fix(iroh-net): poll_send should drop transmits that we dont have a …

…`dest` for (#2393)

## Description

We've learned that if we return anything other than `Ok` in `poll_send`,
we will be blocking the endpoint and ruining all connections. We should
not return an error just because we don't have a proper address for the
`dest`, instead we should log an error and wait for the connection to
timeout. By returning `Poll::Ready(Ok(n))`, we are telling quinn to drop
the `n` transmits that are associated with `dest`. When quinn doesn't
receive any `ACK`s for those transmits, quinn end the connection with a
timeout error.

## Change checklist

- [x] Self-review.
- [x] Documentation updates if relevant.

iroh-net/src/magicsock.rs

@@ -612,11 +612,20 @@ impl MagicSock {
                 Poll::Ready(Ok(transmits_sent))
             }
             None => {
+                // Returning an error here would lock up the entire `Endpoint`.
+                //
+                // If we returned `Poll::Pending`, the waker driving the `poll_send` will never get woken up.
+                //
+                // Our best bet here is to log an error and return `Poll::Ready(Ok(n))`.
+                //
+                // `n` is the number of consecutive transmits in this batch that are meant for the same destination (a destination that we have no node state for, and so we can never actually send).
+                //
+                // When we return `Poll::Ready(Ok(n))`, we are effectively dropping those n messages, by lying to QUIC and saying they were sent.
+                // (If we returned `Poll::Ready(Ok(0))` instead, QUIC would loop to attempt to re-send those messages, blocking other traffic.)
+                //
+                // When `QUIC` gets no `ACK`s for those messages, the connection will eventually timeout.
                 error!(dst=%dest, "no node_state for mapped address");
-                Poll::Ready(Err(io::Error::new(
-                    io::ErrorKind::NotConnected,
-                    "trying to send to unknown node",
-                )))
+                Poll::Ready(Ok(n))
             }
         }
     }