x/tools/go/packages: Load in NeedName|NeedFiles mode can produce incorrect results for certain modules

[dmitshur]

This issue is related to #29452, #31087, and #31893, but is more narrowly focused on the correctness of the packages.NeedName | packages.NeedFiles mode.

Background

Both go/packages and go/build are concerned with loading Go packages and returning information about them.

Go packages may be inside modules that are tidy and compile successfully, or they may be incomplete and fail to build in certain configurations. Loading information about a package should succeed independently from whether or not building said package (in a specific configuration) can be done without build errors.

Environment

$ go version
go version go1.12.5 darwin/amd64

Issue

Incorrect results are produced when using the go/packages package in packages.NeedName | packages.NeedFiles mode. Correct results are produced by the go/build package.

To reproduce the issue, create a target package to be imported in a temporary directory /tmp/target with the following files:

-- go.mod --
module m

go 1.12

require github.com/google/go-github v0.0.0-20181014183319-aa7423eb7970

-- main.go --
// +build ignore

package main

func main() {}

-- p.go --
package p

import "github.com/google/go-github/github"

var _ *github.Client

Then, inside another directory, create the following test command:

-- go.mod --
module m

go 1.12

require golang.org/x/tools v0.0.0-20190506145303-2d16b83fe98c

-- main.go --
package main

import (
	"fmt"
	"go/build"

	"golang.org/x/tools/go/packages"
)

func main() {
	for driver, f := range map[string]func(dir string) (name string, goFiles []string, _ error){
		"go/build":    useGoBuild,
		"go/packages": useGoPackages,
	} {
		name, goFiles, error := f("/tmp/target")

		fmt.Println(driver)
		fmt.Println("  error:", error)
		if error == nil {
			fmt.Println("  name:", name)
			fmt.Println("  goFiles:", goFiles)
		}
		fmt.Println()
	}
}

func useGoBuild(dir string) (name string, goFiles []string, _ error) {
	p, err := build.ImportDir(dir, 0)
	if err != nil {
		return "", nil, fmt.Errorf("build.ImportDir: %v", err)
	}
	return p.Name, p.GoFiles, nil
}

func useGoPackages(dir string) (name string, goFiles []string, _ error) {
	cfg := &packages.Config{
		Mode: packages.NeedName | packages.NeedFiles,
		Dir:  dir,
	}
	pkgs, err := packages.Load(cfg, ".")
	if err != nil {
		return "", nil, fmt.Errorf("packages.Load: %v", err)
	}
	if len(pkgs) != 1 {
		return "", nil, fmt.Errorf("got %d packages, want exactly 1 package", len(pkgs))
	}
	p := pkgs[0]
	if len(p.Errors) > 0 {
		return "", nil, fmt.Errorf("p.Errors: %v", p.Errors)
	}
	return p.Name, p.GoFiles, nil
}

Run it as follows:

$ export GO111MODULE=on
$ export GOPROXY=direct
$ export GOPATH=$(mktemp -d)
$ go build -o /tmp/loadpkg
$ /tmp/loadpkg
go/build
  error: <nil>
  name: p
  goFiles: [p.go]

go/packages
  error: packages.Load: go [list -e -json -compiled=false -test=false -export=false -deps=false -find=true -- .]: exit status 1: go: github.com/google/go-github@v0.0.0-20181014183319-aa7423eb7970: go.mod has post-v0 module path "github.com/google/go-github/v18" at revision aa7423eb7970
go: error loading module requirements

Notice that go/build produced correct results, while go/packages produced incorrect results (an error).

/cc @matloob per owners.

[matloob]

Unfortunately this is one of those cases where we're bound by what go list does. If go list can successfully produce a package, then go/packages will be able to also. But this would require us to pass extra information about what we want to go list.

In any case, as things are now, this is infeasible to fix.