x/tools/cmd/goimports: Panics when GO111MODULE=on and there is no go.mod

[epk]

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

$ go version
go version go1.12 darwin/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/adi/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/adi"
GOPROXY=""
GORACE=""
GOROOT="/Users/adi/.dev/go/1.12"
GOTMPDIR=""
GOTOOLDIR="/Users/adi/.dev/go/1.12/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/dev/null"
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/7c/qpccms_j3v38zgnhczjy8f5c0000gn/T/go-build273490665=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

bar.go

package main

import (
	"github.com/paranoidaditya/foo/pkg/middleware"
)

func main() {
	middleware, err := middleware.New()
	client, err := client.New(middleware)
}

GO111MODULE=on goimports -w bar.go

What did you expect to see?

bar.go

package main

import (
	"github.com/paranoidaditya/foo/pkg/middleware"
        "github.com/paranoidaditya/foo/pkg/client"
)

func main() {
	middleware, err := middleware.New()
	client, err := client.New(middleware)
}

What did you see instead?

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x60 pc=0x1286bce]

goroutine 1 [running]:
golang.org/x/tools/imports.(*moduleResolver).scan(0xc000100280, 0xc0001455f0, 0xc000100280, 0x11453b0, 0x1, 0x1, 0xc0001455f0)
	/Users/adi/pkg/mod/golang.org/x/tools@v0.0.0-20190314010720-1286b2016bb1/imports/mod.go:205 +0x11e
golang.org/x/tools/imports.addExternalCandidates(0xc0001821b0, 0xc0001455f0, 0x7ffeefbff295, 0x47, 0x0, 0x0)
	/Users/adi/pkg/mod/golang.org/x/tools@v0.0.0-20190314010720-1286b2016bb1/imports/fix.go:683 +0x73
golang.org/x/tools/imports.fixImportsDefault(0xc0000247c0, 0xc00010c200, 0x7ffeefbff295, 0x47, 0xc00010c080, 0xe00, 0x15cfa80)
	/Users/adi/pkg/mod/golang.org/x/tools@v0.0.0-20190314010720-1286b2016bb1/imports/fix.go:504 +0x2fb
golang.org/x/tools/imports.process(0x7ffeefbff295, 0x47, 0xc00018a000, 0x747, 0xe00, 0x15cfa80, 0xc00010c080, 0x0, 0xc0000b3d98, 0x10344b4, ...)
	/Users/adi/pkg/mod/golang.org/x/tools@v0.0.0-20190314010720-1286b2016bb1/imports/imports.go:72 +0x753
golang.org/x/tools/imports.Process(...)
	/Users/adi/pkg/mod/golang.org/x/tools@v0.0.0-20190314010720-1286b2016bb1/imports/imports.go:50
main.processFile(0x7ffeefbff295, 0x47, 0x0, 0x0, 0x13c2b40, 0xc000094000, 0x1, 0x0, 0x0)
	/Users/adi/pkg/mod/golang.org/x/tools@v0.0.0-20190314010720-1286b2016bb1/cmd/goimports/goimports.go:136 +0x1d4
main.gofmtMain()
	/Users/adi/pkg/mod/golang.org/x/tools@v0.0.0-20190314010720-1286b2016bb1/cmd/goimports/goimports.go:280 +0x230
main.main()
	/Users/adi/pkg/mod/golang.org/x/tools@v0.0.0-20190314010720-1286b2016bb1/cmd/goimports/goimports.go:196 +0x33

creating a go.mod which looks like:

module foobar

require (
	github.com/paranoidaditya/foo v0.0.0
)

replace github.com/paranoidaditya/foo => /Users/adi/src/github.com/paranoidaditya/foo

Fixes the panic

[bradfitz]

/cc @heschik @jayconrod @ianthehat @bcmills

[heschi]

I guess there's no main module when GOMOD=/dev/null. I believe that you can still do go build file.go, but not go build . Maybe goimports should be able to handle it as well as the go command itself does? I think that would mean scanning the module cache, but nothing else.

@bcmills, is that right?

[bcmills]

Yep, that sounds right.

[gopherbot]

Change https://golang.org/cl/167860 mentions this issue: imports: handle missing main module