cmd/go: install -linkshared invokes link commands for existing .so files since go1.10

[yunabe]

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

go version go1.10 linux/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="/home/yunabe/.cache/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/yunabe/local/gocode"
GORACE=""
GOROOT="/home/yunabe/go1.10"
GOTMPDIR=""
GOTOOLDIR="/home/yunabe/go1.10/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build331277736=/tmp/go-build -gno-record-gcc-switches"

What did you do?

cd $TMPDIR
go get -d golang.org/x/net/context
go install -buildmode=shared -linkshared -pkgdir `pwd`/tmppkg -linkshared std
go install -x -buildmode=shared -pkgdir `pwd`/tmppkg -linkshared golang.org/x/net/context

What did you expect to see?

The second go install command compiles context.a and link libgolang.org-x-net-context.so. It takes less than 1 sec to finish.

What did you see instead?

From go1.10, the second go install run link command to create libstd.so though libstd.so already exists in -pkgdir.
It makes the second go install command 3-4 times slower (0.77sec in go1.9.4 vs 2.8sec in go1.10)

I debugged the behavior of go tools and it seems like b.useCache in linkShared returns false to libstd.so target (https://github.com/golang/go/blob/master/src/cmd/go/internal/work/exec.go#L1032).

[FiloSottile]

@gopherbot please open a backport tracking issue, as this is a 1.10 regression, although just a performance one.

[gopherbot]

Backport issue(s) opened: #25044 (for 1.10).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://golang.org/wiki/MinorReleases.