From a4881b949cbf8995e6a6524e780ce58c39efa33c Mon Sep 17 00:00:00 2001
From: Jonathan Hui <jonhui@google.com>
Date: Fri, 10 Jul 2026 06:51:15 -0700
Subject: [PATCH] [posix] drain TREL UDP packets per mainloop iteration
 (#13324)

This commit updates `ReceivePacket()` in the POSIX TREL platform
implementation (`trel.cpp`) to drain all available UDP datagrams
in a loop whenever the TREL socket (`sSocket`) is readable.

Previously, `ReceivePacket()` called `recvfrom()` exactly once per
`platformTrelProcess()` iteration. When multiple TREL packets (such as
an `AddressNotify` immediately followed by MLE or UDP traffic) arrived
in rapid succession, only the first datagram was read, leaving the
remaining packets buffered in the kernel until subsequent mainloop
cycles.

If an interleaved packet on another interface (such as `NcpSpinel` on
15.4) was processed before the next mainloop iteration, the neighbor's
`mLastRxFragmentTag` advanced, causing the delayed TREL datagram
(`AddressNotify`) to be dropped as `kErrorDuplicated` due to
`!neighbor->IsLastRxFragmentTagAfter(tag)`.

By looping over `recvfrom()` until `EAGAIN` or `EWOULDBLOCK` is returned
(`sSocket` is non-blocking), all queued TREL packets are read and
processed immediately in receipt order. A loop budget
(`kMaxRxPacketsPerIteration = 64`) is included to prevent mainloop
starvation during high-rate UDP floods. Additionally, a typo in the
buffer overflow check (`sizeof(sRxPacketLength)` vs
`sizeof(sRxPacketBuffer)`) is corrected.
---
 src/posix/platform/trel.cpp | 61 ++++++++++++++++++++++---------------
 1 file changed, 37 insertions(+), 24 deletions(-)

diff --git a/src/posix/platform/trel.cpp b/src/posix/platform/trel.cpp
index f2d2d8e0d..eb5eecacc 100644
--- a/src/posix/platform/trel.cpp
+++ b/src/posix/platform/trel.cpp
@@ -276,37 +276,50 @@ exit:
 
 static void ReceivePacket(int aSocket, otInstance *aInstance)
 {
-    struct sockaddr_in6 sockAddr;
-    socklen_t           sockAddrLen = sizeof(sockAddr);
-    ssize_t             ret;
+    const uint16_t kMaxRxPacketsPerIteration = 64;
 
-    memset(&sockAddr, 0, sizeof(sockAddr));
-
-    ret = recvfrom(aSocket, (char *)sRxPacketBuffer, sizeof(sRxPacketBuffer), 0, (struct sockaddr *)&sockAddr,
-                   &sockAddrLen);
-    VerifyOrDie(ret >= 0, OT_EXIT_ERROR_ERRNO);
-
-    sRxPacketLength = (uint16_t)(ret);
-
-    if (sRxPacketLength > sizeof(sRxPacketBuffer))
+    for (uint16_t i = 0; i < kMaxRxPacketsPerIteration;)
     {
-        sRxPacketLength = sizeof(sRxPacketLength);
-    }
+        struct sockaddr_in6 sockAddr;
+        socklen_t           sockAddrLen = sizeof(sockAddr);
+        ssize_t             ret;
 
-    LogDebg("ReceivePacket() - received from [%s]:%d, id:%d, pkt:%s", Ip6AddrToString(&sockAddr.sin6_addr),
-            ntohs(sockAddr.sin6_port), sockAddr.sin6_scope_id, BufferToString(sRxPacketBuffer, sRxPacketLength));
+        memset(&sockAddr, 0, sizeof(sockAddr));
 
-    if (sEnabled)
-    {
-        otSockAddr senderAddr;
+        ret = recvfrom(aSocket, (char *)sRxPacketBuffer, sizeof(sRxPacketBuffer), 0, (struct sockaddr *)&sockAddr,
+                       &sockAddrLen);
+        if (ret < 0)
+        {
+            if (errno == EAGAIN || errno == EWOULDBLOCK)
+            {
+                break;
+            }
+            if (errno == EINTR)
+            {
+                continue;
+            }
+            VerifyOrDie(false, OT_EXIT_ERROR_ERRNO);
+        }
 
-        ++sCounters.mRxPackets;
-        sCounters.mRxBytes += sRxPacketLength;
+        sRxPacketLength = (uint16_t)(ret);
 
-        memcpy(&senderAddr.mAddress, &sockAddr.sin6_addr, sizeof(otIp6Address));
-        senderAddr.mPort = ntohs(sockAddr.sin6_port);
+        LogDebg("ReceivePacket() - received from [%s]:%d, id:%d, pkt:%s", Ip6AddrToString(&sockAddr.sin6_addr),
+                ntohs(sockAddr.sin6_port), sockAddr.sin6_scope_id, BufferToString(sRxPacketBuffer, sRxPacketLength));
 
-        otPlatTrelHandleReceived(aInstance, sRxPacketBuffer, sRxPacketLength, &senderAddr);
+        if (sEnabled)
+        {
+            otSockAddr senderAddr;
+
+            ++sCounters.mRxPackets;
+            sCounters.mRxBytes += sRxPacketLength;
+
+            memcpy(&senderAddr.mAddress, &sockAddr.sin6_addr, sizeof(otIp6Address));
+            senderAddr.mPort = ntohs(sockAddr.sin6_port);
+
+            otPlatTrelHandleReceived(aInstance, sRxPacketBuffer, sRxPacketLength, &senderAddr);
+        }
+
+        i++;
     }
 }