3cfc3d411b3d94838c597c3de137e7-d
/usr/local/google/home/hakim/golang/go/pkg/tool/linux_amd64/buildid -w $WORK/b001/exe/a.out # internal
mv $WORK/b001/exe/a.out modtest7
rm -r $WORK/b001/

I suspect this is related to #28083.

bcmills · 2019-01-17T17:43:57Z

It looks like the linker is stripping out the debug information for some reason: if I modify the program to access it at run-time, then goversion also detects it successfully.

modtest7$ git checkout HEAD main.go

modtest7$ rm ./modtest7 && gotip build

modtest7$ $GOPATH/bin/goversion -m ./modtest7
./modtest7 devel +6e9f664 Thu Jan 17 17:07:23 2019 +0000

modtest7$ cat >main.go <<EOF
> package main

import (
        "fmt"
        "runtime/debug"

        "github.com/hyangah/modtest7/hello"
)

func main() {
        hello.Hi()
        bi, ok := debug.ReadBuildInfo()
        if !ok {
                panic("no build info")
        }
        fmt.Println(bi.Main.Path, bi.Main.Version)
        for _, d := range bi.Deps {
                fmt.Println(d.Path, d.Version)
        }
}
> EOF

modtest7$ rm ./modtest7 && gotip build

modtest7$ $GOPATH/bin/goversion -m ./modtest7
./modtest7 devel +6e9f664 Thu Jan 17 17:07:23 2019 +0000
        path  github.com/hyangah/modtest7
        mod   github.com/hyangah/modtest7        (devel)
        dep   github.com/hyangah/modtest7/hello  v0.0.0-20181112233247-afc59526b1e9

bcmills · 2019-01-17T17:47:55Z

Yeah, it's definitely being pruned out for some reason. An explicit runtime.KeepAlive is all it takes to make goversion work:

modtest7$ cat >main.go <<EOF
package main

import (
        "runtime"

        "github.com/hyangah/modtest7/hello"
)

func main() {
        hello.Hi()
        runtime.KeepAlive(__debug_modinfo__)
}
EOF

modtest7$ rm ./modtest7 && gotip build

modtest7$ $GOPATH/bin/goversion -m ./modtest7
./modtest7 devel +6e9f664 Thu Jan 17 17:07:23 2019 +0000
        path  github.com/hyangah/modtest7
        mod   github.com/hyangah/modtest7        (devel)
        dep   github.com/hyangah/modtest7/hello  v0.0.0-20181112233247-afc59526b1e9

bcmills · 2019-01-17T17:51:42Z

@ianlancetaylor, @randall77, @josharian, @jayconrod, @heschik:
Any idea what might have changed to cause the compiler and/or linker to prune out the __debug_modinfo__ string from the final binary?

It is unused, but defined in package main using //go:linkname __debug_modinfo__ runtime/debug.modinfo.

heschi · 2019-01-17T18:34:38Z

I guess https://golang.org/cl/157097 removed the only use of the variable, allowing it to be eliminated?

bcmills · 2019-01-17T18:51:17Z

Maybe, but @hyangah filed this issue back in November, and that commit only landed this week. I guess it was fixed incidentally and then regressed again!

gopherbot · 2019-01-17T20:15:55Z

Change https://golang.org/cl/158357 mentions this issue: cmd/go/internal/modload: ensure that __debug_modinfo__ is not discarded during linking

hyangah added the modules label Nov 12, 2018

bcmills added this to the Go1.12 milestone Jan 17, 2019

bcmills added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Jan 17, 2019

bcmills changed the title ~~cmd/go: submodule info is omitted in runtime/debug.modinfo~~ cmd/go: module info (from runtime/debug.modinfo) is dropped if unreachable Jan 17, 2019

bcmills changed the title ~~cmd/go: module info (from runtime/debug.modinfo) is dropped if unreachable~~ cmd/go: module info (from runtime/debug.modinfo) dropped from final binary if unused Jan 17, 2019

gopherbot closed this as completed in 9d23975 Jan 23, 2019

golang locked and limited conversation to collaborators Jan 23, 2020

gopherbot added the FrozenDueToAge label Jan 23, 2020

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cmd/go: module info (from runtime/debug.modinfo) dropped from final binary if unused #28753

cmd/go: module info (from runtime/debug.modinfo) dropped from final binary if unused #28753

hyangah commented Nov 12, 2018

bcmills commented Jan 17, 2019 •

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heschi commented Jan 17, 2019

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gopherbot commented Jan 17, 2019

cmd/go: module info (from runtime/debug.modinfo) dropped from final binary if unused #28753

cmd/go: module info (from runtime/debug.modinfo) dropped from final binary if unused #28753

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hyangah commented Nov 12, 2018

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