cmd/go: go test takes extra time to run even for packages that do not have any test files

[cyli]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.7.1 darwin/amd64

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/cyli/Go"
GORACE=""
GOROOT="/usr/local/Cellar/go/1.7.1/libexec"
GOTOOLDIR="/usr/local/Cellar/go/1.7.1/libexec/pkg/tool/darwin_amd64"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/v9/wzpq8hm56b943960j1qrnblm0000gp/T/go-build685341822=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"

What did you do?

time go test $(go list ./... | grep -v vendor)
time go test $(go list -f '{{ if .TestGoFiles }}{{ .ImportPath }}{{ end }}' ./... | grep -v vendor)

What did you expect to see?

I expected they would take the same mostly the same amount of time

What did you see instead?

time go test $(go list -f '{{ if .TestGoFiles }}{{ .ImportPath }}{{ end }}' ./... | grep -v vendor) is reliably faster. I've also tried running time for i in {1..10}; do <command>; done and on average it is faster than not first excluding the packages without tests.

This is the case even if -parallel # is set, whether or not -race or if coverage numbers are generated. For instance, when running the tests for https://github.com/docker/swarmkit:

$  time go test -parallel 8 -race $(go list -f '{{ if .TestGoFiles }}{{ .ImportPath }}{{ end }}' ./... | grep -v vendor)
...
real    0m17.380s
user    0m51.668s
sys 0m7.717s

$  time go test -parallel 8 -race $(go list ./... | grep -v vendor)
...
real    0m27.161s
user    1m26.491s
sys 0m15.368s

$  time go test $(go list -f '{{ if .TestGoFiles }}{{ .ImportPath }}{{ end }}' ./... | grep -v vendor)
...
real    0m15.005s
user    0m41.354s
sys 0m6.796s

$  time go test $(go list  ./... | grep -v vendor)
...
real    0m26.743s
user    1m4.753s
sys 0m13.017s

In case time is excluding how long go list takes:

$  time (go list  ./... | grep -v vendor)
...
real    0m0.308s
user    0m0.239s
sys 0m0.136s

$  time (go list -f '{{ if .TestGoFiles }}{{ .ImportPath }}{{ end }}' ./... | grep -v vendor)
...
real    0m0.318s
user    0m0.248s
sys 0m0.141s

I'm not sure if this is the issue, but maybe go test can automatically exclude packages provided to it that don't have tests first, and not even start building the test binary for those?

[minux]

Working as intended. Because go test will report an error if the package doesn't built, even if the package doesn't have any test files. (This is to fix #7108).

[cyli]

Oh, that makes sense. Thanks for linking the other issue!