cmd/go: 'go mod why' should have an answer for every module in 'go list -m all'

[bcmills]

trafficstars

Currently, if you run go mod why -m on the output of every module in go list -m all, some modules may come back with the answer (main module does not need module […]), even after a go mod tidy.

The reason is that the module graph is conservative: go list -m all answers the question “which module versions are implied by the requirements of the main module?”, not “which modules does the main module need to download to complete go list all?”.¹

In contrast, go mod why explicitly ties its answer to go list all.

We should have some flag to go mod why to answer the related go list -m all question, “what path of requirements imposes the version requirement on module x?”

---

¹The command to answer the latter question, as it turns out, is:

go list -f '{{with .Module}}{{.Path}} {{.Version}}{{end}}' all | sort -u

[myitcv]

In contrast, go mod why explicitly ties its answer to go list all.

I don't think this is true, because go list all is subject to build constraints. The statement would be true if we were to say the union of go list all for all possible build constraints, or the hypothetical go list -nobuild all which would enable go list to list package irrespective of build constraints.

Consider github.com/myitcvscratch/depanalysis/cmd/analyzer. The main package whose import path coincides with the module path effectively sits behind a js,wasm build constraint. As does one of its dependencies.

This is therefore unsurprising:

$ go list
can't load package: package github.com/myitcvscratch/depanalysis/cmd/analyzer: build constraints exclude all Go files in /tmp/depanalysis/cmd/analyzer

If we provide the constraints we can list the main package:

$ GOOS=js GOARCH=wasm go list
github.com/myitcvscratch/depanalysis/cmd/analyzer

Similarly, unless we satisfy the constraints, go list all won't work either:

$ go list -f "{{if not .Standard}}{{.ImportPath}}{{end}}" all | sort -u
go: warning: "all" matched no packages

Again, fixed:

$ GOOS=js GOARCH=wasm go list -f "{{if not .Standard}}{{.ImportPath}}{{end}}" all | sort -u
github.com/myitcvscratch/depanalysis/cmd/analyzer
github.com/myitcvscratch/depanalysis/jsdep
github.com/myitcvscratch/depanalysis/normdep

go list -m is not subject to build constraints; it's operating at the module level, and therefore outside the build:

$ go list -m all
github.com/myitcvscratch/depanalysis/cmd/analyzer
github.com/myitcvscratch/depanalysis v0.0.1
github.com/myitcvscratch/depanalysis/distant v0.0.1

Hence it is not equivalent to a go list -f where we output the module path:

$ go list -f "{{if not .Standard}}{{.Module.Path}}{{end}}" all | sort -u
go: warning: "all" matched no packages

Because when listing packages we need to consider build constraints:

$ GOOS=js GOARCH=wasm go list -f "{{if not .Standard}}{{.Module.Path}}{{end}}" all | sort -u
github.com/myitcvscratch/depanalysis
github.com/myitcvscratch/depanalysis/cmd/analyzer

go list -m gives the nodes of the requirement graph output by go mod graph:

$ go mod graph
github.com/myitcvscratch/depanalysis/cmd/analyzer github.com/myitcvscratch/[email protected]
github.com/myitcvscratch/[email protected] github.com/myitcvscratch/depanalysis/[email protected]

The docs for go mod why say:

By default, why queries the graph of packages matched by "go list all", which includes tests for reachable packages.

But that does not appear to be the case. go mod why instead seems to correspond to the hypothetical go list -nobuild all mentioned above:

$ go mod why github.com/myitcvscratch/depanalysis/jsdep
# github.com/myitcvscratch/depanalysis/jsdep
github.com/myitcvscratch/depanalysis/cmd/analyzer
github.com/myitcvscratch/depanalysis/jsdep

$ go mod why github.com/myitcvscratch/depanalysis/normdep
# github.com/myitcvscratch/depanalysis/normdep
github.com/myitcvscratch/depanalysis/cmd/analyzer
github.com/myitcvscratch/depanalysis/normdep

$ go mod why github.com/myitcvscratch/depanalysis/unused
# github.com/myitcvscratch/depanalysis/unused
(main module does not need package github.com/myitcvscratch/depanalysis/unused)

$ go mod why image/jpeg
# image/jpeg
github.com/myitcvscratch/depanalysis/cmd/analyzer
fmt
reflect
reflect.test
encoding/json
encoding/json.test
image
image.test
image/jpeg

$ go mod why -m github.com/myitcvscratch/depanalysis
# github.com/myitcvscratch/depanalysis
github.com/myitcvscratch/depanalysis/cmd/analyzer
github.com/myitcvscratch/depanalysis/jsdep

$ go mod why -m github.com/myitcvscratch/depanalysis/distant
# github.com/myitcvscratch/depanalysis/distant
(main module does not need module github.com/myitcvscratch/depanalysis/distant)

As you say, we need the ability to answer the question "why does github.com/myitcvscratch/depanalysis/distant appear in our module requirement graph?"

I'm less clear we need something like a -nobuild flag for go list. Instead, on the (rather dangerous) assumption my analysis correct, I think the docs for go mod why need a slight tweak.

[mwf]

Please, keep in mind the case with test dependencies - would be cool if go mod why could tell if the dependency is used transitively in tests - #30206

[aofei]

And I think the note (main module does not need module ...) is flawed. If a module isn't needed by our main module, then it shouldn't cause our main module to be broken (see #36423 for example).

[gopherbot]

Change https://golang.org/cl/220080 mentions this issue: design/36460: add design for lazy module loading

[stapelberg]

“which modules does the main module need to download to complete go list all?” … ¹The command to answer the latter question, as it turns out, is:

go list -f '{{with .Module}}{{.Path}} {{.Version}}{{end}}' all | sort -u

I don’t think that’s correct. When I run this command in ~/distri, and ensure that the reported modules are present in the reported versions in my build environment, I’m encountering an error when running go install ./cmd/...:

go: golang.org/x/[email protected] requires
	dmitri.shuralyov.com/gpu/[email protected]: module lookup disabled by GOPROXY=off

Package dmitri.shuralyov.com/gpu/mtl is only used from files behind the darwin build tag (I’m working with linux), so that’s likely why go list all doesn’t show it, but it seems like go install still requires it to be present. https://github.com/golang/go/issues/29410#issuecomment-515203754 is related.

[OJFord]

I assume this is the same issue - I was trying to update to fix a dependabot issue; but I was still left with the unpatched major version I was hoping to remove. go mod why on it said it's not needed, so I thought ok it's just for some reason left in go.sum; but if I manually remove it then go get -u puts it back.

Is there any (not intensively manual) workaround to find which main package dependency leads to it if why won't tell me?

[mvdan]

@OJFord you want go mod why -m, as go mod why works on packages. If go mod why -m still tells you that a module is not needed even when it's in your go list -m all, then it is this bug.

[OJFord]

If go mod why -m still tells you that a module is not needed even when it's in your go list -m all, then it is this bug.

Yes, that is the case. Thanks.

[LeGEC]

@OJFord you want go mod why -m, as go mod why works on packages. If go mod why -m still tells you that a module is not needed even when it's in your go list -m all, then it is this bug.

@mvdan : I think I have reached that situation, which you describe as a bug :

I was looking into a way to cut the dependency on go.opencensus.io from googleapis/google-api-go-client.

My current attempt is here : https://github.com/LeGEC/google-api-go-client/ (at commit 3005284e02)

On my laptop, I am currently in the following state :

$ go version
go version go1.18.1 linux/amd64
$ go mod tidy
$ grep -l -e go.opencensus.io -r .   # the only files where 'go.opencensus.io' is mentioned are go.sum files
./internal/kokoro/discogen/go.sum
./go.sum

# go.opencensus.io is listed in 'go list -m all'
$ go list -m all | grep opencensus
github.com/census-instrumentation/opencensus-proto v0.2.1
go.opencensus.io v0.23.0

# both go mod why and go mod why -m say that package is not needed :
$ go mod why go.opencensus.io
# go.opencensus.io
(main module does not need package go.opencensus.io)
$ go mod why -m go.opencensus.io
# go.opencensus.io
(main module does not need module go.opencensus.io)

I'm trying to clear the various caches I am aware of (GOCACHE and GOMODCACHE), I still have this package mentioned in go.sum and go list -m all, but not used by the code.

[bcmills]

@LeGEC, I'm happy to help with that problem but it's a bit off-topic for this issue.

Consider starting a thread on golang-nuts, or find me in the #modules channel on the Gophers Slack.

[jan--f]

@LeGEC, I'm happy to help with that problem but it's a bit off-topic for this issue.

Consider starting a thread on golang-nuts, or find me in the #modules channel on the Gophers Slack.

@bcmills any chance you can link a follow-up here is there was one? I'm in a similar situation as described in https://github.com/golang/go/issues/27900#issuecomment-1169637103 where a CVE is involved and its unclear if our package is impacted. The vulnerable module is mentioned in go.sum and go list -m all, but go mod why -m x claims (main module does not need module x)

[bcmills]

@jan--f, if the CVE in question is included in https://pkg.go.dev/vuln/, I would suggest using govulncheck to investigate its impact on your module.

[jan--f]

It's not yet merged unfortunately, though proposed. In any case I experience the same situation @LeGEC describes, so I'd be thankful for pointer to follow up on any threads of slack conversations.

[tonglil]

@OJFord you want go mod why -m, as go mod why works on packages. If go mod why -m still tells you that a module is not needed even when it's in your go list -m all, then it is this bug.

This is the case.

[oshoval]

If go mod why -m still tells you that a module is not needed even when it's in your go list -m all, then it is this bug.

This is the case.

Same here, can give a reproducible env for it if needed even after go clean -modcache

[OJFord]

Can we quote the rest of that paragraph without pinging me going forwards please 😅

[vikblom]

Hello, I've bumped into this here and there and think I've reproduced what is causing some confusion for me: https://github.com/vikblom/go-issue-27900

There is a module dependency on goldmark, but since the package which needs it isn't used, it will be pruned from the package tree, but not the module tree, if that makes sense. But then go mod why -m has no explanation for it.

@mvdan Would this example line up with what you meant here?

If go mod why -m still tells you that a module is not needed even when it's in your go list -m all, then it is this bug.

I think it would be very useful to get an go mod why answer for every module in go.mod as well as go list -m all, perhaps they are equivalent.

[tonecool]

I ended up here because I was trying to find a way to identify why dependency is listed in my go.mod file. It is being identified by snyk.io security tool as an vulnerability, but I have no clue why it is listed in my go.mod. By deleting it manually, go mod tidy is bringing it back to life. Calling go mod why has no clue why it is there saying: main module does not need package . So why it is there if "main module does need it" ? It looks like go mod tidy is so much smarter than go mod why. Why we don't use its dependency tree calculation logic for go mod why as well?

[dmitris]

@tonecool try running go mod why -m <module> on the <module> dependency you are interested in, also go mod graph > /tmp/graph and then grep <module> /tmp/graph. Hope this helps! 😄

[tonecool]

Thanks @dmitris, adding -m as part of go mod why helps indeed.

[shantyk]

Still running into this problem in 2025. The origin of a module that is in the output of go list -m all cannot be found through go mod why or go mod graph outputs.

This issue has been open since 2018, is there a workaround/official advice? Should the output of go mod why be considered more accurate than go list -m all?