patches bug in recv_mmsg when npkts != nrecv

If recv_mmsg receives 2 packets where the first one is filtered out,
then it returns npkts == 1:
https://github.com/solana-labs/solana/blob/01a096adc/streamer/src/recvmmsg.rs#L104-L115

But then streamer::packet::recv_from will erroneously keep the 1st
packet and drop the 2nd one:
https://github.com/solana-labs/solana/blob/01a096adc/streamer/src/packet.rs#L34-L49

To avoid this bug, this commit updates recv_mmsg to always return total
number of received packets. If socket address cannot be correctly
obtained, it is left as the default value which is UNSPECIFIED:
https://github.com/solana-labs/solana/blob/01a096adc/sdk/src/packet.rs#L145

streamer/src/packet.rs

@@ -112,6 +112,10 @@ mod tests {
         }
         send_to(&batch, &send_socket, &SocketAddrSpace::Unspecified).unwrap();
 
+        batch
+            .packets
+            .iter_mut()
+            .for_each(|pkt| pkt.meta = Meta::default());
         let recvd = recv_from(&mut batch, &recv_socket, 1).unwrap();
 
         assert_eq!(recvd, batch.packets.len());

streamer/src/recvmmsg.rs

@@ -2,7 +2,7 @@
 
 pub use solana_perf::packet::NUM_RCVMMSGS;
 use {
-    crate::packet::Packet,
+    crate::packet::{Meta, Packet},
     std::{cmp, io, net::UdpSocket},
 };
 #[cfg(target_os = "linux")]
@@ -15,6 +15,7 @@ use {
 
 #[cfg(not(target_os = "linux"))]
 pub fn recv_mmsg(socket: &UdpSocket, packets: &mut [Packet]) -> io::Result</*num packets:*/ usize> {
+    debug_assert!(packets.iter().all(|pkt| pkt.meta == Meta::default()));
     let mut i = 0;
     let count = cmp::min(NUM_RCVMMSGS, packets.len());
     for p in packets.iter_mut().take(count) {
@@ -66,6 +67,8 @@ fn cast_socket_addr(addr: &sockaddr_storage, hdr: &mmsghdr) -> Option<InetAddr>
 #[cfg(target_os = "linux")]
 #[allow(clippy::uninit_assumed_init)]
 pub fn recv_mmsg(sock: &UdpSocket, packets: &mut [Packet]) -> io::Result</*num packets:*/ usize> {
+    // Assert that there are no leftovers in packets.
+    debug_assert!(packets.iter().all(|pkt| pkt.meta == Meta::default()));
     const SOCKADDR_STORAGE_SIZE: usize = mem::size_of::<sockaddr_storage>();
 
     let mut hdrs: [mmsghdr; NUM_RCVMMSGS] = unsafe { mem::zeroed() };
@@ -93,23 +96,18 @@ pub fn recv_mmsg(sock: &UdpSocket, packets: &mut [Packet]) -> io::Result</*num p
     };
     let nrecv =
         unsafe { libc::recvmmsg(sock_fd, &mut hdrs[0], count as u32, MSG_WAITFORONE, &mut ts) };
-    if nrecv < 0 {
+    let nrecv = if nrecv < 0 {
         return Err(io::Error::last_os_error());
+    } else {
+        usize::try_from(nrecv).unwrap()
+    };
+    for (addr, hdr, pkt) in izip!(addrs, hdrs, packets.iter_mut()).take(nrecv) {
+        pkt.meta.size = hdr.msg_len as usize;
+        if let Some(addr) = cast_socket_addr(&addr, &hdr) {
+            pkt.meta.set_addr(&addr.to_std());
+        }
     }
-    let mut npkts = 0;
-
-    izip!(addrs, hdrs, packets.iter_mut())
-        .take(nrecv as usize)
-        .filter_map(|(addr, hdr, pkt)| {
-            let addr = cast_socket_addr(&addr, &hdr)?.to_std();
-            Some((addr, hdr, pkt))
-        })
-        .for_each(|(addr, hdr, pkt)| {
-            pkt.meta.size = hdr.msg_len as usize;
-            pkt.meta.set_addr(&addr);
-            npkts += 1;
-        });
-    Ok(npkts)
+    Ok(nrecv)
 }
 
 #[cfg(test)]
@@ -176,6 +174,9 @@ mod tests {
                 assert_eq!(packet.meta.addr(), saddr);
             }
 
+            packets
+                .iter_mut()
+                .for_each(|pkt| pkt.meta = Meta::default());
             let recv = recv_mmsg(&reader, &mut packets[..]).unwrap();
             assert_eq!(sent - TEST_NUM_MSGS, recv);
             for packet in packets.iter().take(recv) {
@@ -216,6 +217,9 @@ mod tests {
         }
         reader.set_nonblocking(true).unwrap();
 
+        packets
+            .iter_mut()
+            .for_each(|pkt| pkt.meta = Meta::default());
         let _recv = recv_mmsg(&reader, &mut packets[..]);
         assert!(start.elapsed().as_secs() < 5);
     }
@@ -256,6 +260,9 @@ mod tests {
             assert_eq!(packet.meta.addr(), saddr2);
         }
 
+        packets
+            .iter_mut()
+            .for_each(|pkt| pkt.meta = Meta::default());
         let recv = recv_mmsg(&reader, &mut packets[..]).unwrap();
         assert_eq!(sent1 + sent2 - TEST_NUM_MSGS, recv);
         for packet in packets.iter().take(recv) {

streamer/tests/recvmmsg.rs

@@ -2,7 +2,7 @@
 
 use {
     solana_streamer::{
-        packet::{Packet, PACKET_DATA_SIZE},
+        packet::{Meta, Packet, PACKET_DATA_SIZE},
         recvmmsg::*,
     },
     std::{net::UdpSocket, time::Instant},
@@ -44,6 +44,9 @@ pub fn test_recv_mmsg_batch_size() {
             if recv >= TEST_BATCH_SIZE {
                 break;
             }
+            packets
+                .iter_mut()
+                .for_each(|pkt| pkt.meta = Meta::default());
         }
         elapsed_in_small_batch += now.elapsed().as_nanos();
         assert_eq!(TEST_BATCH_SIZE, recv);