go/build: Import does not respect FindOnly flag in module mode

[dmitshur]

Issue #26504 was about making it possible for go/build's Import and Context.Import to find packages in module mode.

The build.FindOnly import mode is documented as:

// If FindOnly is set, Import stops after locating the directory
// that should contain the sources for a package. It does not
// read any files in the directory.

For example, consider the following Go program:

package main

import (
	"fmt"
	"go/build"
	"log"
	"os"
)

func main() {
	wd, err := os.Getwd()
	if err != nil {
		log.Fatalln(err)
	}
	p, err := build.Import("golang.org/x/exp/shiny/example/goban", wd, build.FindOnly)
	fmt.Printf("dir=%q err=%v\n", p.Dir, err)
}

It works as expected in GOPATH mode:

$ cd $(mktemp -d)
$ cat >main.go <<EOF
package main

import (
    "fmt"
    "go/build"
    "log"
    "os"
)

func main() {
    wd, err := os.Getwd()
    if err != nil {
        log.Fatalln(err)
    }
    p, err := build.Import("golang.org/x/exp/shiny/example/goban", wd, build.FindOnly)
    fmt.Printf("dir=%q err=%v\n", p.Dir, err)
}
EOF

# Run in GOPATH mode.
$ GO111MODULE=off go run .
dir="/Users/gopher/go/src/golang.org/x/exp/shiny/example/goban" err=<nil>

But build.FindOnly has no effect in module mode:

# Run in module mode.
$ go mod init m
go: creating new go.mod: module m
$ GO111MODULE=on go run .
dir="" err=go/build: importGo golang.org/x/exp/shiny/example/goban: exit status 1
can't load package: package golang.org/x/exp/shiny/example/goban: build constraints exclude all Go files in /Users/gopher/go/pkg/mod/golang.org/x/exp@v0.0.0-20190419195159-b8972e603456/shiny/example/goban

[gopherbot]

Change https://golang.org/cl/218817 mentions this issue: go/build: populate partial package information in importGo