fuzz: Add differential fuzzing for BTC VarInt

DarkFi's VarInt struct is meant to be equivalent to the one used by BTC.
Most of the source code is extremeley similar. This commit adds very
basic tests to ensure equivalence between DarkFi's implementation and
the one used by the BTC crate.
The tests included here are quite basic. Future work can expand on
the testing done on the deserialized values.
This harness can also be used as as guide or template for future
differential fuzzing.

fuzz/honggfuzz/Cargo.toml

@@ -9,6 +9,7 @@ cargo-fuzz = true
 
 [dependencies]
 honggfuzz = "0.5"
+bitcoin = "0.30.1"
 
 [dependencies.darkfi]
 path = "../.."
@@ -39,3 +40,9 @@ name = "serial-decode-string"
 path = "src/serial_decode_string.rs"
 test = false
 doc = false
+
+[[bin]]
+name = "varint-differential"
+path = "src/varint_differential.rs"
+test = false
+doc = false

fuzz/honggfuzz/src/varint_differential.rs

@@ -0,0 +1,56 @@
+/* This file is part of DarkFi (https://dark.fi)
+ *
+ * Copyright (C) 2020-2023 Dyne.org foundation
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+extern crate darkfi_serial;
+extern crate bitcoin;
+
+use honggfuzz::fuzz;
+use darkfi_serial::VarInt;
+use bitcoin::VarInt as BTCVarInt;
+// use bitcoin::consensus::serialize;
+// use bitcoin::psbt::serialize;
+// use darkfi_serial::{serialize, deserialize};
+
+fn main() {
+    loop {
+        fuzz!(|data: u64 | {
+            let dark_vi = VarInt(data.clone());
+            let btc_vi = BTCVarInt(data.clone());
+            assert_eq!(
+                dark_vi.length(),
+                btc_vi.len(),
+            );
+
+            let dark_ser = darkfi_serial::serialize(&dark_vi);
+            let btc_ser = bitcoin::consensus::serialize(&btc_vi);
+            assert_eq!(dark_ser, btc_ser);
+
+            let dark_des: VarInt = darkfi_serial::deserialize(&dark_ser).unwrap();
+            let btc_des: BTCVarInt = bitcoin::consensus::deserialize(&btc_ser).unwrap();
+            assert_eq!(
+                dark_des.length(),
+                btc_des.len(),
+            );
+
+            // assert_eq!(
+            //     darkfi_serial::decode(&dark_ser).unwrap(),
+            //     bitcoin::consensus::decode(&btc_ser).unwrap(),
+            // );
+        });
+    }
+}