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fsenart opened this issue Mar 6, 2017 · 2 comments
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plugin: ambiguous/undocumented ldflags="-pluginpath ... -X ..." #19418

fsenart opened this issue Mar 6, 2017 · 2 comments
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Documentation Issues describing a change to documentation. FrozenDueToAge
Milestone
Go1.10

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@fsenart
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fsenart commented Mar 6, 2017

Please answer these questions before submitting your issue. Thanks!

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

go version go1.8 linux/amd64

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/fsenart/projects"
GORACE=""
GOROOT="/home/fsenart/tools/go1.8"
GOTOOLDIR="/home/fsenart/tools/go1.8/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build076969156=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

What did you do?

$GOPATH/src/issue_ldflags/host.go

// +build host

package main

import "plugin"

var hostDummy string = "EMPTY"

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

	println(hostDummy)

	f.(func())()
}

$GOPATH/src/issue_ldflags/guest.go

// +build guest

package main

var guestDummy string = "EMPTY"

func F() {
	println(guestDummy)
}

$GOPATH/src/issue_ldflags/Makefile

scenario_1:
	# INSIDE GOPATH & compile WITHOUT specifying source file
	@echo Host
	@echo ===
	@go build -tags host -ldflags='-X main.hostDummy=dummyHost' -o host
	@nm host | grep hostDummy
	@echo Guest
	@echo =====
	@go build -tags guest -buildmode=plugin -ldflags='-X $(shell basename $(CURDIR)).guestDummy=dummyGuest' -o guest.so
	@nm guest.so | grep guestDummy
	@echo Run
	@echo ===
	@./host
	@echo ===
	# WITH custom plugin path
	# https://github.com/golang/go/blob/master/src/cmd/go/internal/work/build.go#L2412
	@go build -tags guest -buildmode=plugin -ldflags='-pluginpath=custompluginpath -X custompluginpath.guestDummy=dummyGuest' -o guest.so
	@nm guest.so | grep guestDummy
	@echo Run
	@echo ===
	@./host

scenario_2:
	# INSIDE GOPATH & compile WITH specifying source file
	@echo Host
	@echo ===
	@go build -tags host -ldflags='-X main.hostDummy=dummyHost' -o host host.go
	@nm host | grep hostDummy
	@echo Guest
	@echo =====
	# If not seeing 2 lines, change the Makefile with the seen id.
	# https://github.com/golang/go/blob/master/src/cmd/go/internal/work/build.go#L2410
	@go build -tags guest -buildmode=plugin -ldflags='-X plugin/unnamed-1edb35ed362d12b8c6c92ff2cc31ad97e4131d95.guestDummy=dummyGuest' -o guest.so guest.go
	@nm guest.so | grep guestDummy
	@echo Run
	@echo ===
	@./host
	@echo ===
	# WITH custom plugin path
	# https://github.com/golang/go/blob/master/src/cmd/go/internal/work/build.go#L2412
	@go build -tags guest -buildmode=plugin -ldflags='-pluginpath=custompluginpath -X custompluginpath.guestDummy=dummyGuest' -o guest.so guest.go
	@nm guest.so | grep guestDummy
	@echo Run
	@echo ===
	@./host

scenario_3:
	# OUTSIDE GOPATH & compile WITHOUT specifying source file
	@echo Host
	@echo ===
	@GOPATH=/nowhere go build -tags host -ldflags='-X main.hostDummy=dummyHost' -o host
	@nm host | grep hostDummy
	@echo Guest
	@echo =====
	@GOPATH=/nowhere go build -tags guest -buildmode=plugin -ldflags='-X _$(CURDIR).guestDummy=dummyGuest' -o guest.so
	@nm guest.so | grep guestDummy
	@echo Run
	@echo ===
	@./host
	@echo ===
	# WITH custom plugin path
	# https://github.com/golang/go/blob/master/src/cmd/go/internal/work/build.go#L2412
	@GOPATH=/nowhere go build -tags guest -buildmode=plugin -ldflags='-pluginpath=custompluginpath -X custompluginpath.guestDummy=dummyGuest' -o guest.so
	@nm guest.so | grep guestDummy
	@echo Run
	@echo ===
	@./host

scenario_4:
	# OUTSIDE GOPATH & compile WITH specifying source file
	@echo Host
	@echo ===
	@GOPATH=/nowhere go build -tags host -ldflags='-X main.hostDummy=dummyHost' -o host host.go
	@nm host | grep hostDummy
	@echo Guest
	@echo =====
	# https://github.com/golang/go/blob/master/src/cmd/go/internal/work/build.go#L2410
	# If not seeing 2 lines, change the Makefile with the seen id.
	@GOPATH=/nowhere go build -tags guest -buildmode=plugin -ldflags='-X plugin/unnamed-1edb35ed362d12b8c6c92ff2cc31ad97e4131d95.guestDummy=dummyGuest' -o guest.so guest.go
	@nm guest.so | grep guestDummy
	@echo Run
	@echo ===
	@./host
	@echo ===
	# WITH custom plugin path
	# https://github.com/golang/go/blob/master/src/cmd/go/internal/work/build.go#L2412
	@GOPATH=/nowhere go build -tags guest -buildmode=plugin -ldflags='-pluginpath=custompluginpath -X custompluginpath.guestDummy=dummyGuest' -o guest.so guest.go
	@nm guest.so | grep guestDummy
	@echo Run
	@echo ===
	@./host

If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.

What did you expect to see?

I expect to be able to use -ldflags="-X importpath.guestDummy" to set a build time variable when compiling with -buildmode=plugin ; as I can do when compiling regular binaries with -ldflags="-X main.hostDummy"

What did you see instead?

In the context of plugins importpath is variable and depends on the building context. I found that I can have a fixed behavior when using -pluginpath ldflags. But this flag is not well documented:

  -pluginpath string
    	full path name for plugin

Can I use it confidently is this context? is this the good pattern to set a build time variable in plugins?
@cristim cristim mentioned this issue Mar 6, 2017
Merged
@gopherbot gopherbot added the Documentation Issues describing a change to documentation. label Mar 21, 2017
@bradfitz bradfitz added this to the Go1.9Maybe milestone Mar 21, 2017
@bradfitz bradfitz modified the milestones: Go1.9Maybe, Go1.10 Jun 7, 2017
@crawshaw
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Ideally no-one but cmd/go sets -pluginpath. But -X is widely used. I suppose for plugins we could rewrite in cmd/link any value passed in as '-X main.val' to '-X whateverthepluginnameis.val'.

@crawshaw crawshaw self-assigned this Sep 29, 2017
@gopherbot
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Change https://golang.org/cl/67432 mentions this issue: cmd/link: support -X values for main.* in plugins

@golang golang locked and limited conversation to collaborators Oct 4, 2018
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@bradfitz @crawshaw @fsenart @gopherbot