plugin: fails to correctly reference type from vendor

[BryceDFisher]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.10.3 linux/amd64

Does this issue reproduce with the latest release?

yes, 1.10.3

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

GOARCH="amd64"
GOBIN="/home/fisherb/go/bin"
GOCACHE="/home/fisherb/.cache/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/fisherb/go"
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
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-build861164804=/tmp/go-build -gno-record-gcc-switches"

What did you do?

When using plugins, in a typical github vendoring approach: $GOPATH/src/github.com/author/repo/package with resources in $GOPATH/src/github.com/author/repo/package/vendor/github.com/author/repoa/package, if the plugin was compiled using a separate package, that is ultimately imported into the main package via vendoring, the types don't match and casting is not possible.

This tree works (rooted at $GOPATH/src/github.com:

plbody
└── pltypes
    ├── plbody
    │   └── plbody.go
    └── pltypes.go
plmain
└── plmain.go
plplugin
└── plplugin
    └── plplugin.go

This tree does not work (rooted at $GOPATH/src/github.com)

plbody
└── pltypes
    ├── plbody
    │   └── plbody.go
    └── pltypes.go
plmain
├── plmain.go
└── vendor
    └── github.com
        └── plbody
            └── pltypes
                ├── plbody
                │   └── plbody.go
                └── pltypes.go
plplugin
└── plplugin
    ├── plplugin.go
    └── vendor
        └── github.com
            └── plbody
                └── pltypes
                    ├── plbody
                    │   └── plbody.go
                    └── pltypes.go

In both the working and not working cases, the imports are the same, just the directory structure is different.
plbody.go:

package plbody

import (
        "log"
        "plugin"

        "github.com/plbody/pltypes"
)

//Test is the test function
var Test func(a pltypes.A, s string) error

func init() {
        pl, err := plugin.Open("plplugin.so")
        if err != nil {
                log.Fatalf("Unable to find plugin: %s", err.Error())
        }
        obj, err := pl.Lookup("Test")
        if err != nil {
                log.Fatalf("Unable to find Test in plugin: %s", err.Error())
        }

        t := obj.(pltypes.Tester)
        Test = t.Test
}

pltypes.go:

package pltypes

//Tester contains a test function
type Tester interface {
        Test(A, string) error
}

//A A is a custom data type
type A struct {
        Data string
}

plmain.go:

package main

import (
        "log"

        "github.com/plbody/pltypes"
        "github.com/plbody/pltypes/plbody"
)

func main() {
        log.Printf("%v", plbody.Test(pltypes.A{}, ""))
}

plplugin.go:

package main

import "github.com/plbody/pltypes"

type test struct{}

//Test is the exported test struct
var Test test

func main() {}

func (t test) Test(a pltypes.A, s string) error {
        return nil
}

What did you expect to see?

2018/08/17 15:03:34 <nil>

What did you see instead?

The types are not castable to each other:

panic: interface conversion: *main.test is not pltypes.Tester: missing method Test

goroutine 1 [running]:
github.com/plmain/vendor/github.com/plbody/pltypes/plbody.init.0()
        /home/fisherb/go/src/github.com/plmain/vendor/github.com/plbody/pltypes/plbody/plbody.go:23 +0xcf

[andybons]

@ianlancetaylor

[BryceDFisher]

Upon further inspection, it appears the reflection does not view the type A as the same type in the function definition, despite the fact that reflect.TypeOf(A) returns the same PkgPath ("") and Name ("A"). If you remove A from the function definition, everything works as expected.

[BryceDFisher]

It looks like the discussion from https://github.com/golang/proposal/blob/master/design/25719-go15vendor.md and the comment in

go/src/go/build/build.go

Line 360 in 3acd257

// By default, Import searches vendor directories

ensure that plugins built with vendor directories will never see types as used above as the same type at run time.

Does anyone have a better method that I could use in my example to load the plugin with the custom type pltypes.A? Or should I assume that plugins will not support this behavior for the foreseeable future and therefore I should move on to other options.