cmd/go: go test cannot find module path in module root directory

[suzmue]

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

go version devel +ecec63c8a1 Tue Jul 31 16:34:38 2018 +0000 darwin/amd64

Does this issue reproduce with the latest release?

testing with tip (issue does not reproduce in go1.11beta2)

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

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/suzmue/Library/Caches/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/suzmue/go"
GOPROXY=""
GORACE=""
GOROOT="/Users/suzmue/tipgo/go"
GOTMPDIR=""
GOTOOLDIR="/Users/suzmue/tipgo/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/suzmue/myproject/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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/c4/982kvfgd4jz6_6sdlzzk2nj400fkl_/T/go-build226914824=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Files in directory a:
a/a.go:
package a
a/a_test.go:
package a; import "testing"; func Test(t *testing.T){}
a/go.mod:
module github.com/user/a

$ cd a/
$ go test

What did you expect to see?

PASS
ok  	github.com/user/a	0.017s

What did you see instead?

build github.com/user/a (test): cannot find module for path github.com/user/a (test)

Additional notes

go test works as expected in any subdirectories of a.

[matloob]

I can reproduce this on tip.

[suzmue]

cc @bcmills

[bcmills]

I suspect that this is the same underlying problem as #26602.

[mfridman]

@suzmue I can repro as well. I did notice ./... works, e.g.,

# go version devel +6b9c782f9f Wed Aug 1 00:57:00 2018 +0000 darwin/amd64

GO111MODULE=on go run main.go
build main: cannot find module for path main

GO111MODULE=on go run ./...
2018/08/01 11:14:49 failed decodeResp: Failed: Invalid Access Code / Password
exit status 1

Maybe the same is true for go test.

[rogpeppe]

I bisected - it seems that https://go-review.googlesource.com/125836 (commit b294fe9) introduced this problem.

[bcmills]

@rogpeppe, thanks for bisecting. In my experience debugging similar issues this week, these kinds of problems are very sensitive to the state of the module cache: did you clear the module and build caches at each commit?

(Just wondering how confident I should be in the bisection result.)

[rogpeppe]

@bcmills I just tried again with clean module and build caches and I got the same results. I might be missing something though.

[bcmills]

Nope, you've probably got it right. I suspect an off-by-one error (a trailing slash).

[rogpeppe]

That is: 68170aa succeeds consistently;
b294fe9 fails.

FWIW I've been using gopkg.in/macaroon.v2 for testing.
Reproduce by:

 cd /tmp
 git clone git@github.com:go-macaroon/macaroon
 cd macaroon
 go test

After downloading the deps, I get:

build gopkg.in/macaroon.v2 (test): cannot find module for path gopkg.in/macaroon.v2 (test)

[bcmills]

I added @suzmue's repro steps as a script test and it fails. (Thanks for the great minimal test case!)

I'll try macaroon too once it's fixed, just to be sure.