go mod attempts to download packages when it detects a VCS, even if a replace directive would override the package

[jeffjsc]

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

go version go1.11.1 darwin/amd64

Does this issue reproduce with the latest release?

Yes

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

GOARCH="amd64"
GOBIN="/Users/jeffrey.johnson2/xxx/Dev/go/bin/"
GOCACHE="/Users/jeffrey.johnson2/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/jeffrey.johnson2/xxx/Dev/go/"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/Cellar/go/1.11.1/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.11.1/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/jeffrey.johnson2/xxx/Dev/go/src/vgotest.com/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/g_/zq5mxm_j0b77gqntfvptq3m80000gp/T/go-build324185808=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I tried creating a local override for a non-existent remote git repository, using go mod's replace directive. Go mod attempts to fetch the remote before the replace is applied, which does not work because the remote does not exist. Go mod decides that the package is a git repository because the package name matches the regex here:

go/src/cmd/go/internal/get/vcs.go

Line 1033 in f2131f6

re: `^(?P<root>(?P<repo>([a-z0-9.\-]+\.)+[a-z0-9.\-]+(:[0-9]+)?(/~?[A-Za-z0-9_.\-]+)+?)\.(?P<vcs>bzr|fossil|git|hg|svn))(/~?[A-Za-z0-9_.\-]+)*$`,

Here's a script to set up test input.

cd $GOPATH/src
mkdir modtest.com
cd modtest.com

# create program main
mkdir program
echo -e "package main\n\nimport \"example.com/baz.git/mypackage\"\n\nfunc main() {\n\tmypackage.Do()\n}" > program/main.go

# create library package in vendor directory
mkdir -p vendor/example.com/baz.git/mypackage/
echo -e "package mypackage\n\nimport \"fmt\"\n\nfunc Do() {\n\tfmt.Println(\"hello\")\n}" > vendor/example.com/baz.git/mypackage/code.go
echo "module example.com/baz.git" > vendor/example.com/baz.git/go.mod

# create mod file with replace
echo -e "module vgotest.com\nrequire example.com/baz.git v0.0.0\nreplace example.com/baz.git => ./vendor/example.com/baz.git" > go.mod

Now run:
vgo run program/main.go

The same issue is exercised by:
go mod install

What did you expect to see?

I expected no download attempt and to see this printed to the console:
hello

What did you see instead?

Go hangs as it attempts to fetch the non-existent remote example.com/baz.git repository using ssh.

Note that if you omit .git from the library path, this works. For example:

cd $GOPATH/src
mkdir modtest-works.com
cd modtest-works.com

# create program main
mkdir program
echo -e "package main\n\nimport \"example.com/baz/mypackage\"\n\nfunc main() {\n\tmypackage.Do()\n}" > program/main.go

# create library package in vendor directory
mkdir -p vendor/example.com/baz/mypackage/
echo -e "package mypackage\n\nimport \"fmt\"\n\nfunc Do() {\n\tfmt.Println(\"hello\")\n}" > vendor/example.com/baz/mypackage/code.go
echo "module example.com/baz" > vendor/example.com/baz/go.mod

# create mod file with replace
echo -e "module vgotest.com\nrequire example.com/baz v0.0.0\nreplace example.com/baz => ./vendor/example.com/baz" > go.mod

vgo run program/main.go

[myitcv]

Probably a dup of #27859 or #26241

[myitcv]

I'm going to mark this as a duplicate of #26241. There are aspects of #26134 and #25982 at play too, but again, those issues are covered elsewhere.