cmd/go: package imported multiple times from GOPATH and vendor

[tiancaiamao]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.11.1 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

➜  test go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/genius/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/media/genius/OS/project"
GOPROXY=""
GORACE=""
GOROOT="/media/genius/OS/project/go"
GOTMPDIR=""
GOTOOLDIR="/media/genius/OS/project/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-build246817751=/tmp/go-build -gno-record-gcc-switches"

What did you do?

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

Package p imports q, q is in p's vendor.

➜  p cd $GOPATH/src/p
➜  p tree
.
├── p.go
└── vendor
    └── q
        └── q.go

2 directories, 2 files

➜  p cat p.go 
package p

import _ "q"

There is also a q package in GOPATH, i.e, q is in both GOPATH and p's vendor:

➜  p cd $GOPATH/src/q
➜  q cat q.go 
package q

import "net/http"

func init() {
	http.HandleFunc("/debug/requests", func(w http.ResponseWriter, req *http.Request) {})
}

I have a test.go, it imports both p and q:

package main

import (
	_ "p"
	_ "q"
)

func main() {}

Then package q will be import twice, and the init function run twice, results in the following error:

➜  test ./test   
panic: http: multiple registrations for /debug/requests

goroutine 1 [running]:
net/http.(*ServeMux).Handle(0x84b560, 0x67a277, 0xf, 0x6b7420, 0x68a390)
	/media/genius/OS/project/go/src/net/http/server.go:2377 +0x221
net/http.(*ServeMux).HandleFunc(0x84b560, 0x67a277, 0xf, 0x68a390)
	/media/genius/OS/project/go/src/net/http/server.go:2395 +0x5a
net/http.HandleFunc(0x67a277, 0xf, 0x68a390)
	/media/genius/OS/project/go/src/net/http/server.go:2407 +0x4b
q.init.0()
	/media/genius/OS/project/src/q/q.go:6 +0x42

What did you expect to see?

Go should handle packages in vendor and GOPATH, don't import them twice.

What did you see instead?

The same package been import twice and their init function conflicts.

[agnivade]

@bcmills

[bcmills]

This seems to be working as designed. If you don't want package p to have its own duplicate copy of q, then don't vendor q within p.

(Consider what would happen if some other package r also vendored in q at a conflicting version: which version would p end up using? Which version would q end up using?)

As far as I can tell, this is exactly one of the problems with vendoring that modules are intended to address.