Merge rust-bitcoin/rust-bitcoin#2841: fuzz: add more coverage for `deserialize_psbt`

55cbae9c2f fuzz: add more coverage for `deserialize_psbt` (Bruno Garcia)

Pull request description:

  This PR adds more coverage for `deserialize_psbt` harness. It uses the `data` to create more than one psbt and then combine them.

ACKs for top commit:
  apoelstra:
    ACK 55cbae9c2f yeah, I like this

Tree-SHA512: 568899be8a82595c07e7b47c3011c093ce4ce49962e14781ea0c2eb9608fbba9920bbc81a296ec49d6ca124b813715bc0b8eb0cf887d44bbf8190f9a10c18845

fuzz/fuzz_targets/bitcoin/deserialize_psbt.rs

@@ -1,14 +1,37 @@
 use honggfuzz::fuzz;
 
+fn consume_random_bytes<'a>(data: &mut &'a [u8]) -> &'a [u8] {
+    if data.is_empty() {
+        return &[];
+    }
+
+    let length = (data[0] as usize) % (data.len() + 1);
+    let (bytes, rest) = data.split_at(length);
+    *data = rest;
+
+    bytes
+}
+
 fn do_test(data: &[u8]) {
-    let psbt: Result<bitcoin::psbt::Psbt, _> = bitcoin::psbt::Psbt::deserialize(data);
+    let mut new_data = data;
+    let bytes = consume_random_bytes(&mut new_data);
+    let psbt: Result<bitcoin::psbt::Psbt, _> = bitcoin::psbt::Psbt::deserialize(bytes);
     match psbt {
         Err(_) => {}
-        Ok(psbt) => {
+        Ok(mut psbt) => {
             let ser = bitcoin::psbt::Psbt::serialize(&psbt);
             let deser = bitcoin::psbt::Psbt::deserialize(&ser).unwrap();
             // Since the fuzz data could order psbt fields differently, we compare to our deser/ser instead of data
             assert_eq!(ser, bitcoin::psbt::Psbt::serialize(&deser));
+
+            let new_bytes = consume_random_bytes(&mut new_data);
+            let psbt_b: Result<bitcoin::psbt::Psbt, _> = bitcoin::psbt::Psbt::deserialize(new_bytes);
+            match psbt_b {
+                Err(_) => {},
+                Ok(mut psbt_b) => {
+                    assert_eq!(psbt_b.combine(psbt.clone()).is_ok(), psbt.combine(psbt_b).is_ok());
+                }
+            }
         }
     }
 }