plugin: unable to load plugin if go mod is not enabled

[life1347]

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

1.11.10

Does this issue reproduce with the latest release?

Yes

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

GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/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-build245942501=/tmp/go-build -gno-record-gcc-switches"

What did you do?

test-go/
├── main.go
└── plugins
    └── main.go

• test-go/plugins/main.go

package main

import (
	"fmt"
	"net/http"
)

func Foobar() {
	fmt.Printf("Foo Bar: %v", http.StatusOK)
}

• test-go/main.go

package main

import (
	"fmt"
	"plugin"
	"net/http"
)

func main() {
	p, err := plugin.Open("plugin.so")
	if err != nil {
		panic(err)
	}

	symbol, err := p.Lookup("Foobar")
	if err != nil {
		panic(err)
	}

	f := symbol.(func())
	f()

	fmt.Printf("%v", http.StatusOK)
}

Steps to reproduce

/go/src/test-go# cd plugins
/go/src/test-go/plugins# export export GO111MODULE=on
/go/src/test-go/plugins# go build -buildmode=plugin -o plugin.so
/go/src/test-go/plugins# mv plugin.so ../
/go/src/test-go/plugins# cd ../
/go/src/test-go# export export GO111MODULE=off
/go/src/test-go# go run main.go

panic: plugin.Open("plugin"): plugin was built with a different version of package net

goroutine 1 [running]:
main.main()
	/go/src/test-go/main.go:12 +0x249
exit status 2

Everything works with 1.12.4 but failed with 1.11.10 (Test with docker golang:1.12.4 and golang:1.11.10).

The workaround is to set GO111MODULE=on then it works again.

/go/src/test-go# export export GO111MODULE=on
/go/src/test-go# go mod init
/go/src/test-go# go run main.go

Foo Bar: 200200

Question

1. Why having different results in 1.12.4 and 1.11.10 if GO111MODULE=off?
2. As long as plugins/main.go doesn't import net/http then it works in 1.11.10. Why?

What did you expect to see?

Main application should be able to load the plugin.

What did you see instead?

In 1.11.10

panic: plugin.Open("plugin"): plugin was built with a different version of package net

goroutine 1 [running]:
main.main()
	/go/src/test-go/main.go:12 +0x249
exit status 2

in 1.12.4

Foo Bar: 200200

[andybons]

@ianlancetaylor @bcmills @jayconrod

[bcmills]

See previously #31354 and #31278.

[bcmills]

The difference you observe between 1.11.10 and 1.12.4 is possibly due to #27285.

[bcmills]

Duplicate of #31278