cmd/go: in module root, go list {module-root}/... behaves differently than go list ./..., yet go mod tidy removes go.sum entry needed for former

[dmitshur]

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

$ go version
go version go1.18rc1 darwin/arm64

Does this issue reproduce with the latest release?

It reproduces with go1.18rc1.

Not really viable to test with Go 1.17.7 because it happens in a module whose go.mod file has go 1.18, so go1.17.7 mod tidy refuses to run on it:

$ go1.17.7 mod tidy
go mod tidy: go.mod file indicates go 1.18, but maximum supported version is 1.17

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

go env Output

$ go env
GO111MODULE=""
GOARCH="arm64"
GOBIN=""
GOCACHE="/Users/dmitri/Library/Caches/go-build"
GOENV="/Users/dmitri/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/dmitri/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/dmitri/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_arm64"
GOVCS=""
GOVERSION="go1.18rc1"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/_0/h0671fcn4rgb5pn9c745dx2h0000gn/T/go-build1161277448=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I ran go mod tidy inside a module that declares go 1.18. It removed one line from go.sum. Then I tried running go list {module-root}/... and go list ./... while being at the module root directory, without using the new workspace mode (that is, go env GOWORK output was blank).

(See the bottom of this issue for a sequence of exact commands that reproduce the issue.)

What did you expect to see?

I expected both go list {module-root}/... and go list ./... to work, and produce identical output, given the current working directory is the module root.

(I also expected that the amount of work needed to process these two relative and full import path patterns would be approximately the same. If that's not the case, I'm interested in learning what makes them different.)

What did you see instead?

Running go mod tidy causes the following line to be removed from go.sum:

golang.org/x/exp v0.0.0-20190731235908-ec7cb31e5a56 h1:estk1glOnSVeJ9tdEZZc5mAMDZk5lNJNyJ6DvrBkTEU=

Then, when I ran go list {module-root}/..., it failed:

$ go list golang.org/x/exp/shiny/...
pattern golang.org/x/exp/shiny/...: golang.org/x/exp@v0.0.0-20190731235908-ec7cb31e5a56: missing go.sum entry

Notably, it's saying the go.sum entry from golang.org/x/exp@v0.0.0-20190731235908-ec7cb31e5a56 is missing, the very same one that go mod tidy removed earlier.

go list ./... works well.

$ go list ./...
golang.org/x/exp/shiny/driver
golang.org/x/exp/shiny/driver/gldriver
[...]
golang.org/x/exp/shiny/widget/node
golang.org/x/exp/shiny/widget/theme
$ go list ./... | shasum
19cedcfaf7f294abf33a6ef54a81560517aa8a8d  -

If I manually add the missing go.sum entry back, then the longer go list invocation produces an identical result:

$ go list golang.org/x/exp/shiny/... | shasum
19cedcfaf7f294abf33a6ef54a81560517aa8a8d  -

Sequence of commands to reproduce

export GOPATH=$(mktemp -d)
cd $(mktemp -d)
git clone https://go.googlesource.com/exp && cd exp
git checkout 6cf2b201936e0673ace5b31d2dffa00dafb3ee58
cd shiny
go mod tidy
go list ./...
go list golang.org/x/exp/shiny/...
echo $?

CC @bcmills, @matloob.

[dmitshur]

Note, this issue was discovered in CL 387014 because the Go build system currently runs go test {module-root}/... invocations in each module root, rather than the shorter (and I expect more commonly used by users) go test ./... invocation. (CC @nigeltao.)

I'm not sure which format is better for TryBots to use (I thought they'd be pretty much interchangeable), and it's not hard for us to change it if desired. But that should be a separate issue.

[dmitshur]

Also worth noting a difference in error text between when go list is given an import path pattern vs the import path corresponding to the module root (which happens not to have a Go package in it):

$ go list golang.org/x/exp/shiny/...
pattern golang.org/x/exp/shiny/...: golang.org/x/exp@v0.0.0-20190731235908-ec7cb31e5a56: missing go.sum entry

$ go list golang.org/x/exp/shiny    
missing go.sum entry for module providing package golang.org/x/exp/shiny; to add:
	go mod download golang.org/x/exp

Notice the helpful go mod download golang.org/x/exp suggestion is included only in the latter case. Running it re-adds the go.sum entry that tidy removed.

---

The Go 1.18 draft release notes mention:

The go mod tidy command now retains additional checksums in the go.sum file for modules whose source code is needed to verify that each imported package is provided by only one module in the build list.

Since x/exp/shiny only recently became a separate module (previously all of its packages were inside x/exp), and neither x/exp nor x/exp/shiny modules provide a package at import path golang.org/x/exp/shiny, this makes me think that's possibly related. Maybe the fix for that tidy issue (#47738) is not handling this edge case?

[bcmills]

go list ./... stops at module boundaries, whereas go list $(GOWORK=off go list -m)/... does not.
(#43733 proposes to change that, but that's still deep in the backlog and may or may not actually happen.)

Since x/exp/shiny only recently became a separate module (previously all of its packages were inside x/exp), and neither x/exp nor x/exp/shiny modules provide a package at import path golang.org/x/exp/shiny, this makes me think that's possibly related.

Indeed. The reason go list golang.org/x/exp/shiny/... fails is because the go command is trying to determine whether the required version of the x/exp module (which may be arbitrarily different from the version contained in the parent of the working directory) provides any package matching golang.org/x/exp/shiny/....

However, go mod tidy does not preserve the checksum for golang.org/x/exp because it isn't needed: all of the x/exp packages actually imported within x/exp/shiny/... are contained within the shiny module itself, and x/exp is only in the module graph of x/exp/shiny at all as an otherwise-irrelevant transitive dependency of x/mobile. Because x/exp isn't explicitly required and x/exp/shiny is at go 1.18, lazy module loading takes effect and the irrelevant dependency is not used at all during package loading, so its missing checksum doesn't cause any problems when building packages within the module.

So unfortunately, I'm not entirely sure about the right path forward here. It seems wasteful in the general case to add a checksum for the selected version of x/exp, since that module isn't actually relevant to the build and (especially if the repo is private) the checksum could potentially be fairly expensive to compute. On the other hand, it also seems wrong for go list to potentially under-report the set of packages matching the requested pattern. 🤔

[dmitshur]

go list ./... stops at module boundaries, whereas go list $(GOWORK=off go list -m)/... does not.

Ah, this distinction is very helpful to be aware of and certainly explains this, thanks.

Do you know if it is already documented somewhere? I tried looking at https://pkg.go.dev/cmd/go#hdr-Relative_import_paths (and the rest of that page) and https://go.dev/ref/mod, but couldn't spot it so far. (Perhaps documenting it is a part of future work, if so, that's not a problem.)

[bcmills]

I expected the difference to be mentioned in https://pkg.go.dev/cmd/go@master#hdr-Package_lists_and_patterns, but it doesn't appear to be there. 🤔

[nigeltao]

So is this all Working As Intended?

Note, this issue was discovered in CL 387014

Getting back to this CL, if "go mod tidy" is still a no-op in my x/exp/shiny directory, should I manually edit the go.sum file, in order to pass the trybots?