cmd/go: module update should drop unused indirect dependencies?

[mwf]

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

Latest go devel: go version devel +d278f09333 Thu Jul 19 05:40:37 2018 +0000 darwin/amd64

Does this issue reproduce with the latest release?

Yes

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

GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/ikorolev/Library/Caches/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/ikorolev/.gvm/pkgsets/go1.11beta1/global:/Users/ikorolev/dev/go"
GOPROXY=""
GORACE=""
GOROOT="/Users/ikorolev/.gvm/gos/go1.11beta1"
GOTMPDIR=""
GOTOOLDIR="/Users/ikorolev/.gvm/gos/go1.11beta1/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
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/_b/d1934m9s587_8t_6ngv3hnc00000gp/T/go-build301560759=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Sorry, can't give you a standalone reproduction - bug is produced only with downloading updated dependencies.

Consider the following flow:

A(v0.0.1) uses B(v0.0.1):

module github.com/mwf/vgo-modules/a

require github.com/mwf/vgo-modules/b v0.0.1

Then A(v0.1.0) drops the dependency.

Here we got a project main which uses A(v0.0.1) and we want to upgrade A to v0.1.0

cd `mktemp -d`
git clone https://github.com/mwf/vgo-modules .
echo "\n$(cat go.mod)"
echo "\n$(cat go.sum)"
go get github.com/mwf/vgo-modules/a@v0.1.0
echo "\n$(cat go.mod)"

We get an unexpected indirect dependency github.com/mwf/vgo-modules/b v0.0.1 // indirect in go.mod, it seems to be taken from go.sum as a side-effect.

What did you expect to see?

go.mod should contain updated a dependency

module github.com/mwf/vgo-modules

require github.com/mwf/vgo-modules/a v0.1.0

What did you see instead?

We get an unexpected indirect dependency from b in go.mod, it seems to be taken from go.sum as a side-effect.

module github.com/mwf/vgo-modules

require (
    github.com/mwf/vgo-modules/a v0.1.0
    github.com/mwf/vgo-modules/b v0.0.1 // indirect
)

[mwf]

Currently there is a workaround to run go mod -sync after go get, so maybe the issue is not so critical. But still looks buggy.

[bcmills]

This is working as intended. (It's a bit confusing, but arguably less confusing than the alternatives.)

In general, go get tries to make the smallest possible change to the build list in order to get the requested modules at the requested versions.

When you started out, you had a direct requirement on a and an indirect one (via a) on b. Upgrading a did not require any changes to b in the build list, so go get did not make them, but that means that the previously-implicit dependency on b must be made explicit in the build file.

In this specific case, we theoretically have enough information to prune out the dependency. However, in the general case that can get expensive: if the main module depends on some other package c that imports b, and c does not have a go.mod of its own, then we don't actually know whether we still need b unless we process all of the (transitive) imports for c. But you didn't tell us to change anything about c and we don't need to touch it otherwise, so if go get started scanning its imports we'd be doing a lot of extra work that you didn't ask for.

$ cd $(mktemp -d)
$ git clone https://github.com/mwf/vgo-modules .

$ go list -m all
github.com/mwf/vgo-modules
github.com/mwf/vgo-modules/a v0.0.1
github.com/mwf/vgo-modules/b v0.0.1

$ go mod -graph
github.com/mwf/vgo-modules github.com/mwf/vgo-modules/a@v0.0.1
github.com/mwf/vgo-modules github.com/mwf/vgo-modules/b@v0.0.1  # bug in mod -graph: #26489
github.com/mwf/vgo-modules/a@v0.0.1 github.com/mwf/vgo-modules/b@v0.0.1

$ go get github.com/mwf/vgo-modules/a@v0.1.0

$ go list -m all
github.com/mwf/vgo-modules
github.com/mwf/vgo-modules/a v0.1.0
github.com/mwf/vgo-modules/b v0.0.1

$ go mod -graph
github.com/mwf/vgo-modules github.com/mwf/vgo-modules/a@v0.1.0
github.com/mwf/vgo-modules github.com/mwf/vgo-modules/b@v0.0.1

[bcmills]

I'm going to close this for now, but we'll keep an eye out for similar issues. Thanks for the great repro steps!

[mwf]

Thanks for a great explanation, Brian!
The logic does make sense now when dealing with dependencies without a go.mod file.

Actually the common usage will be

go get module1
go get module2
...
go mod -sync

Which leads to the minimal case of upgrading a single dependency with go get module1 && go mod-sync.
It's really unobvious and a bit confusing why do you need to run 2 commands instead of a single one, but for a bunch of consequent updates it could be really faster than if go get would always traverse the full tree.

I checked go help module-get - this nuance is not covered even in a minimal way (that one may want to run go mod -sync after get).

Let's reopen the issue to update the docs?

And @bcmills, could you please take a look at #26048 (comment) about go install behaviour? Maybe I'm missing there some similar nuances, but I can't see them yet :)

[bcmills]

Let's reopen the issue to update the docs?

Done.

[rsc]

I don't think this is working as intended, fwiw. Maybe I'm wrong. Will return to this.

[rsc]

I think this is probably incorrect behavior but it's not a showstopper for Go 1.11 because 'go mod tidy' will correct it. Don't want to make changes now but we should investigate for Go 1.12.

[bcmills]

Contrast #29702.

[bcmills]

This still reproduces as of Go 1.14 RC1.
(It is somewhat related to #26902, which was fixed in 1.13, but the behavior here is “not removing transitive dependencies that were never actually relevant” rather than “upgrading dependencies that are not relevant”.)

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

$ git clone https://github.com/mwf/vgo-modules
…

$ cd vgo-modules/

vgo-modules$ cat go.mod
module github.com/mwf/vgo-modules

require github.com/mwf/vgo-modules/a v0.0.1

vgo-modules$ cat go.sum
github.com/mwf/vgo-modules/a v0.0.1 h1:BaK3DzgdA4LxVXd42qID7F9G+KFxQ4SL69hm+BNcjA0=
github.com/mwf/vgo-modules/a v0.0.1/go.mod h1:nK31MtN2feKpYgPBywpeTy/Om+1x3OxQHQAsyi95AQ0=
github.com/mwf/vgo-modules/b v0.0.1 h1:aEPPx6pEuQ1/Wu9SzeLkiYltlir2h1fDJmKRv1ckarY=
github.com/mwf/vgo-modules/b v0.0.1/go.mod h1:7kI8RJ5+IPwKWU1MrE/kTA9AdWBNzMmucdpAiWXATOs=

vgo-modules$ go get github.com/mwf/vgo-modules/a@v0.1.0
go: downloading github.com/mwf/vgo-modules/a v0.1.0

vgo-modules$ cat go.mod
module github.com/mwf/vgo-modules

go 1.14

require (
        github.com/mwf/vgo-modules/a v0.1.0
        github.com/mwf/vgo-modules/b v0.0.1 // indirect
)

vgo-modules$ cat go.sum
github.com/mwf/vgo-modules/a v0.0.1 h1:BaK3DzgdA4LxVXd42qID7F9G+KFxQ4SL69hm+BNcjA0=
github.com/mwf/vgo-modules/a v0.0.1/go.mod h1:nK31MtN2feKpYgPBywpeTy/Om+1x3OxQHQAsyi95AQ0=
github.com/mwf/vgo-modules/a v0.1.0 h1:QD+ijrXwAYk4CMTOqEAzcogJoF4zLfit2alZVmT80EM=
github.com/mwf/vgo-modules/a v0.1.0/go.mod h1:XGJvSKC62ygHgRNmDf4RqXyp0zngXIJMLc6BaSfyTfI=
github.com/mwf/vgo-modules/b v0.0.1 h1:aEPPx6pEuQ1/Wu9SzeLkiYltlir2h1fDJmKRv1ckarY=
github.com/mwf/vgo-modules/b v0.0.1/go.mod h1:7kI8RJ5+IPwKWU1MrE/kTA9AdWBNzMmucdpAiWXATOs=

vgo-modules$ go mod tidy

vgo-modules$ cat go.mod
module github.com/mwf/vgo-modules

go 1.14

require github.com/mwf/vgo-modules/a v0.1.0

vgo-modules$ cat go.sum
github.com/mwf/vgo-modules/a v0.1.0 h1:QD+ijrXwAYk4CMTOqEAzcogJoF4zLfit2alZVmT80EM=
github.com/mwf/vgo-modules/a v0.1.0/go.mod h1:XGJvSKC62ygHgRNmDf4RqXyp0zngXIJMLc6BaSfyTfI=