fix: /base64 endpoint decodes both URL-safe and standard b64 encodings (#153)

As reported in #152, the `/base64` endpoint can only decode the
"URL-safe" base64 encoding, but the error it returns is not very useful
if you're not already familiar with different base64 encoding variants.

Here we follow [Postel's law][1] and accept either the URL-safe or
standard encodings, while continuing to use the URL-safe variant
when encoding ourselves.

Fixes #152.

[1]: https://en.wikipedia.org/wiki/Robustness_principle

Author: mccutchen

httpbin/handlers_test.go

@@ -2707,6 +2707,13 @@ func TestBase64(t *testing.T) {
 			"/base64/decode/YWJjMTIzIT8kKiYoKSctPUB-",
 			"abc123!?$*&()'-=@~",
 		},
+		{
+			// Std base64 is also supported for decoding (+ instead of - in
+			// encoded input string). See also:
+			// https://github.com/mccutchen/go-httpbin/issues/152
+			"/base64/decode/8J+Ziywg8J+MjSEK4oCm",
+			"🙋, 🌍!\n…",
+		},
 		{
 			// URL-safe base64 is used for encoding (note the - instead of + in
 			// encoded output string)
@@ -2764,12 +2771,6 @@ func TestBase64(t *testing.T) {
 			"/base64/unknown/dmFsaWRfYmFzZTY0X2VuY29kZWRfc3RyaW5n",
 			"invalid operation: unknown",
 		},
-		{
-			// we only support URL-safe base64 encoded strings (note the +
-			// instead of - in encoded input string)
-			"/base64/decode/YWJjMTIzIT8kKiYoKSctPUB+",
-			"illegal base64 data",
-		},
 	}
 
 	for _, test := range errorTests {

httpbin/helpers.go

@@ -429,16 +429,20 @@ func newBase64Helper(path string) (*base64Helper, error) {
 	return &b, nil
 }
 
-// Encode - encode data as base64
+// Encode - encode data as URL-safe base64
 func (b *base64Helper) Encode() ([]byte, error) {
 	buff := make([]byte, base64.URLEncoding.EncodedLen(len(b.data)))
 	base64.URLEncoding.Encode(buff, []byte(b.data))
 	return buff, nil
 }
 
-// Decode - decode data from base64
+// Decode - decode data from base64, attempting both URL-safe and standard
+// encodings.
 func (b *base64Helper) Decode() ([]byte, error) {
-	return base64.URLEncoding.DecodeString(b.data)
+	if result, err := base64.URLEncoding.DecodeString(b.data); err == nil {
+		return result, nil
+	}
+	return base64.StdEncoding.DecodeString(b.data)
 }
 
 func wildCardToRegexp(pattern string) string {