cmd/go: executables are sometimes named after their module's last element, not directory

[spenczar]

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

go1.11

Does this issue reproduce with the latest release?

yes

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

darwin amd64

What did you do?

In a package main's directory (specifically, github.com/twitchtv/twirp/protoc-gen-twirp), I had a go.mod like this:

module github.com/twitchtv/twirp/protoc-gen-twirp/v5

I then ran GO111MODULE=on go build . from within $GOPATH/src/github.com/twitchtv/twirp/protoc-gen-twirp.

I expected an executable named protoc-gen-twirp, because that is the last element of the directory. This is the documented behavior in https://tip.golang.org/cmd/go/#hdr-Compile_packages_and_dependencies:

When compiling a single main package, build writes the resulting executable to an output file named after the first source file ('go build ed.go rx.go' writes 'ed' or 'ed.exe') or the source code directory ('go build unix/sam' writes 'sam' or 'sam.exe').

Instead, an executable was produced named v5.

However, if I invoke go build with a full path, I get the expected name. GO111MODULE=on go build github.com/twitchtv/twirp/protoc-gen-twirp produces an executable named protoc-gen-twirp.

This is either a documentation error, or a bug.

[mvdan]

cc @bcmills @rsc @myitcv

[myitcv]

Confirmed.

export GOPATH=$(mktemp -d)
cd $(mktemp -d)
mkdir hello
cd hello
go mod init example.com/hello/v6
cat <<EOD > main.go

package main

func main() {}
EOD
go build .

Now:

$ ls
go.mod  main.go  v6

But:

$ go install
$ ls $GOPATH/bin
hello

go install was fixed in #24667; sounds like a similar fix needed for go build.

[gopherbot]

Change https://golang.org/cl/140863 mentions this issue: cmd/go: fix the default build output name for versioned binaries

[FiloSottile]

If this does not land before RC1 (in the next couple of days), open a backport issue if it should be cherry-picked into 1.12.

/cc @hyangah