api: massage protojson's output to use hex rather than base64

Fixes #1354.

api/helpers.go

@@ -1,12 +1,15 @@
 package api
 
 import (
+	"bytes"
+	"encoding/base64"
 	"encoding/hex"
 	"encoding/json"
 	"errors"
 	"fmt"
 	"math"
 	"math/big"
+	"reflect"
 	"strconv"
 	"strings"
 	"time"
@@ -71,9 +74,55 @@ func protoTxAsJSON(tx []byte) []byte {
 	if err != nil {
 		panic(err)
 	}
+	// protojson follows protobuf's json mapping behavior,
+	// which requires bytes to be encoded as base64:
+	// https://protobuf.dev/programming-guides/proto3/#json
+	//
+	// We want hex rather than base64 for consistency with our REST API
+	// protojson does not expose any option to configure its behavior,
+	// and as the Go types are code generated, we cannot use our HexBytes type.
+	//
+	// Do a bit of a hack: walk the protobuf value with reflection,
+	// find all []byte values, and search-and-replace their base64 encoding with hex
+	// in the protojson bytes. This can be slow if we have many byte slices,
+	// but in general the protobuf message will be small so there will be few.
+	bytesValues := collectBytesValues(nil, reflect.ValueOf(&ptx))
+	for _, bv := range bytesValues {
+		asBase64 := base64.StdEncoding.AppendEncode(nil, bv)
+		asHex := hex.AppendEncode(nil, bv)
+		println(string(asBase64), string(asHex))
+		asJSON = bytes.Replace(asJSON, asBase64, asHex, 1)
+	}
 	return asJSON
 }
 
+var typBytes = reflect.TypeFor[[]byte]()
+
+func collectBytesValues(result [][]byte, val reflect.Value) [][]byte {
+	typ := val.Type()
+	if typ == typBytes {
+		return append(result, val.Bytes())
+	}
+	switch typ.Kind() {
+	case reflect.Pointer, reflect.Interface:
+		if !val.IsNil() {
+			result = collectBytesValues(result, val.Elem())
+		}
+	case reflect.Struct:
+		for i := 0; i < val.NumField(); i++ {
+			if !typ.Field(i).IsExported() {
+				continue
+			}
+			result = collectBytesValues(result, val.Field(i))
+		}
+	case reflect.Slice, reflect.Array:
+		for i := 0; i < val.Len(); i++ {
+			result = collectBytesValues(result, val.Index(i))
+		}
+	}
+	return result
+}
+
 // isTransactionType checks if the given transaction is of the given type.
 // t is expected to be a pointer to a protobuf transaction message.
 func isTransactionType[T any](signedTxBytes []byte) (bool, error) {

api/helpers_test.go

@@ -161,8 +161,8 @@ func TestProtoTxAsJSON(t *testing.T) {
 	"setProcess": {
 		"txtype": "SET_PROCESS_CENSUS",
 		"nonce": 1,
-		"processId": "sx3/YYFNq5DWw6m2XWyQgwSA5Lda0y50eUICAAAAAAA=",
-		"censusRoot": "zUU9BcTLBCnuXuGu/tAW9VO4AmtM7VsMNSkFv6U8foE=",
+		"processId": "b31dff61814dab90d6c3a9b65d6c90830480e4b75ad32e747942020000000000",
+		"censusRoot": "cd453d05c4cb0429ee5ee1aefed016f553b8026b4ced5b0c352905bfa53c7e81",
 		"censusURI": "ipfs://bafybeicyfukarcryrvy5oe37ligmxwf55sbfiojori4t25wencma4ymxfa",
 		"censusSize": "1000"
 	}