support/fuzz: Add xdr.ClaimPredicate JSON fuzzing (#3181)

A bug in `xdr.ClaimPredicate.MarshalJSON` (#3179) stopped ingestion in
testnet. I wrote a fuzzer to ensure there are no more bugs. Added files
to separate `xdr/fuzz` package to add more fuzzers in the future.

xdr/fuzz/jsonclaimpredicate/fuzz.go

@@ -0,0 +1,78 @@
+// +build gofuzz
+
+package jsonclaimpredicate
+
+import (
+	"bytes"
+	"encoding/json"
+
+	"github.com/stellar/go/xdr"
+)
+
+// Fuzz is go-fuzz function for fuzzing xdr.ClaimPredicate JSON
+// marshaller and unmarshaller.
+func Fuzz(data []byte) int {
+	// Ignore malformed ClaimPredicate
+	var p xdr.ClaimPredicate
+	err := xdr.SafeUnmarshal(data, &p)
+	if err != nil {
+		return -1
+	}
+
+	// Ignore invalid predicates: (and/or length != 2, nil not)
+	if !validate(p) {
+		return -1
+	}
+
+	j, err := json.Marshal(p)
+	if err != nil {
+		panic(err)
+	}
+
+	var p2 xdr.ClaimPredicate
+	err = json.Unmarshal(j, &p2)
+	if err != nil {
+		panic(err)
+	}
+
+	j2, err := json.Marshal(p2)
+	if err != nil {
+		panic(err)
+	}
+
+	if !bytes.Equal(j, j2) {
+		panic("not equal " + string(j) + " " + string(j2))
+	}
+
+	return 1
+}
+
+func validate(p xdr.ClaimPredicate) bool {
+	switch p.Type {
+	case xdr.ClaimPredicateTypeClaimPredicateUnconditional:
+		return true
+	case xdr.ClaimPredicateTypeClaimPredicateAnd:
+		and := *p.AndPredicates
+		if len(and) != 2 {
+			return false
+		}
+		return validate(and[0]) && validate(and[1])
+	case xdr.ClaimPredicateTypeClaimPredicateOr:
+		or := *p.OrPredicates
+		if len(or) != 2 {
+			return false
+		}
+		return validate(or[0]) && validate(or[1])
+	case xdr.ClaimPredicateTypeClaimPredicateNot:
+		if *p.NotPredicate == nil {
+			return false
+		}
+		return validate(**p.NotPredicate)
+	case xdr.ClaimPredicateTypeClaimPredicateBeforeAbsoluteTime:
+		return *p.AbsBefore >= 0
+	case xdr.ClaimPredicateTypeClaimPredicateBeforeRelativeTime:
+		return *p.RelBefore >= 0
+	}
+
+	panic("Invalid type")
+}