cmd/go: don't accept a relative path as GOBIN

Fixes #12907.

Change-Id: I5925852fe6962d4ec7dbb3ea5323e8ddfaf9d576
Reviewed-on: https://go-review.googlesource.com/15755Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/alldocs.go

@@ -886,7 +886,7 @@ DIR/bin/quux, not DIR/bin/foo/quux.  The "foo/" prefix is stripped
 so that you can add DIR/bin to your PATH to get at the
 installed commands.  If the GOBIN environment variable is
 set, commands are installed to the directory it names instead
-of DIR/bin.
+of DIR/bin. GOBIN must be an absolute path.
 
 Here's an example directory layout:
 

src/cmd/go/build.go

@@ -501,6 +501,10 @@ func libname(args []string) string {
 }
 
 func runInstall(cmd *Command, args []string) {
+	if gobin != "" && !filepath.IsAbs(gobin) {
+		fatalf("cannot install, GOBIN must be an absolute path")
+	}
+
 	raceInit()
 	buildModeInit()
 	pkgs := pkgsFilter(packagesForBuild(args))

src/cmd/go/go_test.go

@@ -1145,6 +1145,15 @@ func TestInstallFailsWithNoBuildableFiles(t *testing.T) {
 	tg.grepStderr("no buildable Go source files", "go install cgotest did not report 'no buildable Go Source files'")
 }
 
+func TestRelativeGOBINFail(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.tempFile("triv.go", `package main; func main() {}`)
+	tg.setenv("GOBIN", ".")
+	tg.runFail("install")
+	tg.grepStderr("cannot install, GOBIN must be an absolute path", "go install must fail if $GOBIN is a relative path")
+}
+
 // Test that without $GOBIN set, binaries get installed
 // into the GOPATH bin directory.
 func TestInstallIntoGOPATH(t *testing.T) {

src/cmd/go/help.go

@@ -307,7 +307,7 @@ DIR/bin/quux, not DIR/bin/foo/quux.  The "foo/" prefix is stripped
 so that you can add DIR/bin to your PATH to get at the
 installed commands.  If the GOBIN environment variable is
 set, commands are installed to the directory it names instead
-of DIR/bin.
+of DIR/bin. GOBIN must be an absolute path.
 
 Here's an example directory layout: