cmd/go: Building a shared library in modules mode produces empty library

[misha-ridge]

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

go version go1.14 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

GO111MODULE=""
GOARCH="amd64"
GOBIN="/Users/dottedmag/go/bin"
GOCACHE="/Users/dottedmag/Library/Caches/go-build"
GOENV="/Users/dottedmag/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GONOPROXY="github.com/ridge"
GONOSUMDB="github.com/ridge"
GOOS="darwin"
GOPATH="/Users/dottedmag/go"
GOPRIVATE="github.com/ridge"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/dottedmag/tectonic/_deps.darwin-amd64/go-1.14"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/dottedmag/tectonic/_deps.darwin-amd64/go-1.14/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/bw/6hyp_vyj68v973qbhlbcvlcr0000gn/T/go-build983884458=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Tried to build a shared library and a binary that links to that shared library in modules mode. A repository with reproducer: https://github.com/misha-ridge/splitlib

Packages:

• github.com/misha-ridge/splitlib/a (imports ./c)
• github.com/misha-ridge/splitlib/b (imports ./c)
• github.com/misha-ridge/splitlib/c
• github.com/misha-ridge/splitlib (imports ./a, ./b, ./c)

./a and ./c ought to end up in a shared library, ./b and splitlib in binary.

Build:

$ go install -buildmode=shared std
$ go install -buildmode=shared -linkshared  -gcflags='-N -l' ./a
$ go build -o main -linkshared  -gcflags='-N -l' .

What did you expect to see?

1. libgithub.com-misha-ridge-splitlib-a.so with code from ./a and ./c packages, dynamically linking to std.
2. splitlib binary with code from ./b and splitlib, dynamically linking to std and libgithub.com-misha-ridge-splitlib-a.so.

What did you see instead?

1. libgithub.com-misha-ridge-splitlib-a.so is empty:

$ go tool nm libgithub.com-misha-ridge-splitlib-a.so  | grep splitlib
  104cd0 D go.link.abihash.libgithub.com-misha-ridge-splitlib-a.so

2. splitlib contains code from ./a, ./b, ./c and splitlib and dynamically links only to std:

$ go tool nm main  | grep splitlib
    2180 T github.com/misha-ridge/splitlib/a.SetC
    2190 T github.com/misha-ridge/splitlib/b.SetC
    2170 T github.com/misha-ridge/splitlib/c.Get
    5210 D github.com/misha-ridge/splitlib/c.i
    2180 t local.github.com/misha-ridge/splitlib/a.SetC
    2190 t local.github.com/misha-ridge/splitlib/b.SetC
    2170 t local.github.com/misha-ridge/splitlib/c.Get

Additional info

Trying to build the same using Go 1.13 triggers #35759.

[toothrot]

/cc @bcmills @jayconrod

[bcmills]

CC @matloob

[seankhliao]

Obsoleted by #47788