feat(transport): accept borrowed instead of owned input datagram

Previously `process_input` (and the like) took a `Datagram<Vec<u8>>` as input.
In other words, it required allocating each UDP datagram payload in a dedicated
`Vec<u8>` before passing it to `process_input`.

With this patch, `process_input` accepts a `Datagram<&[u8]>`. In other words, it
accepts a `Datagram` with a borrowed view into an existing buffer (`&[u8]`),
e.g. a long lived receive buffer.

fuzz/fuzz_targets/client_initial.rs

@@ -17,7 +17,7 @@ fuzz_target!(|data: &[u8]| {
     };
 
     let mut client = default_client();
-    let ci = client.process(None, now()).dgram().expect("a datagram");
+    let ci = client.process_output(now()).dgram().expect("a datagram");
     let Some((header, d_cid, s_cid, payload)) = decode_initial_header(&ci, Role::Client) else {
         return;
     };
@@ -60,7 +60,7 @@ fuzz_target!(|data: &[u8]| {
     let fuzzed_ci = Datagram::new(ci.source(), ci.destination(), ci.tos(), ciphertext);
 
     let mut server = default_server();
-    let _response = server.process(Some(&fuzzed_ci), now());
+    let _response = server.process(Some(fuzzed_ci), now());
 });
 
 #[cfg(any(not(fuzzing), windows))]

fuzz/fuzz_targets/server_initial.rs

@@ -17,12 +17,9 @@ fuzz_target!(|data: &[u8]| {
     };
 
     let mut client = default_client();
-    let ci = client.process(None, now()).dgram().expect("a datagram");
+    let ci = client.process_output(now()).dgram().expect("a datagram");
     let mut server = default_server();
-    let si = server
-        .process(Some(&ci), now())
-        .dgram()
-        .expect("a datagram");
+    let si = server.process(Some(ci), now()).dgram().expect("a datagram");
 
     let Some((header, d_cid, s_cid, payload)) = decode_initial_header(&si, Role::Server) else {
         return;
@@ -64,7 +61,7 @@ fuzz_target!(|data: &[u8]| {
         (header_enc.len() - 1)..header_enc.len(),
     );
     let fuzzed_si = Datagram::new(si.source(), si.destination(), si.tos(), ciphertext);
-    let _response = client.process(Some(&fuzzed_si), now());
+    let _response = client.process(Some(fuzzed_si), now());
 });
 
 #[cfg(any(not(fuzzing), windows))]

neqo-bin/src/client/http09.rs

@@ -183,7 +183,7 @@ impl super::Client for Connection {
 
     fn process_multiple_input<'a, I>(&mut self, dgrams: I, now: Instant)
     where
-        I: IntoIterator<Item = &'a Datagram>,
+        I: IntoIterator<Item = Datagram<&'a [u8]>>,
     {
         self.process_multiple_input(dgrams, now);
     }

neqo-bin/src/client/http3.rs

@@ -134,7 +134,7 @@ impl super::Client for Http3Client {
 
     fn process_multiple_input<'a, I>(&mut self, dgrams: I, now: Instant)
     where
-        I: IntoIterator<Item = &'a Datagram>,
+        I: IntoIterator<Item = Datagram<&'a [u8]>>,
     {
         self.process_multiple_input(dgrams, now);
     }

neqo-bin/src/client/mod.rs

@@ -376,7 +376,7 @@ trait Client {
     fn process_output(&mut self, now: Instant) -> Output;
     fn process_multiple_input<'a, I>(&mut self, dgrams: I, now: Instant)
     where
-        I: IntoIterator<Item = &'a Datagram>;
+        I: IntoIterator<Item = Datagram<&'a [u8]>>;
     fn has_events(&self) -> bool;
     fn close<S>(&mut self, now: Instant, app_error: AppError, msg: S)
     where
@@ -462,7 +462,7 @@ impl<'a, H: Handler> Runner<'a, H> {
                 break;
             }
             self.client
-                .process_multiple_input(dgrams.iter(), Instant::now());
+                .process_multiple_input(dgrams.iter().map(Datagram::borrow), Instant::now());
             self.process_output().await?;
         }