Add a bytes round-trip test for compactsize encoding.

zebra-chain/proptest-regressions/serialization.txt

@@ -5,3 +5,4 @@
 # It is recommended to check this file in to source control so that
 # everyone who runs the test benefits from these saved cases.
 cc ddefa5145f08994fb873af028344fc03d656dcd1f9287fac71dfa98cbd6a5090 # shrinks to s = 65536
+cc c04b21f87727bdd1fd9eb7a1616104fd65fa617f507bb6e8163be6542ff2ed68 # shrinks to bytes = [253, 0, 0, 0, 0, 0, 0, 0, 0]

zebra-chain/src/serialization.rs

@@ -282,16 +282,30 @@ mod tests {
     use std::io::Cursor;
 
     proptest! {
-        // The test below is cheap so we can run it a lot.
+        // The tests below are cheap so we can run them a lot.
         #![proptest_config(ProptestConfig::with_cases(100_000))]
 
         #[test]
-        fn compactsize_round_trip(s in 0u64..0x2_0000u64) {
+        fn compactsize_write_then_read_round_trip(s in 0u64..0x2_0000u64) {
             // Maximum encoding size of a compactsize is 9 bytes.
             let mut buf = [0u8; 8+1];
             Cursor::new(&mut buf[..]).write_compactsize(s).unwrap();
             let expect_s = Cursor::new(&buf[..]).read_compactsize().unwrap();
             prop_assert_eq!(s, expect_s);
         }
+
+        #[test]
+        fn compactsize_read_then_write_round_trip(bytes in prop::array::uniform9(0u8..)) {
+            let s = Cursor::new(&bytes[..]).read_compactsize().unwrap();
+            // The compactsize encoding is variable-length, so we may not even
+            // read all of the input bytes, and therefore we can't expect that
+            // the encoding will reproduce bytes that were never read. Instead,
+            // copy the input bytes, and overwrite them with the encoding of s,
+            // so that if the encoding is different, we'll catch it on the part
+            // that's written.
+            let mut expect_bytes = bytes.clone();
+            Cursor::new(&mut expect_bytes[..]).write_compactsize(s).unwrap();
+            prop_assert_eq!(bytes, expect_bytes);
+        }
     }
 }