call go env GOPATH if env var is not found (Refs #60)

common/os.go

@@ -6,6 +6,7 @@ import (
 	"io"
 	"io/ioutil"
 	"os"
+	"os/exec"
 	"os/signal"
 	"path/filepath"
 	"strings"
@@ -13,9 +14,22 @@ import (
 )
 
 var (
-	GoPath = os.Getenv("GOPATH")
+	GoPath = gopath()
 )
 
+func gopath() string {
+	path := os.Getenv("GOPATH")
+	if len(path) == 0 {
+		goCmd := exec.Command("go", "env", "GOPATH")
+		out, err := goCmd.Output()
+		if err != nil {
+			panic(fmt.Sprintf("failed to determine gopath: %v", err))
+		}
+		path = string(out)
+	}
+	return path
+}
+
 func TrapSignal(cb func()) {
 	c := make(chan os.Signal, 1)
 	signal.Notify(c, os.Interrupt, syscall.SIGTERM)

common/os_test.go

@@ -27,3 +27,26 @@ func TestWriteFileAtomic(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestGoPath(t *testing.T) {
+	// restore original gopath upon exit
+	path := os.Getenv("GOPATH")
+	defer func() {
+		_ = os.Setenv("GOPATH", path)
+	}()
+
+	err := os.Setenv("GOPATH", "~/testgopath")
+	if err != nil {
+		t.Fatal(err)
+	}
+	path = gopath()
+	if path != "~/testgopath" {
+		t.Fatalf("gopath should return GOPATH env var if set, 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)
+	}
+}