Fix vsock test flake

lib/propolis/src/vsock/poller.rs

@@ -1465,6 +1465,10 @@ mod test {
         let mut accepted = listener.accept().unwrap().0;
         accepted.set_nonblocking(false).unwrap();
         accepted.set_read_timeout(Some(Duration::from_secs(5))).unwrap();
+
+        // Wait for the RESPONSE packet to arrive. The poller takes a desc from
+        // the rx_avail ring, writes the packet into it, and publishes it via
+        // `push_used` updating rx_used.
         wait_for_condition(|| harness.rx_used_idx() >= 1, 5000);
 
         // Send enough data to exceed half the buffer capacity (64KB).
@@ -1473,8 +1477,14 @@ mod test {
         let payload = vec![0xAB_u8; chunk_size];
         let total_sent = num_chunks * chunk_size;
 
+        // This is the vsock poller copying the REQUEST data and calling
+        // `push_used` which bumps the tx ring's used index.
+        let initial_tx_used = harness.tx_used_idx();
+
         for tx_consumed in (1u16..).take(num_chunks) {
-            // Reuse descriptor slots each iteration
+            // Reuse descriptor slots each iteration. This is only safe because
+            // this test ensures that all tx_avail descriptors are pushed back
+            // to us via tx_used.
             harness.reset_tx_cursors();
 
             let mut rw_hdr = VsockPacketHeader::new();
@@ -1495,7 +1505,13 @@ mod test {
             harness.publish_tx(d_hdr);
             notify.queue_notify(VSOCK_TX_QUEUE).unwrap();
 
-            wait_for_condition(|| harness.tx_used_idx() >= tx_consumed, 5000);
+            // Wait for the poller to release the desc chain back to us via
+            // tx_used. This is the running total of the REQUEST and n RW
+            // packets.
+            wait_for_condition(
+                || harness.tx_used_idx() >= initial_tx_used + tx_consumed,
+                5000,
+            );
         }
 
         // Drain the data from the accepted socket to confirm it arrived