cmd/go: go clean <package> downloads modules

[hyangah]

#28680 is fixed and go clean --modcache command no longer donwloads modules before cleaning if no argument is given to the command.

Note however 'go clean' accepts packages parameter as an argument(which isn't relevant to -modcache but is still meaningful for other purposes of go clean). If they are given, go clean still downloads the modules and even updates the go.mod file.

$ go help clean
usage: go clean [clean flags] [build flags] [packages]
...
$ export GOPATH=$(mktemp -d)
$ go1.13beta1 clean -testcache -cache -modcache
$ go1.13beta1 mod init scratch
go: creating new go.mod: module scratch
$ go1.13beta1 clean  golang.org/x/text
go: finding golang.org/x/text v0.3.2
go: downloading golang.org/x/text v0.3.2
go: extracting golang.org/x/text v0.3.2
$ cat go.mod
module scratch

go 1.13

require golang.org/x/text v0.3.2 // indirect

And the go clean -modcache <package> behaves differently.

$ go1.13beta1 clean -modcache golang.org/x/text
$ go1.13beta1 clean -modcache golang.org/x/text
go: downloading golang.org/x/text v0.3.2
go: extracting golang.org/x/text v0.3.2
go: finding golang.org/x/text v0.3.2

[bcmills]

Probably the best we could do here is to detect that the argument to clean is not in the main module and error out.

Per https://tip.golang.org/cmd/go/#hdr-Remove_object_files_and_cached_files:

If a package argument is given or the -i or -r flag is set, clean removes the following files from each of the source directories corresponding to the import paths: […]

But of course we cannot remove files from the (read-only) module cache.

CC @jayconrod

[hitzhangjie]

I have following project:

• greeter
  |- go.mod: defines module a.b.c
  |- *.go

after I run go build it generates a.b.c, after I run go test it generates a.b.c.test.

But when I run go clean, a.b.c and a.b.c.test not removed, while in go 1.12.6 it works fine.

[bcmills]

@hitzhangjie, that seems mostly unrelated to the issue reported here. If you are seeing something you believe to be a regression, please file a new issue with complete steps to reproduce it.