cmd/go: ignore empty path elements in GOPATH

go command refuses to use GOPATH with empty path elements
(like %GOPATH%=C:\go;). But environment variable change dialog
on Windows 10 produces strings ending with ; (see issue #21928
for a picture). Just accept GOPATH with empty path elements,
and ignore all empty path elements.

Fixes #21928

Change-Id: I1d3c3a19274ed69204d29ae06c3e8ff8c57c1ca0
Reviewed-on: https://go-review.googlesource.com/65151
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/go_test.go

@@ -1689,6 +1689,27 @@ func TestRejectRelativePathsInGOPATHCommandLinePackage(t *testing.T) {
 	tg.grepStderr("GOPATH entry is relative", "expected an error message rejecting relative GOPATH entries")
 }
 
+// Issue 21928.
+func TestRejectBlankPathsInGOPATH(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	sep := string(filepath.ListSeparator)
+	tg.setenv("GOPATH", " "+sep+filepath.Join(tg.pwd(), "testdata"))
+	tg.runFail("build", "go-cmd-test")
+	tg.grepStderr("GOPATH entry is relative", "expected an error message rejecting relative GOPATH entries")
+}
+
+// Issue 21928.
+func TestIgnoreEmptyPathsInGOPATH(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.creatingTemp("testdata/bin/go-cmd-test" + exeSuffix)
+	sep := string(filepath.ListSeparator)
+	tg.setenv("GOPATH", ""+sep+filepath.Join(tg.pwd(), "testdata"))
+	tg.run("install", "go-cmd-test")
+	tg.wantExecutable("testdata/bin/go-cmd-test"+exeSuffix, "go install go-cmd-test did not write to testdata/bin/go-cmd-test")
+}
+
 // Issue 4104.
 func TestGoTestWithPackageListedMultipleTimes(t *testing.T) {
 	tg := testgo(t)

src/cmd/go/main.go

@@ -89,6 +89,11 @@ func main() {
 		fmt.Fprintf(os.Stderr, "warning: GOPATH set to GOROOT (%s) has no effect\n", gopath)
 	} else {
 		for _, p := range filepath.SplitList(gopath) {
+			// Some GOPATHs have empty directory elements - ignore them.
+			// See issue 21928 for details.
+			if p == "" {
+				continue
+			}
 			// Note: using HasPrefix instead of Contains because a ~ can appear
 			// in the middle of directory elements, such as /tmp/git-1.8.2~rc3
 			// or C:\PROGRA~1. Only ~ as a path prefix has meaning to the shell.