config/module: file paths require pwd

config/module/detect_file.go

@@ -16,6 +16,11 @@ func (d *FileDetector) Detect(src, pwd string) (string, bool, error) {
 	// Make sure we're using "/" even on Windows. URLs are "/"-based.
 	src = filepath.ToSlash(src)
 	if !filepath.IsAbs(src) {
+		if pwd == "" {
+			return "", true, fmt.Errorf(
+				"relative paths require a module with a pwd")
+		}
+
 		src = filepath.Join(pwd, src)
 	}
 

config/module/detect_file_test.go

@@ -30,3 +30,31 @@ func TestFileDetector(t *testing.T) {
 		}
 	}
 }
+
+func TestFileDetector_noPwd(t *testing.T) {
+	cases := []struct {
+		Input  string
+		Output string
+		Err    bool
+	}{
+		{"./foo", "", true},
+		{"foo", "", true},
+		{"/foo", "file:///foo", false},
+	}
+
+	pwd := ""
+	f := new(FileDetector)
+	for i, tc := range cases {
+		output, ok, err := f.Detect(tc.Input, pwd)
+		if (err != nil) != tc.Err {
+			t.Fatalf("%d: err: %s", i, err)
+		}
+		if !ok {
+			t.Fatal("not ok")
+		}
+
+		if output != tc.Output {
+			t.Fatalf("%d: bad: %#v", i, output)
+		}
+	}
+}