chore(relay): fix flaky proptest (#4157)

This turned out to be a user error in how I was using proptest.

Related: https://github.com/proptest-rs/proptest/issues/72.
Resolves: #3965.

rust/relay/src/proptest.rs

@@ -1,6 +1,5 @@
 use crate::Binding;
 use proptest::arbitrary::any;
-use proptest::collection::vec;
 use proptest::strategy::Just;
 use proptest::strategy::Strategy;
 use proptest::string::string_regex;
@@ -30,11 +29,16 @@ pub fn channel_number() -> impl Strategy<Value = ChannelNumber> {
     (ChannelNumber::MIN..ChannelNumber::MAX).prop_map(|n| ChannelNumber::new(n).unwrap())
 }
 
-pub fn channel_payload() -> impl Strategy<Value = (Vec<u8>, usize)> {
-    vec(any::<u8>(), 0..(u16::MAX as usize)).prop_flat_map(|vec| {
-        let len = vec.len();
-        (Just(vec), 0..len)
-    })
+pub fn channel_payload() -> impl Strategy<Value = (Vec<u8>, u16)> {
+    any::<Vec<u8>>()
+        .prop_filter("payload does not fit into u16", |vec| {
+            vec.len() <= u16::MAX as usize
+        })
+        .prop_map(|vec| {
+            let len = vec.len();
+
+            (vec, len as u16)
+        })
 }
 
 pub fn username_salt() -> impl Strategy<Value = String> {

rust/relay/src/server/channel_data.rs

@@ -97,12 +97,12 @@ mod tests {
     #[test_strategy::proptest]
     fn channel_data_decoding(
         #[strategy(crate::proptest::channel_number())] channel: ChannelNumber,
-        #[strategy(crate::proptest::channel_payload())] payload: (Vec<u8>, usize),
+        #[strategy(crate::proptest::channel_payload())] payload: (Vec<u8>, u16),
     ) {
-        let encoded = to_bytes(channel.value(), payload.1 as u16, &payload.0);
+        let encoded = to_bytes(channel.value(), payload.1, &payload.0);
         let parsed = ChannelData::parse(&encoded).unwrap();
 
         assert_eq!(channel.value(), parsed.channel);
-        assert_eq!(&payload.0[..payload.1], parsed.data)
+        assert_eq!(&payload.0[..(payload.1 as usize)], parsed.data)
     }
 }