cmd/go: fix install target name for versioned binaries

For a package in the module root, using the containing directory name
might mean the directory in the module cache, in which case the
executable has a final @v1.2.3 in it, which is no good. Fix that.

While we're here, change go install example.com/cmd/foo/v2 to
install foo instead of the less useful "v2".

Fixes #24667.
Fixes #26869.

Change-Id: Ie40ca1bc9e27955441f1cdb7abd3a1f69034c9f5
Reviewed-on: https://go-review.googlesource.com/128900Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/load/pkg.go

@@ -1241,6 +1241,37 @@ func (p *Package) load(stk *ImportStack, bp *build.Package, err error) {
 			return
 		}
 		_, elem := filepath.Split(p.Dir)
+		if cfg.ModulesEnabled {
+			// NOTE(rsc): Using p.ImportPath instead of p.Dir
+			// makes sure we install a package in the root of a
+			// cached module directory as that package name
+			// not name@v1.2.3.
+			// Using p.ImportPath instead of p.Dir
+			// is probably correct all the time,
+			// even for non-module-enabled code,
+			// but I'm not brave enough to change the
+			// non-module behavior this late in the
+			// release cycle. Maybe for Go 1.12.
+			// See golang.org/issue/26869.
+			_, elem = pathpkg.Split(p.ImportPath)
+
+			// If this is example.com/mycmd/v2, it's more useful to install it as mycmd than as v2.
+			// See golang.org/issue/24667.
+			isVersion := func(v string) bool {
+				if len(v) < 2 || v[0] != 'v' || v[1] < '1' || '9' < v[1] {
+					return false
+				}
+				for i := 2; i < len(v); i++ {
+					if c := v[i]; c < '0' || '9' < c {
+						return false
+					}
+				}
+				return true
+			}
+			if isVersion(elem) {
+				_, elem = pathpkg.Split(pathpkg.Dir(p.ImportPath))
+			}
+		}
 		full := cfg.BuildContext.GOOS + "_" + cfg.BuildContext.GOARCH + "/" + elem
 		if cfg.BuildContext.GOOS != base.ToolGOOS || cfg.BuildContext.GOARCH != base.ToolGOARCH {
 			// Install cross-compiled binaries to subdirectories of bin.

src/cmd/go/testdata/mod/rsc.io_fortune_v1.0.0.txt

+rsc.io/fortune v1.0.0
+written by hand
+
+-- .mod --
+module rsc.io/fortune
+-- .info --
+{"Version":"v1.0.0"}
+-- fortune.go --
+package main
+
+import "rsc.io/quote"
+
+func main() {
+	println(quote.Hello())
+}

src/cmd/go/testdata/mod/rsc.io_fortune_v2_v2.0.0.txt

+rsc.io/fortune v2.0.0
+written by hand
+
+-- .mod --
+module rsc.io/fortune/v2
+-- .info --
+{"Version":"v2.0.0"}
+-- fortune.go --
+package main
+
+import "rsc.io/quote"
+
+func main() {
+	println(quote.Hello())
+}

src/cmd/go/testdata/script/mod_install.txt

-env GO111MODULE=on
-
-go mod init example.com/m
-
-# get of a binary should install it to $GOPATH/bin
-# BUG: vgo-tour should be installed as vgo-tour, not vgo-tour@v1.0.0.
-go get research.swtch.com/vgo-tour
-exec $GOPATH/bin/vgo-tour@v1.0.0
-stdout 'Hello, world.'
-rm $GOPATH/bin/vgo-tour@v1.0.0
-
-# install of a binary should install it to $GOPATH/bin
-# BUG: vgo-tour should be installed as vgo-tour, not vgo-tour@v1.0.0.
-go install research.swtch.com/vgo-tour
-exec $GOPATH/bin/vgo-tour@v1.0.0
-stdout 'Hello, world.'

src/cmd/go/testdata/script/mod_install_versioned.txt

+env GO111MODULE=on
+
+go list -f '{{.Target}}' rsc.io/fortune
+! stdout fortune@v1
+stdout 'fortune(\.exe)?$'
+
+go list -f '{{.Target}}' rsc.io/fortune/v2
+! stdout v2
+stdout 'fortune(\.exe)?$'
+
+-- go.mod --
+module m