cmd/go: go clean -modcache should not download modules

[perillo]

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

$ go version
go version go1.14beta1 linux/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
GO111MODULE="on"
GOARCH="amd64"
GOBIN="/home/manlio/.local/bin"
GOCACHE="/home/manlio/.cache/go-build"
GOENV="/home/manlio/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GONOPROXY="github.com/perillo"
GONOSUMDB="github.com/perillo"
GOOS="linux"
GOPATH="/home/manlio/.local/lib/go:/home/manlio/src/go"
GOPRIVATE="github.com/perillo"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/manlio/sdk/go1.14beta1"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/manlio/sdk/go1.14beta1/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build740722259=/tmp/go-build -gno-record-gcc-switches"
GOROOT/bin/go version: go version go1.14beta1 linux/amd64
GOROOT/bin/go tool compile -V: compile version go1.14beta1
uname -sr: Linux 5.4.15-arch1-1
/usr/lib/libc.so.6: GNU C Library (GNU libc) stable release version 2.30.
gdb --version: GNU gdb (GDB) 8.3.1

What did you do?

From inside an empty module (a directory with a go.mod file but no .go source files:

go clean -modcache -n github.com/perillo/i3workon

What did you expect to see?

I was expecting to see only the commands used to remove the module cache.

What did you see instead?

go: downloading github.com/perillo/i3workon v0.3.0
go: downloading golang.org/x/tools v0.0.0-20200115044656-831fdb1e1868
cd /home/manlio/.local/lib/go/pkg/mod/github.com/perillo/[email protected]
rm -f [email protected] [email protected] [email protected] [email protected] main main.exe
rm -rf /home/manlio/.local/lib/go/pkg/mod

go clean is downloading the module, since it is not present in the cache, and then removing it.

There is also something strange with the commands go clean is using.
If, in the end, it is going to remove the entire $GOPATH/pkg/mod directory, what's the use of first trying to remove the specified package first?

I also don't really understand what

rm -f [email protected] [email protected] [email protected] [email protected] main main.exe

is trying to do.
These files do not exist.

[perillo]

After having seen the output of go clean -modcache -n -r github.com/perillo/iworkon, it seems that the go clean command thinks that its cleaning a newly built go distribution.

[perillo]

I think go clean -modcache <modpath> should only remove all the versions of <modpath> from the module cache, instead of cleaning the entire module cache.

[bcmills]

go clean is unfortunately pretty overloaded. (See https://tip.golang.org/cmd/go/#hdr-Remove_object_files_and_cached_files for the full details.)

#28835 proposes a -m flag to tell go clean to remove only the requested module, which is semantically different from the meaning of package arguments to go clean today.

CC @jayconrod @matloob

[bcmills]

#32976 is a more focused request for module arguments to go clean.

[bcmills]

Duplicate of #32977