config: support arbitrary nested config structs (#4440)

Update bindEnvs() to add support for binding nested fields of the
Options struct to environment variables. The variable names are formed
by joining the nested fields' mapstructure tags with underscores (after
first converting to uppercase).

This is in preparation for adding a new nested struct for downstream
mTLS settings that will look something like this:

  downstream_mtls:
    ca_file: /path/to/CA/cert.pem
    enforcement: reject_connection

With this change, these fields would be bound to the variables
DOWNSTREAM_MTLS_CA_FILE and DOWNSTREAM_MTLS_ENFORCEMENT.

config/options_test.go

@@ -11,6 +11,8 @@ import (
 	"net/url"
 	"os"
 	"path/filepath"
+	"reflect"
+	"strings"
 	"sync"
 	"testing"
 	"time"
@@ -94,7 +96,7 @@ func Test_bindEnvs(t *testing.T) {
 	t.Setenv("POMERIUM_DEBUG", "true")
 	t.Setenv("POLICY", "LSBmcm9tOiBodHRwczovL2h0dHBiaW4ubG9jYWxob3N0LnBvbWVyaXVtLmlvCiAgdG86IAogICAgLSBodHRwOi8vbG9jYWxob3N0OjgwODEsMQo=")
 	t.Setenv("HEADERS", `{"X-Custom-1":"foo", "X-Custom-2":"bar"}`)
-	err := bindEnvs(o, v)
+	err := bindEnvs(v)
 	if err != nil {
 		t.Fatalf("failed to bind options to env vars: %s", err)
 	}
@@ -117,6 +119,83 @@ func Test_bindEnvs(t *testing.T) {
 	}
 }
 
+type Foo struct {
+	FieldOne Bar    `mapstructure:"field_one"`
+	FieldTwo string `mapstructure:"field_two"`
+}
+type Bar struct {
+	Baz  int    `mapstructure:"baz"`
+	Quux string `mapstructure:"quux"`
+}
+
+func Test_bindEnvsRecursive(t *testing.T) {
+	v := viper.New()
+	_, err := bindEnvsRecursive(reflect.TypeOf(Foo{}), v, "", "")
+	require.NoError(t, err)
+
+	t.Setenv("FIELD_ONE_BAZ", "123")
+	t.Setenv("FIELD_ONE_QUUX", "hello")
+	t.Setenv("FIELD_TWO", "world")
+
+	var foo Foo
+	v.Unmarshal(&foo)
+	assert.Equal(t, Foo{
+		FieldOne: Bar{
+			Baz:  123,
+			Quux: "hello",
+		},
+		FieldTwo: "world",
+	}, foo)
+}
+
+func Test_bindEnvsRecursive_Override(t *testing.T) {
+	v := viper.New()
+	v.SetConfigType("yaml")
+	v.ReadConfig(strings.NewReader(`
+field_one:
+  baz: 10
+  quux: abc
+field_two: hello
+`))
+
+	// Baseline: values populated from config file.
+	var foo1 Foo
+	v.Unmarshal(&foo1)
+	assert.Equal(t, Foo{
+		FieldOne: Bar{
+			Baz:  10,
+			Quux: "abc",
+		},
+		FieldTwo: "hello",
+	}, foo1)
+
+	_, err := bindEnvsRecursive(reflect.TypeOf(Foo{}), v, "", "")
+	require.NoError(t, err)
+
+	// Environment variables should selectively override config file keys.
+	t.Setenv("FIELD_ONE_QUUX", "def")
+	var foo2 Foo
+	v.Unmarshal(&foo2)
+	assert.Equal(t, Foo{
+		FieldOne: Bar{
+			Baz:  10,
+			Quux: "def",
+		},
+		FieldTwo: "hello",
+	}, foo2)
+
+	t.Setenv("FIELD_TWO", "world")
+	var foo3 Foo
+	v.Unmarshal(&foo3)
+	assert.Equal(t, Foo{
+		FieldOne: Bar{
+			Baz:  10,
+			Quux: "def",
+		},
+		FieldTwo: "world",
+	}, foo3)
+}
+
 func Test_parseHeaders(t *testing.T) {
 	// t.Parallel()
 	tests := []struct {