From 4e0574e7d4e3a8972a6ee41a537f82a8b20ce8e8 Mon Sep 17 00:00:00 2001
From: Christian Schwarz <me@cschwarz.com>
Date: Sun, 5 Apr 2020 20:12:32 +0200
Subject: [PATCH] envconst.Var

---
 util/envconst/envconst.go      | 31 +++++++++++++++++++++
 util/envconst/envconst_test.go | 50 ++++++++++++++++++++++++++++++++++
 2 files changed, 81 insertions(+)
 create mode 100644 util/envconst/envconst_test.go

diff --git a/util/envconst/envconst.go b/util/envconst/envconst.go
index 8b8e56f..8c965fb 100644
--- a/util/envconst/envconst.go
+++ b/util/envconst/envconst.go
@@ -1,7 +1,10 @@
 package envconst
 
 import (
+	"flag"
+	"fmt"
 	"os"
+	"reflect"
 	"strconv"
 	"sync"
 	"time"
@@ -85,3 +88,31 @@ func String(varname string, def string) string {
 	cache.Store(varname, e)
 	return e
 }
+
+func Var(varname string, def flag.Value) interface{} {
+
+	// use def's type to instantiate a new object of that same type
+	// and call flag.Value.Set() on it
+	defType := reflect.TypeOf(def)
+	if defType.Kind() != reflect.Ptr {
+		panic(fmt.Sprintf("envconst var must be a pointer, got %T", def))
+	}
+	defElemType := defType.Elem()
+
+	if v, ok := cache.Load(varname); ok {
+		return v.(string)
+	}
+	e := os.Getenv(varname)
+	if e == "" {
+		return def
+	}
+
+	newInstance := reflect.New(defElemType)
+	if err := newInstance.Interface().(flag.Value).Set(e); err != nil {
+		panic(err)
+	}
+
+	res := newInstance.Interface()
+	cache.Store(varname, res)
+	return res
+}
diff --git a/util/envconst/envconst_test.go b/util/envconst/envconst_test.go
new file mode 100644
index 0000000..bc4af67
--- /dev/null
+++ b/util/envconst/envconst_test.go
@@ -0,0 +1,50 @@
+package envconst_test
+
+import (
+	"fmt"
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/zrepl/zrepl/util/envconst"
+)
+
+type ExampleVarType struct{ string }
+
+var (
+	Var1 = ExampleVarType{"var1"}
+	Var2 = ExampleVarType{"var2"}
+)
+
+func (m ExampleVarType) String() string { return string(m.string) }
+func (m *ExampleVarType) Set(s string) error {
+	switch s {
+	case Var1.String():
+		*m = Var1
+	case Var2.String():
+		*m = Var2
+	default:
+		return fmt.Errorf("unknown var %q", s)
+	}
+	return nil
+}
+
+const EnvVarName = "ZREPL_ENVCONST_UNIT_TEST_VAR"
+
+func TestVar(t *testing.T) {
+	_, set := os.LookupEnv(EnvVarName)
+	require.False(t, set)
+	defer os.Unsetenv(EnvVarName)
+
+	val := envconst.Var(EnvVarName, &Var1)
+	if &Var1 != val {
+		t.Errorf("default value shut be same address")
+	}
+
+	err := os.Setenv(EnvVarName, "var2")
+	require.NoError(t, err)
+
+	val = envconst.Var(EnvVarName, &Var1)
+	require.Equal(t, &Var2, val, "only structural identity is required for non-default vars")
+}