udp remote pbuf helper: honor fragmented packets (#6222)

fix for #5960 didn't take fragmented packets into account
fixes #6218

Author: d-a-v

libraries/ESP8266WiFi/src/include/UdpContext.h

@@ -34,6 +34,7 @@ void esp_schedule();
 
 #define PBUF_ALIGNER_ADJUST 4
 #define PBUF_ALIGNER(x) ((void*)((((intptr_t)(x))+3)&~3))
+#define PBUF_HELPER_FLAG 0xff // lwIP pbuf flag: u8_t
 
 class UdpContext
 {
@@ -243,21 +244,24 @@ class UdpContext
 
         if (_rx_buf)
         {
-            // we have interleaved informations on addresses within reception pbuf chain:
-            // before: (data-pbuf) -> (data-pbuf) -> (data-pbuf) -> ... in the receiving order
-            // now: (address-info-pbuf -> data-pbuf) -> (address-info-pbuf -> data-pbuf) -> ...
+            if (_rx_buf->flags == PBUF_HELPER_FLAG)
+            {
+                // we have interleaved informations on addresses within reception pbuf chain:
+                // before: (data-pbuf) -> (data-pbuf) -> (data-pbuf) -> ... in the receiving order
+                // now: (address-info-pbuf -> data-pbuf) -> (address-info-pbuf -> data-pbuf) -> ...
 
-            // so the first rx_buf contains an address helper,
-            // copy it to "current address"
-            auto helper = (AddrHelper*)PBUF_ALIGNER(_rx_buf->payload);
-            _currentAddr = *helper;
+                // so the first rx_buf contains an address helper,
+                // copy it to "current address"
+                auto helper = (AddrHelper*)PBUF_ALIGNER(_rx_buf->payload);
+                _currentAddr = *helper;
 
-            // destroy the helper in the about-to-be-released pbuf
-            helper->~AddrHelper();
+                // destroy the helper in the about-to-be-released pbuf
+                helper->~AddrHelper();
 
-            // forward in rx_buf list, next one is effective data
-            // current (not ref'ed) one will be pbuf_free'd with deleteme
-            _rx_buf = _rx_buf->next;
+                // forward in rx_buf list, next one is effective data
+                // current (not ref'ed) one will be pbuf_free'd with deleteme
+                _rx_buf = _rx_buf->next;
+            }
 
             // this rx_buf is not nullptr by construction,
             // ref'ing it to prevent release from the below pbuf_free(deleteme)
@@ -471,6 +475,7 @@ class UdpContext
             }
             // construct in place
             new(PBUF_ALIGNER(pb_helper->payload)) AddrHelper(srcaddr, TEMPDSTADDR, srcport);
+            pb->flags = PBUF_HELPER_FLAG; // mark helper pbuf
             // chain it
             pbuf_cat(_rx_buf, pb_helper);