cmd/go: go list should not add a dependency to go.mod

[glinton]

Possibly related to #36812
Re-open of #28014 which was not a duplicate of 27643

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

$ go version
go version go1.13.7 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=""
GOCACHE="/home/me/.cache/go-build"
GOENV="/home/me/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/opt/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/opt/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/tmp/listTest/go.mod"
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-build352932148=/tmp/go-build -gno-record-gcc-switches"

What did you do?

mkdir /tmp/listTest
cd /tmp/listTest

go mod init test
# go: creating new go.mod: module test

cat go.mod
# module test
# 
# go 1.13

go list github.com/golang/protobuf/proto
# github.com/golang/protobuf/proto

cat go.mod
# module test
# 
# go 1.13
# 
# require github.com/golang/protobuf v1.3.3 // indirect

go mod tidy

cat go.mod 
# module test
# 
# go 1.13

What did you expect to see?

I expected to see if the package argument to go list was present in my go environment. No file writes.

What did you see instead?

I saw that the package argument to go list was present, but I also saw an intrusive update to my go.mod.

[gopherbot]

Change https://golang.org/cl/219237 mentions this issue: cmd/go: eliminate empty '()' when passing -mod=readonly explicitly to 'go list'

[bcmills]

This is closely related to #28692 and #26977, and somewhat more distantly related to #29869.

But the use case here is a bit different from #28692: in that issue, the package mentioned on the command line is in a module already present in the module graph, and go build and go list are just resolving transitive dependencies of that package.

And this use case is also a bit different from #26977: there, go mod why is being asked why a specific package is already in the dependency graph, but here you're explicitly asking go list to tell you about a new package. (If you run go build or go test instead, then probably we should just build or test that package.)

Note that -mod=readonly or -mod=vendor would successfully prevent go list from adding the dependency:

test$ go version
go version devel +363bcd00 Wed Feb 12 18:22:50 2020 +0000 linux/amd64

test$ go mod init test
go: creating new go.mod: module test

test$ cat go.mod
module test

go 1.14

test$ go list -mod=readonly github.com/golang/protobuf/proto
can't load package: cannot find module providing package github.com/golang/protobuf/proto: import lookup disabled by -mod=readonly

test$ go list -mod=vendor github.com/golang/protobuf/proto
can't load package: package github.com/golang/protobuf/proto: cannot find package "." in:
        /tmp/tmp.TideYgX4n4/test/vendor/github.com/golang/protobuf/proto

test$ go list -mod=mod github.com/golang/protobuf/proto
go: finding module for package github.com/golang/protobuf/proto
go: downloading github.com/golang/protobuf v1.3.3
go: found github.com/golang/protobuf/proto in github.com/golang/protobuf v1.3.3
github.com/golang/protobuf/proto

Given that, this seems like a usability question — is it better for go list to resolve the packages that you ask it about, or to reject them if they aren't already in the module graph? But to answer that, we need to examine the context: what was your higher-level goal in running this command? (What led you to run go list for this particular package path without either -mod=readonly or -mod=vendor?)

CC @matloob @jayconrod

[gopherbot]

Timed out in state WaitingForInfo. Closing.

(I am just a bot, though. Please speak up if this is a mistake or you have the requested information.)