bitcoin: add "extreme object size" deserialization unit test

As promised in rust-bitcoin#4762

I am not convinced that our decoding of large transactions and blocks is
properly tested to make sure that we don't accidentally start rejecting
very large objects. Add a unit test specifically for this.

Author: apoelstra

bitcoin/src/consensus/encode.rs

@@ -1058,4 +1058,45 @@ mod tests {
             FromHexError::Decode(DecodeError::Unconsumed)
         ));
     }
+
+    #[test]
+    fn deserialize_extreme_tx() {
+        use crate::{ScriptSigBuf, Witness};
+
+        // Start with transaction from `deserialize_tx_hex`
+        let hex = include_str!("../../tests/data/previous_tx_0_hex"); // An arbitrary transaction.
+        let tx = deserialize_hex::<Transaction>(hex).unwrap();
+
+        assert_eq!(tx.inputs.len(), 1);
+        assert_eq!(tx.outputs.len(), 2);
+        assert_eq!(tx.inputs[0].witness.len(), 2);
+
+        // 1. Test with 4 million witnesses.
+        let mut tx_copy = tx.clone();
+        tx_copy.inputs[0].witness = Witness::from_slice(&vec![vec![]; 4_000_000]);
+        let roundtrip = deserialize(&serialize(&tx_copy)).unwrap();
+        assert_eq!(tx_copy, roundtrip);
+
+        // 2. Test with a single large witness. (Size of 4 megs, including length prefix)
+        let mut tx_copy = tx.clone();
+        tx_copy.inputs[0].witness = Witness::from_slice(&vec![vec![0; 4_000_000 - 9]; 1]);
+        let roundtrip = deserialize(&serialize(&tx_copy)).unwrap();
+        assert_eq!(tx_copy, roundtrip);
+
+        // 3. Combine these; with the witness stack we can exceed a total size of 4M but
+        //    only by a tiny bit. (It is not part of our API guarantee that such things
+        //    will round-trip, but we unit test them anyway to help notice changes.)
+        let mut tx_copy = tx.clone();
+        tx_copy.inputs[0].witness = Witness::from_slice(&vec![vec![0; 997]; 4_000]);
+        let roundtrip = deserialize(&serialize(&tx_copy)).unwrap();
+        assert_eq!(tx_copy, roundtrip);
+
+        // 4. Test with a large script sig. With scriptsigs there is no limit on how large
+        //    an object we can parse, which is inconsistent with witnesses. Also not an
+        //    API guarantee.
+        let mut tx_copy = tx.clone();
+        tx_copy.inputs[0].script_sig = ScriptSigBuf::from(vec![0; 8_000_001]);
+        let roundtrip = deserialize(&serialize(&tx_copy)).unwrap();
+        assert_eq!(tx_copy, roundtrip);
+    }
 }