cmd/go: crashes when -buildmode=shared is set and modules enabled

[shawndx]

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

$ go version
go version devel +d75b7f9648 Sat Oct 24 15:08:50 2020 +0000 linux/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
GO111MODULE="auto"

GOARCH="amd64"

GOBIN=""

GOCACHE="/home/jixiangdong/.cache/go-build"

GOENV="/home/jixiangdong/.config/go/env"

GOEXE=""

GOFLAGS=""

GOHOSTARCH="amd64"

GOHOSTOS="linux"

What did you do?

Two crashes could be produced when running -buildmode=shared with 'go module' being enabled, depending on the package name, here are details with a reduced test-case:

step 1: create hello.go (outside $GOPATH and $GOROOT)

package main
import "fmt"
func main() {
	fmt.Println("Hello")
}

step 2: create a go.mod, picking up a module name without hostname
$ go mod init mymodule

step 3:
$ go install -buildmode=shared std
$ go install -buildmode=shared -linkshared mymodule

panic: runtime error: index out of range [0] with length 0

goroutine 1 [running]:
cmd/go/internal/work.(*Builder).linkSharedAction.func2(0x997f80)
	/home/jixiangdong/util/go1.15/src/cmd/go/internal/work/action.go:818 +0x8e5
cmd/go/internal/work.(*Builder).cacheAction(0xc000540a00, 0xc000242180, 0x14, 0x0, 0xc0006cfb08, 0xc000242180)
	/home/jixiangdong/util/go1.15/src/cmd/go/internal/work/action.go:371 +0x9e
cmd/go/internal/work.(*Builder).linkSharedAction(0xc000540a00, 0x1, 0x1, 0xc00053d20a, 0x6, 0xc00038b540, 0xc00053d20a)
...

step 4:
If changing the package name in hello.go to anything else other than 'main' the compiler crashes with a different stack.

# /tmp/go-build927715808/b035/libmymodule.so
panic: runtime error: makeslice: cap out of range

goroutine 1 [running]:
cmd/link/internal/loader.(*Loader).LoadSyms(0xc000606000, 0x872300)
	/home/jixiangdong/util/go1.15/src/cmd/link/internal/loader/loader.go:2161 +0x125
cmd/link/internal/ld.(*Link).loadlib(0xc00014c000)
	/home/jixiangdong/util/go1.15/src/cmd/link/internal/ld/lib.go:550 +0x3d0

step 5:
turns out the module name is the direct reason, if changing it to something like example.com/mymodule then the compiler simply errors out:

         package mymodule is not in GOROOT (/home/jixiangdong/util/go1.15/src/mymodule)

What did you expect to see?

no crash, and meaningful error message

What did you see instead?

various crashes.

[shawndx]

Update:
I could produce the same crash as that of 'step 3' with a more complicated case even if the module has a hostname as its prefix.

[cagedmantis]

/cc @bcmills @jayconrod @matloob

[bcmills]

The attached go env output appears to be truncated, and I'm a bit confused at why the path in the panic traceback indicates go1.15.

@shawn-xdji, can you confirm the version of the go command you are using, and how to installed it (i.e. built from source, from an release downloaded from https://golang.org/dl/, or from some other package manager)?

[shawndx]

@bcmills sorry for the confusion, /home/jixiangdong/util/go1.15 is built from source with the latest commit (d75b7f9 when I filed the issue), I named it go1.15 since it's synced after 1.15 was released, should be something like go.latest.

[seankhliao]

Obsoleted by #47788