cmd/go: reject relative values for GOPATH

Fixes #4062.

R=rsc, dave, r
CC=golang-dev, nicksaika
https://golang.org/cl/6488129

src/cmd/go/main.go

@@ -127,6 +127,13 @@ func main() {
 	// which is not what most people want when they do it.
 	if gopath := os.Getenv("GOPATH"); gopath == runtime.GOROOT() {
 		fmt.Fprintf(os.Stderr, "warning: GOPATH set to GOROOT (%s) has no effect\n", gopath)
+	} else {
+		for _, p := range strings.Split(gopath, ":") {
+			if build.IsLocalImport(p) {
+				fmt.Fprintf(os.Stderr, "go: GOPATH entry is relative; must be absolute path: %q.\nRun 'go help gopath' for usage.\n", p)
+				os.Exit(2)
+			}
+		}
 	}
 
 	for _, cmd := range commands {

src/cmd/go/test.bash

@@ -125,6 +125,17 @@ elif ! test -x testdata/bin1/helloworld; then
 	ok=false
 fi
 
+# Reject relative paths in GOPATH.
+if GOPATH=. ./testgo build testdata/src/go-cmd-test/helloworld.go; then
+    echo 'GOPATH="." go build should have failed, did not'
+    ok=false
+fi
+
+if GOPATH=:$(pwd)/testdata:. ./testgo build go-cmd-test; then
+    echo 'GOPATH=":$(pwd)/testdata:." go build should have failed, did not'
+    ok=false
+fi
+
 if $ok; then
 	echo PASS
 else