cmd/go: go get creates go.mod and go.sum files inconsistently, depending on directory

[douglarek]

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

~ $ go version
go version go1.12rc1 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

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/lingchax/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/lingchax/.go"
GOPROXY="https://athens.azurefd.net/"
GORACE=""
GOROOT="/Users/lingchax/sdk/go1.12rc1"
GOTMPDIR=""
GOTOOLDIR="/Users/lingchax/sdk/go1.12rc1/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/dev/null"
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/pq/lsz6sm8956vfcgd42rk4rfn00000gn/T/go-build922124980=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

$ export GO111MODULE=on
$ go get -x github.com/saibing/bingo

What did you expect to see?

github.com/saibing/bingo should be installed as bingo to $GOPATH/bin/ without creating go.mod and go.sum.

What did you see instead?

When I run it in a git repo, it will create go.mod and go.sum:

~/.g/s/g/s/gocode (master|✔) $ go get -x github.com/saibing/bingo
go: creating new go.mod: module github.com/stamblerre/gocode
WORK=/var/folders/pq/lsz6sm8956vfcgd42rk4rfn00000gn/T/go-build121973882

but when I run it in a non-git directory:

~ $ go get -x github.com/saibing/bingo
WORK=/var/folders/pq/lsz6sm8956vfcgd42rk4rfn00000gn/T/go-build044041777

It is very confusing.

[katiehockman]

I think this is expected behavior for the go command running in modules mode. The go command documentation for module downloading and verification describes this - namely that it creates a go.mod and go.sum in the main module's root directory, and will be updated/created when a new dependency is installed. I don't think it has anything to do with being in a git repository specifically, but being in a directory that contains go code.

This also may be a more appropriate question to post at https://golang.org/wiki/Questions.

/cc @bcmills to correct me or add extra context.

[bcmills]

Dup of #29433, for which I've just submitted a (partial) fix.

(The remaining work for that is to make go mod init choose the correct module path for repos with v2+ tags.)