Merge pull request #1040 from jonboulle/1040_config_env_vars

etcd needs to be configurable through env vars

main.go

@@ -48,6 +48,8 @@ func init() {
 func main() {
 	flag.Parse()
 
+	setFlagsFromEnv()
+
 	if *proxyMode {
 		startProxy()
 	} else {
@@ -198,3 +200,25 @@ func (as *Addrs) Set(s string) error {
 func (as *Addrs) String() string {
 	return strings.Join(*as, ",")
 }
+
+// setFlagsFromEnv parses all registered flags in the global flagset,
+// and if they are not already set it attempts to set their values from
+// environment variables. Environment variables take the name of the flag but
+// are UPPERCASE, have the prefix "ETCD_", and any dashes are replaced by
+// underscores - for example: some-flag => ETCD_SOME_FLAG
+func setFlagsFromEnv() {
+	alreadySet := make(map[string]bool)
+	flag.Visit(func(f *flag.Flag) {
+		alreadySet[f.Name] = true
+	})
+	flag.VisitAll(func(f *flag.Flag) {
+		if !alreadySet[f.Name] {
+			key := "ETCD_" + strings.ToUpper(strings.Replace(f.Name, "-", "_", -1))
+			val := os.Getenv(key)
+			if val != "" {
+				flag.Set(f.Name, val)
+			}
+		}
+
+	})
+}

main_test.go

@@ -0,0 +1,43 @@
+package main
+
+import "os"
+import "flag"
+import "testing"
+
+func TestSetFlagsFromEnv(t *testing.T) {
+	os.Clearenv()
+	// flags should be settable using env vars
+	os.Setenv("ETCD_DATA_DIR", "/foo/bar")
+	// and command-line flags
+	if err := flag.Set("peer-bind-addr", "1.2.3.4"); err != nil {
+		t.Fatal(err)
+	}
+	// command-line flags take precedence over env vars
+	os.Setenv("ETCD_ID", "woof")
+	if err := flag.Set("id", "quack"); err != nil {
+		t.Fatal(err)
+	}
+
+	// first verify that flags are as expected before reading the env
+	for f, want := range map[string]string{
+		"data-dir":       "",
+		"peer-bind-addr": "1.2.3.4",
+		"id":             "quack",
+	} {
+		if got := flag.Lookup(f).Value.String(); got != want {
+			t.Fatalf("flag %q=%q, want %q", f, got, want)
+		}
+	}
+
+	// now read the env and verify flags were updated as expected
+	setFlagsFromEnv()
+	for f, want := range map[string]string{
+		"data-dir":       "/foo/bar",
+		"peer-bind-addr": "1.2.3.4",
+		"id":             "quack",
+	} {
+		if got := flag.Lookup(f).Value.String(); got != want {
+			t.Errorf("flag %q=%q, want %q", f, got, want)
+		}
+	}
+}