make GoPath a function

otherwise it could try to execute go binary and panic if no go binary
found. See https://github.com/tendermint/tendermint/issues/782

common/os.go

@@ -13,11 +13,15 @@ import (
 	"syscall"
 )
 
-var (
-	GoPath = gopath()
-)
+var gopath string
+
+// GoPath returns GOPATH env variable value. If it is not set, this function
+// will try to call `go env GOPATH` subcommand.
+func GoPath() string {
+	if gopath != "" {
+		return gopath
+	}
 
-func gopath() string {
 	path := os.Getenv("GOPATH")
 	if len(path) == 0 {
 		goCmd := exec.Command("go", "env", "GOPATH")
@@ -27,6 +31,7 @@ func gopath() string {
 		}
 		path = string(out)
 	}
+	gopath = path
 	return path
 }
 

common/os_test.go

@@ -39,14 +39,31 @@ func TestGoPath(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	path = gopath()
+	path = GoPath()
 	if path != "~/testgopath" {
-		t.Fatalf("gopath should return GOPATH env var if set, got %v", path)
+		t.Fatalf("should get GOPATH env var value, got %v", path)
 	}
 	os.Unsetenv("GOPATH")
 
-	path = gopath()
-	if path == "~/testgopath" || path == "" {
-		t.Fatalf("gopath should return go env GOPATH result if env var does not exist, got %v", path)
+	path = GoPath()
+	if path != "~/testgopath" {
+		t.Fatalf("subsequent calls should return the same value, got %v", path)
+	}
+}
+
+func TestGoPathWithoutEnvVar(t *testing.T) {
+	// restore original gopath upon exit
+	path := os.Getenv("GOPATH")
+	defer func() {
+		_ = os.Setenv("GOPATH", path)
+	}()
+
+	os.Unsetenv("GOPATH")
+	// reset cache
+	gopath = ""
+
+	path = GoPath()
+	if path == "" || path == "~/testgopath" {
+		t.Fatalf("should get nonempty result of calling go env GOPATH, got %v", path)
 	}
 }